JVM Troubleshooting Guide

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Troubleshoot the JVM like never before

JVM Troubleshooting
Guide
Pierre-Hugues Charbonneau
Ilias Tsagklis

JVM Troubleshooting Handbook

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Table of Contents
Oracle HotSpot JVM Memory...................................................................................................................3
Java HotSpot VM Heap space...............................................................................................................3
Java HotSpot VM PermGen space........................................................................................................4
IBM JVM Memory.................................................................................................................................... 6
Oracle JRockit JVM Memory....................................................................................................................7
Tips for proper Java Heap size...................................................................................................................8
Java Threading: JVM Retained memory analysis....................................................................................14
Java 8: From PermGen to Metaspace.......................................................................................................21
HPROF - Memory leak analysis with Eclipse Memory Analyzer Tool (MAT).......................................26
JVM verbose GC output tutorial..............................................................................................................33
Analyzing thread dumps.......................................................................................................................... 40
Introduction to thread dump analysis.................................................................................................. 40
Thread Dump: Thread Stack Trace analysis........................................................................................47
Java Thread CPU analysis on Windows...................................................................................................49
Case Study - Too many open files............................................................................................................54
GC overhead limit exceeded – Analysis and Patterns..............................................................................58
Java deadlock troubleshooting and analysis............................................................................................ 69
Java Thread deadlock - Case Study......................................................................................................... 73
Java concurrency: the hidden thread deadlocks.......................................................................................79
OutOfMemoryError patterns....................................................................................................................85
OutOfMemoryError: Java heap space - what is it?.............................................................................86
OutOfMemoryError: Out of swap space - Problem Patterns..............................................................87
OutOfMemoryError: unable to create new native thread....................................................................89
ClassNotFoundException: How to resolve..............................................................................................93
NoClassDefFoundError Problem patterns............................................................................................... 99
NoClassDefFoundError – How to resolve........................................................................................ 103
NoClassDefFoundError problem case 1 - missing JAR file............................................................. 105
NoClassDefFoundError problem case 2 - static initializer failure.................................................... 113

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Oracle HotSpot JVM Memory
Java HotSpot VM Heap space
This section will provide you with a high level overview of the different Java Heap memory spaces of
the Oracle Java HotSpot VM. This understanding is quite important for any individual involved in
production support given how frequent memory problems are observed. Proper knowledge of the Java
VM Heap Space is critical.
Your Java VM is basically the foundation of your Java program which provides you with dynamic
memory management services, garbage collection, Threads, IO and native operations and more.
The Java Heap Space is the memory "container" of you runtime Java program which provides to your
Java program the proper memory spaces it needs (Java Heap, Native Heap) and managed by the
JVM itself.
The JVM HotSpot memory is split between 3 memory spaces:
• The Java Heap
• The PermGen (permanent generation) space
• The Native Heap (C-Heap)
Here is the breakdown for each one of them:
Memory Space
Java Heap

Start-up arguments
and tuning

Monitoring strategies

-Xmx (maximum Heap
space)

- verbose GC
- JMX API
- JConsole
- Other monitoring tools

The Java Heap is
storing your primary
Java program Class
instances.

- verbose GC
- JMX API
- JConsole
- Other monitoring tools

The Java HotSpot VM
permanent generation
space is the JVM
storage used mainly to
store your Java Class
objects such as names
and method of the
Classes, internal JVM
objects and other JIT
optimization related

-Xms (minimum Heap
size)

Description

EX:
-Xmx1024m
-Xms1024m
PermGen

-XX:MaxPermSize
(maximum size)
-XX:PermSize
(minimum size)
EX:
XX:MaxPermSize=512

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Native Heap
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m
-XX:PermSize=256m

data.

Not configurable
directly.

The C-Heap is storing
objects such as MMAP
file, other JVM and third
party native code
objects.

- Total process size
check in Windows and
Linux
For a 32-bit VM, the C- - pmap command on
Heap capacity = 4 Gig – Solaris & Linux
Java Heap - PermGen - svmon command on
AIX
For a 64-bit VM, the CHeap capacity =
Physical server total
RAM & virtual memory
– Java Heap - PermGen

Java Heap Space - Overview & life cycle
Your Java program life cycle typically looks like this:
•
•
•

Java program coding (via Eclipse IDE etc.) e.g. HelloWorld.java
Java program compilation (Java compiler or third party build tools such as Apache Ant, Apache
Maven..) e.g. HelloWord.class
Java program start-up and runtime execution e.g. via your HelloWorld.main() method

Now let's dissect your HelloWorld.class program so you can better understand.
•

•
•

•

At start-up, your JVM will load and cache some of your static program and JDK libraries to the
Native Heap, including native libraries, Mapped Files such as your program Jar file(s), Threads
such as the main start-up Thread of your program etc.
Your JVM will then store the "static" data of your HelloWorld.class Java program to the
PermGen space (Class metadata, descriptors, etc.).
Once your program is started, the JVM will then manage and dynamically allocate the memory
of your Java program to the Java Heap (YoungGen & OldGen). This is why it is so important
that you understand how much memory your Java program needs to you can properly finetuned the capacity of your Java Heap controlled via -Xms & -Xmx JVM parameters. Profiling,
Heap Dump analysis allow you to determine your Java program memory footprint.
Finally, the JVM has to also dynamically release the memory from the Java Heap Space that
your program no longer need; this is called the garbage collection process. This process can
be easily monitored via the JVM verbose GC or a monitoring tool of your choice such as
Jconsole.

Java HotSpot VM PermGen space
The Java HotSpot VM permanent generation space is the JVM storage used mainly to store your Java
Class objects. The Java Heap is the primary storage that is storing the actual short and long term

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instances of your PermGen Class objects.
The PermGen space is fairly static by nature unless using third party tool and/or Java Reflection API
which relies heavily on dynamic class loading.
It is important to note that this memory storage is applicable only for a Java HotSpot VM; other JVM
vendors such as IBM and Oracle JRockit do not have such fixed and configurable PermGen storage
and are using other techniques to manage the non Java Heap memory (native memory).
Find below a graphical view of a JVM HotSpot Java Heap vs. PermGen space breakdown along with
its associated attributes and capacity tuning arguments.

Apart from the Oracle HotSpot JVM, there are other virtual machines provided by differented vendors.
The following sections examine the memory configurations used by other JVMs. Understanding those
is quite important given the implementation and naming convention differences between HotSpot and

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the other JVMs.

IBM JVM Memory
The IBM VM memory is split between 2 memory spaces:
•
•

The Java Heap (nursery and tenured spaces)
The Native Heap (C-Heap)

Here is the breakdown for each one of them:
Memory Space
Java Heap

Start-up arguments
and tuning
-Xmx (maximum Heap
space)

Monitoring strategies
- verbose GC
- JMX API
- IBM monitoring tools

-Xms (minimum Heap
size)- verbose GC
- JMX API
- IBM monitoring tools

Description
The IBM Java Heap is
typically split between
the nursery and tenured
space (YoungGen,
OldGen).
The gencon GC policy
(combo of concurrent
and generational GC) is
typically used for Java
EE platforms in order to
minimize the GC pause
time.

EX:
-Xmx1024m
-Xms1024m
GC policy Ex:
-Xgcpolicy:gencon
(enable gencon GC
policy)
Native Heap
(C-Heap)

Not configurable
directly.
For a 32-bit VM, the CHeap capacity = 4 Gig –
Java Heap

- svmon command

The C-Heap is storing
class metadata objects
including library files,
other JVM and third
party native code
objects.

For a 64-bit VM, the CHeap capacity =
Physical server total
RAM & virtual memory
– Java Heap
As you might noticed, there is no PermGen space for the IBM VM. The PermGen space is only
applicable to the HotSpot VM. The IBM VM is using the Native Heap for Class metadata related data.

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Also note that Oracle is also starting to remove the PermGen space for the HotSpot VM, as we will
discuss in a next section.

Oracle JRockit JVM Memory
The JRockit VM memory is split between 2 memory spaces:
•
•

The Java Heap (YoungGen and OldGen)
The Native memory space (Classes pool, C-Heap, Threads...)
Memory Space

Java Heap

Start-up arguments
and tuning
-Xmx (maximum Heap
space)
-Xms (minimum Heap
size)

Monitoring strategies

Description

- verbose GC
- JMX API
- JRockit Mission
Control tools suite

The JRockit Java Heap
is typically split between
the YoungGen (shortlived objects), OldGen
(long-lived objects).

- Total process size
check in Windows and
Linux
- pmap command on
Solaris & Linux
- JRockit JRCMD tool

The JRockit Native
memory space is
storing the Java
Classes metadata,
Threads and objects
such as library files,
other JVM and third
party native code
objects.

EX:
-Xmx1024m
-Xms1024m
Native memory space

Not configurable
directly.
For a 32-bit VM, the
native memory space
capacity = 2-4 Gig –
Java Heap
** Process size limit of 2
GB, 3 GB or 4 GB
depending of your OS
**
For a 64-bit VM, the
native memory space
capacity = Physical
server total RAM &
virtual memory – Java
Heap

Similar to the IBM VM, there is no PermGen space for the JRockit VM. The PermGen space is only
applicable to the HotSpot VM. The JRockit VM is using the Native Heap for Class metadata related
data.

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The JRockit VM tend to uses more native memory in exchange for better performance. JRockit does
not have an interpretation mode, compilation only, so due to its additional native memory needs the
process size tends to use a couple of hundred MB larger than the equivalent Sun JVM size. This
should not be a big problem unless you are using a 32-bit JRockit with a large Java Heap requirement;
in this scenario, the risk of OutOfMemoryError due to Native Heap depletion is higher for a JRockit VM
(e.g. for a 32-bit VM, bigger is the Java Heap, smaller is memory left for the Native Heap).
Oracle's strategy, being the vendor for both HotSpot and JRockit product lines, is to merge the two
Vms to a single JVM project that will include the best features of each one. This will also simplify JVM
tuning since right now failure to understand the differences between these 2 VM's can lead to bad
tuning recommendations and performance problems.

Tips for proper Java Heap size
Determination of proper Java Heap size for a production system is not a straightforward exercise.
Multiple performance problem can occur due to inadequate Java Heap capacity and tuning. This
section will provide some tips that can help you determine the optimal Java heap size, as a starting
point, for your current or new production environment. Some of these tips are also very useful
regarding the prevention and resolution of OutOfMemoryError problems, including memory leaks.
Please note that these tips are intended to “help you” determine proper Java Heap size. Since each IT
environment is unique, you are actually in the best position to determine precisely the required Java
Heap specifications of your client’s environment.
#1 - JVM: you always fear what you don't understand
How can you expect to configure, tune and troubleshoot something that you don't understand? You
may never have the chance to write and improve Java VM specifications but you are still free to learn
its foundation in order to improve your knowledge and troubleshooting skills. Some may disagree, but
from my perspective, the thinking that Java programmers are not required to know the internal JVM
memory management is an illusion.
Java Heap tuning and troubleshooting can especially be a challenge for Java & Java EE beginners.
Find below a typical scenario:
•
•
•
•

Your client production environment is facing OutOfMemoryError on a regular basis and causing
lot of business impact. Your support team is under pressure to resolve this problem.
A quick Google search allows you to find examples of similar problems and you now believe
(and assume) that you are facing the same problem.
You then grab JVM -Xms and -Xmx values from another person OutOfMemoryError problem
case, hoping to quickly resolve your client's problem.
You then proceed and implement the same tuning to your environment. 2 days later you realize
problem is still happening (even worse or little better)...the struggle continues...

What went wrong?

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You failed to first acquire proper understanding of the root cause of your problem.
You may also have failed to properly understand your production environment at a deeper level
(specifications, load situation etc.). Web searches is a great way to learn and share knowledge
but you have to perform your own due diligence and root cause analysis.
You may also be lacking some basic knowledge of the JVM and its internal memory
management, preventing you to connect all the dots together.

My #1 tip and recommendation to you is to learn and understand the basic JVM principles along with
its different memory spaces. Such knowledge is critical as it will allow you to make valid
recommendations to your clients and properly understand the possible impact and risk associated with
future tuning considerations.
As a reminder, the Java VM memory is split up to 3 memory spaces:
•
•
•

The Java Heap: Applicable for all JVM vendors, usually split between YoungGen (nursery) &
OldGen (tenured) spaces.
The PermGen (permanent generation): Applicable to the Sun HotSpot VM only (PermGen
space will be removed in future Java updates)
The Native Heap (C-Heap): Applicable for all JVM vendors.

As you can see, the Java VM memory management is more complex than just setting up the biggest
value possible via –Xmx. You have to look at all angles, including your native and PermGen space
requirement along with physical memory availability (and # of CPU cores) from your physical host(s).
It can get especially tricky for 32-bit JVM since the Java Heap and native Heap are in a race. The
bigger your Java Heap, the smaller the native Heap. Attempting to setup a large Heap for a 32-bit VM
e.g .2.5 GB+ increases risk of native OutOfMemoryError depending of your application(s) footprint,
number of Threads etc. 64-bit JVM resolves this problem but you are still limited to physical resources
availability and garbage collection overhead (cost of major GC collections go up with size). The bottom
line is that the bigger is not always the better so please do not assume that you can run all your 20
Java EE applications on a single 16 GB 64-bit JVM process.

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#2 - Data and application is king: review your static footprint requirement
Your application(s) along with its associated data will dictate the Java Heap footprint requirement. By
static memory, I mean "predictable" memory requirements as per below.
•

•

•

•

Determine how many different applications you are planning to deploy to a single JVM process
e.g. number of EAR files, WAR files, jar files etc. The more applications you deploy to a single
JVM, higher demand on native Heap.
Determine how many Java classes will be potentially loaded at runtime; including third part
API's. The more class loaders and classes that you load at runtime, higher demand on the
HotSpot VM PermGen space and internal JIT related optimization objects.
Determine data cache footprint e.g. internal cache data structures loaded by your application
(and third party API's) such as cached data from a database, data read from a file etc. The
more data caching that you use, higher demand on the Java Heap OldGen space.
Determine the number of Threads that your middleware is allowed to create. This is very
important since Java threads require enough native memory or OutOfMemoryError will be
thrown.

For example, you will need much more native memory and PermGen space if you are planning to
deploy 10 separate EAR applications on a single JVM process vs. only 2 or 3. Data caching not

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serialized to a disk or database will require extra memory from the OldGen space.
Try to come up with reasonable estimates of the static memory footprint requirement. This will be very
useful to setup some starting point JVM capacity figures before your true measurement exercise (e.g.
tip #4). For 32-bit JVM, I usually do not recommend a Java Heap size high than 2 GB (-Xms2048m,
-Xmx2048m) since you need enough memory for PermGen and native Heap for your Java EE
applications and threads.
This assessment is especially important since too many applications deployed in a single 32-bit JVM
process can easily lead to native Heap depletion; especially in a multi threads environment.
For a 64-bit JVM, a Java Heap size of 3 GB or 4 GB per JVM process is usually my recommended
starting point.
#3 - Business traffic set the rules: review your dynamic footprint requirement
Your business traffic will typically dictate your dynamic memory footprint. Concurrent users & requests
generate the JVM GC "heartbeat" that you can observe from various monitoring tools due to very
frequent creation and garbage collections of short & long lived objects. As you saw from the above
JVM diagram, a typical ratio of YoungGen vs. OldGen is 1:3 or 33%.
For a typical 32-bit JVM, a Java Heap size setup at 2 GB (using generational & concurrent collector)
will typically allocate 500 MB for YoungGen space and 1.5 GB for the OldGen space.
Minimizing the frequency of major GC collections is a key aspect for optimal performance so it is very
important that you understand and estimate how much memory you need during your peak volume.
Again, your type of application and data will dictate how much memory you need. Shopping cart type
of applications (long lived objects) involving large and non-serialized session data typically need large
Java Heap and lot of OldGen space. Stateless and XML processing heavy applications (lot of short
lived objects) require proper YoungGen space in order to minimize frequency of major collections.
Example:
•
•
•
•
•
•
•

You have 5 EAR applications (~2 thousands of Java classes) to deploy (which include
middleware code as well...).
Your native heap requirement is estimated at 1 GB (has to be large enough to handle Threads
creation etc.).
Your PermGen space is estimated at 512 MB.
Your internal static data caching is estimated at 500 MB.
Your total forecast traffic is 5000 concurrent users at peak hours.
Each user session data footprint is estimated at 500 K.
Total footprint requirement for session data alone is 2.5 GB under peak volume.

As you can see, with such requirement, there is no way you can have all this traffic sent to a single
JVM 32-bit process. A typical solution involves splitting (tip #5) traffic across a few JVM processes and
/ or physical host (assuming you have enough hardware and CPU cores available).

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However, for this example, given the high demand on static memory and to ensure a scalable
environment in the long run, I would also recommend 64-bit VM but with a smaller Java Heap as a
starting point such as 3 GB to minimize the GC cost. You definitely want to have extra buffer for the
OldGen space so I typically recommend up to 50% memory footprint post major collection in order to
keep the frequency of Full GC low and enough buffer for fail-over scenarios.
Most of the time, your business traffic will drive most of your memory footprint, unless you need
significant amount of data caching to achieve proper performance which is typical for portal (media)
heavy applications. Too much data caching should raise a yellow flag that you may need to revisit
some design elements sooner than later.
#4 - Don't guess it, measure it!
At this point you should:
•
•
•
•
•

Understand the basic JVM principles and memory spaces
Have a deep view and understanding of all applications along with their characteristics (size,
type, dynamic traffic, stateless vs. stateful objects, internal memory caches etc.)
Have a very good view or forecast on the business traffic (# of concurrent users etc.) and for
each application
Some ideas if you need a 64-bit VM or not and which JVM settings to start with
Some ideas if you need more than one JVM (middleware) processes

But wait, your work is not done yet. While this above information is crucial and great for you to come
up with "best guess" Java Heap settings, it is always best and recommended to simulate your
application(s) behaviour and validate the Java Heap memory requirement via proper profiling, load &
performance testing.
You can learn and take advantage of tools such as JProfiler. From my perspective, learning how to
use a profiler is the best way to properly understand your application memory footprint. Another
approach I use for existing production environments is heap dump analysis using the Eclipse MAT
tool. Heap Dump analysis is very powerful and allow you to view and understand the entire memory
footprint of the Java Heap, including class loader related data and is a must do exercise in any
memory footprint analysis; especially memory leaks.

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Java profilers and heap dump analysis tools allow you to understand and validate your application
memory footprint, including detection and resolution of memory leaks. Load and performance testing
is also a must since this will allow you to validate your earlier estimates by simulating your forecast
concurrent users. It will also expose your application bottlenecks and allow you to further fine tune
your JVM settings. You can use tools such as Apache JMeter which is very easy to learn and use or
explore other commercial products.
Finally, I have seen quite often Java EE environments running perfectly fine until the day where one
piece of the infrastructure start to fail e.g. hardware failure. Suddenly the environment is running at
reduced capacity (reduced # of JVM processes) and the whole environment goes down. What
happened?
There are many scenarios that can lead to domino effects but lack of JVM tuning and capacity to
handle fail-over (short term extra load) is very common. If your JVM processes are running at 80%+
OldGen space capacity with frequent garbage collections, how can you expect to handle any fail-over
scenario?
Your load and performance testing exercise performed earlier should simulate such scenario and you

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should adjust your tuning settings properly so your Java Heap has enough buffer to handle extra load
(extra objects) at short term. This is mainly applicable for the dynamic memory footprint since fail-over
means redirecting a certain % of your concurrent users to the available JVM processes (middleware
instances).
#5 - Divide and conquer
At this point you have performed dozens of load testing iterations. You know that your JVM is not
leaking memory. Your application memory footprint cannot be reduced any further. You tried several
tuning strategies such as using a large 64-bit Java Heap space of 10 GB+, multiple GC policies but
still not finding your performance level acceptable?
In my experience I found that, with current JVM specifications, proper vertical and horizontal scaling
which involved creating a few JVM processes per physical host and across several hosts will give you
the throughput and capacity that you are looking for. Your IT environment will also more fault tolerant if
you break your application list in a few logical silos, with their own JVM process, Threads and tuning
values.
This "divide and conquer" strategy involves splitting your application(s) traffic to multiple JVM
processes and will provide you with:
•
•
•
•
•

Reduced Java Heap size per JVM process (both static & dynamic footprint)
Reduced complexity of JVM tuning
Reduced GC elapsed and pause time per JVM process
Increased redundancy and fail-over capabilities
Aligned with latest Cloud and IT virtualization strategies

The bottom line is that when you find yourself spending too much time in tuning that single elephant
64-bit JVM process, it is time to revisit your middleware and JVM deployment strategy and take
advantage of vertical & horizontal scaling. This implementation strategy is more taxing for the
hardware but will really pay off in the long run.

Java Threading: JVM Retained memory analysis
Having discussed the various heap spaces of the JVM, this section will provide you with a tutorial
allowing you to determine how much and where Java heap space is retained from your active
application Java threads. A true case study from an Oracle Weblogic 10.0 production environment will
be presented in order for you to better understand the analysis process.
We will also attempt to demonstrate that excessive garbage collection or Java heap space memory
footprint problems are often not caused by true memory leaks but instead due to thread execution
patterns and high amount of short lived objects.
Background
Java threads are part of the JVM fundamentals. Your Java heap space memory footprint is driven not

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only by static and long lived objects but also by short lived objects.
OutOfMemoryError problems are often wrongly assumed to be due to memory leaks. We often
overlook faulty thread execution patterns and short lived objects they "retain" on the Java heap until
their executions are completed. In this problematic scenario:
•

•
•
•
•
•

Your "expected" application short lived / stateless objects (XML, JSON data payload etc.)
become retained by the threads for too long (thread lock contention, huge data payload, slow
response time from remote system etc.).
Eventually such short lived objects get promoted to the long lived object space e.g.
OldGen/tenured space by the garbage collector.
As a side effect, this is causing the OldGen space to fill up rapidly, increasing the Full GC
(major collections) frequency.
Depending of the severity of the situation this can lead to excessive GC garbage collection,
increased JVM paused time and ultimately “OutOfMemoryError: Java heap space”.
Your application is now down, you are now puzzled on what is going on.
Finally, you are thinking to either increase the Java heap or look for memory leaks...are you
really on the right track?

In the above scenario, you need to look at the thread execution patterns and determine how much
memory each of them retain at a given time.
OK I get the picture but what about the thread stack size?
It is very important to avoid any confusion between thread stack size and Java memory retention. The
thread stack size is a special memory space used by the JVM to store each method call. When a
thread calls method A, it "pushes" the call onto the stack. If method A calls method B, it gets also
pushed onto the stack. Once the method execution completes, the call is "popped" off the stack.
The Java objects created as a result of such thread method calls are allocated on the Java heap
space. Increasing the thread stack size will definitely not have any effect. Tuning of the thread stack
size is normally required when dealing with java.lang.stackoverflowerror or “OutOfMemoryError:
unable to create new native thread” problems.

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Case study and problem context
The following analysis is based on a true production problem we investigated recently.
1. Severe performance degradation was observed from a Weblogic 10.0 production environment
following some changes to the user web interface (using Google Web Toolkit and JSON as
data payload).
2. Initial analysis did reveal several occurrences of “OutOfMemoryError: Java heap space” errors
along with excessive garbage collection. Java heap dump files were generated automatically (XX:+HeapDumpOnOutOfMemoryError) following OOM events.
3. Analysis of the verbose:gc logs did confirm full depletion of the 32-bit HotSpot JVM OldGen
space (1 GB capacity).
4. Thread dump snapshots were also generated before and during the problem.
5. The only problem mitigation available at that time was to restart the affected Weblogic server
when problem was observed.
6. A rollback of the changes was eventually performed which did resolve the situation.
The team first suspected a memory leak problem from the new code introduced.
Thread dump analysis: looking for suspects
The first step we took was to perform an analysis of the generated thread dump data. Thread dump
will often show you the culprit threads allocating memory on the Java heap. It will also reveal any
hogging or stuck thread attempting to send and receive data payload from a remote system.
The first pattern we noticed was a good correlation between OOM events and STUCK threads
observed from the Weblogic managed servers (JVM processes). Find below the primary thread

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pattern found:
<10-Dec-2012 1:27:59 o'clock PM EST>  
<[STUCK] ExecuteThread: '22' for queue:
'weblogic.kernel.Default (self-tuning)'
has been busy for "672" seconds working on the request
which is more than the configured time of "600" seconds.

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As you can see, the above thread appears to be STUCK or taking very long time to read and receive
the JSON response from the remote server. Once we found that pattern, the next step was to correlate
this finding with the JVM heap dump analysis and determine how much memory these stuck threads
were taking from the Java heap.
Heap dump analysis: retained objects exposed!
The Java heap dump analysis was performed using MAT. We will now list the different analysis steps
which did allow us to pinpoint the retained memory size and source.
1. Load the HotSpot JVM heap dump

2. Select the HISTOGRAM view and filter by "ExecuteThread"
* ExecuteThread is the Java class used by the Weblogic kernel for thread creation & execution *

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As you can see, this view was quite revealing. We can see a total of 210 Weblogic threads created.
The total retained memory footprint from these threads is 806 MB. This is pretty significant for a 32-bit
JVM process with 1 GB OldGen space. This view alone is telling us that the core of the problem and
memory retention originates from the threads themselves.
3. Deep dive into the thread memory footprint analysis
The next step was to deep dive into the thread memory retention. To do this, simply right click over the
ExecuteThread class and select: List objects > with outgoing references.

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As you can see, we were able to correlate STUCK threads from the thread dump analysis with high
memory retention from the heap dump analysis. The finding was quite surprising.
4. Thread Java Local variables identification
The final analysis step did require us to expand a few thread samples and understand the primary
source of memory retention.

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As you can see, this last analysis step did reveal huge JSON response data payload at the root cause.
That pattern was also exposed earlier via the thread dump analysis where we found a few threads
taking very long time to read & receive the JSON response; a clear symptom of huge data payload
footprint.
It is crucial to note that short lived objects created via local method variables will show up in the heap
dump analysis. However, some of those will only be visible from their parent threads since they are not
referenced by other objects, like in this case. You will also need to analyze the thread stack trace in
order to identify the true caller, followed by a code review to confirm the root cause.
Following this finding, our delivery team was able to determine that the recent JSON faulty code
changes were generating, under some scenarios, huge JSON data payload up to 45 MB+. Given the
fact that this environment is using a 32-bit JVM with only 1 GB of OldGen space, you can understand
that only a few threads were enough to trigger severe performance degradation.
This case study is clearly showing the importance of proper capacity planning and Java heap analysis,
including the memory retained from your active application & Java EE container threads.

Java 8: From PermGen to Metaspace
Java 8 will introduce some new language and runtime features. One of these features is the complete
removal of the Permanent Generation (PermGen) space which has been announced by Oracle since
the release of JDK 7. Interned strings, for example, have already been removed from the PermGen
space since JDK 7. The JDK 8 release finalizes its decommissioning. In this section, we shall discuss
the PermGen successor: Metaspace.
We will also compare the runtime behavior of the HotSpot 1.7 vs. HotSpot 1.8 (b75) when executing a
Java program “leaking” class metadata objects.
The final specifications, tuning flags and documentation around Metaspace should be available once

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Java 8 is officially released.
Metaspace: A new memory space is born
The JDK 8 HotSpot JVM is now using native memory for the representation of class metadata and is
called Metaspace; similar to the Oracle JRockit and IBM JVM's.
The good news is that it means no more java.lang.OutOfMemoryError: PermGen space problems and
no need for you to tune and monitor this memory space anymore…not so fast. While this change is
invisible by default, we will show you next that you will still need to worry about the class metadata
memory footprint. Please also keep in mind that this new feature does not magically eliminate class
and classloader memory leaks. You will need to track down these problems using a different approach
and by learning the new naming convention.
In summary:
PermGen space situation
•
•

This memory space is completely removed.
The PermSize and MaxPermSize JVM arguments are ignored and a warning is issued if
present at start-up.

Metaspace memory allocation model
•
•

Most allocations for the class metadata are now allocated out of native memory.
The classes that were used to describe class metadata have been removed.

Metaspace capacity
•

•

By default class metadata allocation is limited by the amount of available native memory
(capacity will of course depend if you use a 32-bit JVM vs. 64-bit along with OS virtual
memory availability).
A new flag is available (MaxMetaspaceSize), allowing you to limit the amount of native
memory used for class metadata. If you don’t specify this flag, the Metaspace will
dynamically re-size depending of the application demand at runtime.

Metaspace garbage collection
•
•

Garbage collection of the dead classes and classloaders is triggered once the class
metadata usage reaches the “MaxMetaspaceSize”.
Proper monitoring & tuning of the Metaspace will obviously be required in order to limit the
frequency or delay of such garbage collections. Excessive Metaspace garbage collections
may be a symptom of classes, classloaders memory leak or inadequate sizing for your
application.

Java heap space impact
Some miscellaneous data has been moved to the Java heap space. This means you may observe

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an increase of the Java heap space following a future JDK 8 upgrade.
Metaspace monitoring
•
•

Metaspace usage is available from the HotSpot 1.8 verbose GC log output.
Jstat & JVisualVM have not been updated at this point based on our testing with b75 and the
old PermGen space references are still present.

Enough theory now, let’s see this new memory space in action via our leaking Java program…
PermGen vs. Metaspace runtime comparison
In order to better understand the runtime behavior of the new Metaspace memory space, we created a
class metadata leaking Java program. You can download the source here.
The following scenarios will be tested:
•
•
•

Run the Java program using JDK 1.7 in order to monitor & deplete the PermGen memory
space set at 128 MB.
Run the Java program using JDK 1.8 (b75) in order to monitor the dynamic increase and
garbage collection of the new Metaspace memory space.
Run the Java program using JDK 1.8 (b75) in order to simulate the depletion of the Metaspace
by setting the MaxMetaspaceSize value at 128 MB.

JDK 1.7 @64-bit – PermGen depletion
•
•
•

Java program with 50K configured iterations
Java heap space of 1024 MB
Java PermGen space of 128 MB (-XX:MaxPermSize=128m)

As you can see form JVisualVM, the PermGen depletion was reached after loading about 30K+
classes. We can also see this depletion from the program and GC output.

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Now let’s execute the program using the HotSpot JDK 1.8 JRE.
JDK 1.8 @64-bit – Metaspace dynamic re-size
•
•
•

Java program with 50K configured iterations
Java heap space of 1024 MB
Java Metaspace space: unbounded (default)

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As you can see from the verbose GC output, the JVM Metaspace did expand dynamically from 20 MB
up to 328 MB of reserved native memory in order to honor the increased class metadata memory
footprint from our Java program. We could also observe garbage collection events in the attempt by
the JVM to destroy any dead class or classloader object. Since our Java program is leaking, the JVM
had no choice but to dynamically expand the Metaspace memory space.
The program was able to run its 50K of iterations with no OOM event and loaded 50K+ Classes.
Let's move to our last testing scenario.
JDK 1.8 @64-bit – Metaspace depletion
•
•
•

Java program with 50K configured iterations
Java heap space of 1024 MB
Java Metaspace space: 128 MB (-XX:MaxMetaspaceSize=128m)

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As you can see form JVisualVM, the Metaspace depletion was reached after loading about 30K+
classes; very similar to the run with the JDK 1.7. We can also see this from the program and GC
output. Another interesting observation is that the native memory footprint reserved was twice as much
as the maximum size specified. This may indicate some opportunities to fine tune the Metaspace
resize policy, if possible, in order to avoid native memory waste.
Capping the Metaspace at 128 MB like we did for the baseline run with JDK 1.7 did not allow us to
complete the 50K iterations of our program. A new OOM error was thrown by the JVM. The above
OOM event was thrown by the JVM from the Metaspace following a memory allocation failure.
Final Words on Metaspace
The current observations definitely indicate that proper monitoring & tuning will be required in order to
stay away from problems such as excessive Metaspace GC or OOM conditions triggered from our last
testing scenario.

HPROF - Memory leak analysis with Eclipse Memory Analyzer
Tool (MAT)
In this section, we will show you how you can analyze a JVM memory leak problem by generating and
analyzing a HotSpot JVM HPROF Heap Dump file.

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A real life case study will be used for that purpose: Weblogic 9.2 memory leak affecting the Weblogic
Admin server.
Environment specifications
•
•
•
•

Java EE server: Oracle Weblogic Server 9.2 MP1
Middleware OS: Solaris 10
Java VM: Sun HotSpot 1.5.0_22
Platform type: Middle tier

Monitoring and troubleshooting tools
•
•
•
•

Quest Foglight (JVM and garbage collection monitoring)
jmap (hprof / Heap Dump generation tool)
Memory Analyzer 1.1 via IBM support assistant (hprof Heap Dump analysis)
Platform type: Middle tier

Step #1 - WLS 9.2 Admin server JVM monitoring and leak confirmation
The Quest Foglight Java EE monitoring tool was quite useful to identify a Java Heap leak from our
Weblogic Admin server. As you can see below, the Java Heap memory is growing over time.
If you are not using any monitoring tool for your Weblogic environment, my recommendation to you is
to at least enable verbose:gc of your HotSpot VM. Please visit the Java 7 verbose:gc tutorial below on
this subject for more detailed instructions.

Step #2 - Generate a Heap Dump from your leaking JVM

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Following the discovery of a JVM memory leak, the goal is to generate a Heap Dump file (binary
format) by using the Sun JDK jmap utility.
** Please note that jmap Heap Dump generation will cause your JVM to become unresponsive so
please ensure that no more traffic is sent to your affected / leaking JVM before running the jmap utility
**
/bin/jmap -heap:format=b 

This command will generate a Heap Dump binary file (heap.bin) of your leaking JVM. The size of the
file and elapsed time of the generation process will depend of your JVM size and machine
specifications / speed.
For our case study, a binary Heap Dump file of ~2GB was generated in about 1 hour elapsed time.
Sun HotSpot 1.5/1.6/1.7 Heap Dump file will also be generated automatically as a result of a
OutOfMemoryError and by adding -XX:+HeapDumpOnOutOfMemoryError in your JVM start-up
arguments.
Step #3 - Load your Heap Dump file in Memory Analyzer tool
It is now time to load your Heap Dump file in the Memory Analyzer tool. The loading process will take
several minutes depending of the size of your Heap Dump and speed of your machine.

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Step #4 - Analyze your Heap Dump
The Memory Analyzer provides you with many features, including a Leak Suspect report. For this case
study, the Java Heap histogram was used as a starting point to analyze the leaking objects and the
source.

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For our case study, java.lang.String and char[] data were found as the leaking Objects. Now question
is what is the source of the leak e.g. references of those leaking Objects. Simply right click over your
leaking objects and select >> List Objects > with incoming references.

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As you can see, javax.management.ObjectName objects were found as the source of the leaking
String & char[] data. The Weblogic Admin server is communicating and pulling stats from its managed
servers via MBeans / JMX which create javax.management.ObjectName for any MBean object type.
Now question is why Weblogic 9.2 is not releasing properly such Objects…
Root cause: Weblogic javax.management.ObjectName leak!
Following our Heap Dump analysis, a review of the Weblogic known issues was performed which did
reveal the following Weblogic 9.2 bug below:
•
•
•
•

Weblogic Bug ID: CR327368
Description: Memory leak of javax.management.ObjectName objects on the Administration
Server used to cause OutOfMemory error on the Administration Server.
Affected Weblogic version(s): WLS 9.2
Fixed in: WLS 10 MP1

This finding was quite conclusive given the perfect match of our Heap Dump analysis, WLS version

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and this known problem description.
Hopefully, this tutorial along with the case study has helped you understand how you can pinpoint the
source of a Java Heap leak using jmap and the Memory Analyzer tool.

JVM verbose GC output tutorial
This section will provide you with a detailed tutorial on how to enable and read Java 7 HotSpot VM
verbose gc output data.
I recommend that you compile and run the sample program on your end as well.
We also recently created a Java verbose GC tutorial video explaining this analysis process.
Tutorial specifications and tools
•
•
•

OS: Windows 7 - 64-bit
Java VM: Sun Java 7 HotSpot (build 21.0-b17)
IDE: Eclipse Java EE IDE for Web Developer v4.1

Step #1 - Compile our sample Java program
We created a sample Java program in order to load the Java Heap and trigger an explicit GC in order
to generate some interesting verbose GC output. This program is simply loading about 3 million
instances of java.lang.String in a static Map data structure and triggers an explicit GC (via
System.gc()) followed by the removal of 2 million instances along with a second explicit GC before
exiting.

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package org.ph.javaee.tools.jdk7;
import java.util.Map;
import java.util.HashMap;
/**
* JavaHeapVerboseGCTest
* @author Pierre-Hugues Charbonneau
*
*/
public class JavaHeapVerboseGCTest {
private static Map mapContainer = new HashMap();
/**
* @param args
*/
public static void main(String[] args) {
System.out.println("Java 7 HotSpot Verbose GC Test Program v1.0");
System.out.println("Author: Pierre-Hugues Charbonneau");
System.out.println("http://javaeesupportpatterns.blogspot.com/");
String stringDataPrefix = "stringDataPrefix";
// Load Java Heap with 3 M java.lang.String instances
for (int i=0; i<3000000; i++) {
String newStringData = stringDataPrefix + i;
mapContainer.put(newStringData, newStringData);
}
System.out.println("MAP size: "+mapContainer.size());
System.gc(); // Explicit GC!
// Remove 2 M out of 3 M
for (int i=0; i<2000000; i++) {
String newStringData = stringDataPrefix + i;
mapContainer.remove(newStringData);
}
System.out.println("MAP size: "+mapContainer.size());
System.gc();
System.out.println("End of program!");
}
}

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Step #2 - Enable verbose GC via the JVM start-up arguments
The next step is to enable the verbose GC via the JVM start-up arguments and specify a name and
location for our GC log file.

Step #3 - Execute our sample Java program
At this point, it is now time to execute our sample program and generate the JVM verbose GC output.

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Verbose GC output high level analysis
It is now time to review the generated GC output.
First, let's start with the raw data. As you can see below, the GC output is divided into 3 main sections:
•
•
•

5 Minor collections (YoungGen space collections) identified as PSYoungGen
2 Major collections (triggered by System.gc()) identified as Full GC (System)
A detailed Java Heap breakdown of each memory space

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0.437: [GC [PSYoungGen: 262208K->43632K(305856K)]
262208K->137900K(1004928K), 0.1396040 secs]
[Times: user=0.45 sys=0.01, real=0.14 secs]
0.785: [GC [PSYoungGen: 305840K->43640K(305856K)]
400108K->291080K(1004928K), 0.2197630 secs]
[Times: user=0.56 sys=0.03, real=0.22 secs]
1.100: [GC [PSYoungGen: 164752K->43632K(305856K)]
412192K->340488K(1004928K), 0.0878209 secs]
[Times: user=0.37 sys=0.00, real=0.09 secs]
1.188: [Full GC (System) [PSYoungGen: 43632K->0K(305856K)]
[PSOldGen: 296856K->340433K(699072K)]
340488K->340433K(1004928K)
[PSPermGen: 1554K->1554K(16384K)], 0.4053311 secs]
[Times: user=0.41 sys=0.00, real=0.40 secs]
1.883: [GC [PSYoungGen: 262208K->16K(305856K)]
602641K->340449K(1004928K), 0.0326756 secs]
[Times: user=0.09 sys=0.00, real=0.03 secs]
2.004: [GC [PSYoungGen: 92122K->0K(305856K)]
432556K->340433K(1004928K), 0.0161477 secs]
[Times: user=0.06 sys=0.00, real=0.02 secs]
2.020: [Full GC (System) [PSYoungGen: 0K->0K(305856K)]
[PSOldGen: 340433K->125968K(699072K)]
340433K->125968K(1004928K)
[PSPermGen: 1555K->1555K(16384K)], 0.2302415 secs]
[Times: user=0.23 sys=0.00, real=0.23 secs]
Heap
PSYoungGen total 305856K, used 5244K [0x3dac0000, 0x53010000, 0x53010000)
eden space 262208K, 2% used [0x3dac0000,0x3dfdf168,0x4dad0000)
from space 43648K, 0% used [0x4dad0000,0x4dad0000,0x50570000)
to space 43648K, 0% used [0x50570000,0x50570000,0x53010000)
PSOldGen total 699072K, used 125968K [0x13010000, 0x3dac0000, 0x3dac0000)
object space 699072K, 18% used [0x13010000,0x1ab140a8,0x3dac0000)
PSPermGen total 16384K, used 1560K [0x0f010000, 0x10010000, 0x13010000)
object space 16384K, 9% used [0x0f010000,0x0f1960b0,0x10010000)

Verbose GC data interpretation and sequence
As you can see from the verbose GC output, the OldGen space was at 340 MB after the initial loading
of 3M String instances in our HashMap. It did go down to 126 MB following the removal of 2M String

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instances.
Now find below explanation and snapshots on how you can read the GC output data in more detail for
each Java Heap space.
## YoungGen space analysis

## OldGen space analysis

## PermGen space analysis

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## Java Heap breakdown analysis

Hopefully this sample Java program and verbose GC output analysis has helped you understand how
to read and interpret this critical data.

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Analyzing thread dumps
Introduction to thread dump analysis
This section will teach you how to analyze a JVM Thread Dump and pinpoint the root cause of your
problem(s). From my perspective, Thread Dump analysis is the most important skillset to master for
any individual involved in Java EE production support. The amount of information that you can derive
from Thread Dump snapshots is often much beyond than what you can think of.
You may find complementary training videos here and here.
Before going deeper into Thread Dump analysis and problem patterns, it is very important that you
understand the fundamentals.
Java VM overview
The Java virtual machine is really the foundation of any Java EE platform. This is where your
middleware and applications are deployed and active.
The JVM provides the middleware software and your Java / Java EE program with:
•
•
•

A runtime environment for your Java / Java EE program (bytecode format).
Several program features and utilities (IO facilities, data structure, Threads management,
security, monitoring etc.).
Dynamic memory allocation and management via the garbage collector.

Your JVM can reside on many OS (Solaris, AIX, Windows etc.) and depending of your physical server
specifications, you can install 1...n JVM processes per physical / virtual server.
JVM and Middleware software interactions
Find below a diagram showing you a high level interaction view between the JVM, middleware and
application(s).

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This is showing you a typical and simple interaction diagram between the JVM, middleware and
application. As you can see, the Threads allocation for a standard Java EE application are done
mainly between the middleware kernel itself and JVM (there are some exceptions when application
itself or some APIs create Threads directly but this is not common and must be done very carefully).
Also, please note that certain Threads are managed internally within the JVM itself such as GC
(garbage collection) Threads in order to handle concurrent garbage collections.
Since most of the Thread allocations are done by the Java EE container, it is important that you
understand and recognize the Thread Stack Trace and identify it properly from the Thread Dump data.
This will allow you to understand quickly the type of request that the Java EE container is attempting
to execute.
From a Thread Dump analysis perspective, you will learn how to differentiate between the different
Thread Pools found from the JVM and identify the request type.

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JVM Thread Dump - what is it?
A JVM Thread Dump is a snapshot taken at a given time which provides you with a complete listing of
all created Java Threads.
Each individual Java Thread found gives you information such as:
- Thread name; often used by middleware vendors to identify the Thread Id along with its associated
Thread Pool name and state (running, stuck etc.)
- Thread type & priority ex: daemon prio=3 ** middleware softwares typically create their Threads
as daemon meaning their Threads are running in background; providing services to its user e.g. your
Java EE application **
- Java Thread ID ex: tid=0x000000011e52a800 ** This is the Java Thread Id obtained via
java.lang.Thread.getId() and usually implemented as an auto-incrementing long 1..n**
- Native Thread ID ex: nid=0x251c** Crucial information as this native Thread Id allows you to
correlate for example which Threads from an OS perspective are using the most CPU within your JVM
etc. **
- Java Thread State and detail ex: waiting for monitor entry [0xfffffffea5afb000]
java.lang.Thread.State: BLOCKED (on object monitor)
** Allows

to quickly learn about Thread state and its potential current blocking condition **

- Java Thread Stack Trace; this is by far the most important data that you will find from the Thread
Dump. This is also where you will spent most of your analysis time since the Java Stack Trace
provides you with 90% of the information that you need in order to pinpoint root cause of many
problem pattern types as you will learn later in the training sessions
- Java Heap breakdown; starting with HotSpot VM 1.6, you will also find at the bottom of the Thread
Dump snapshot a breakdown of the HotSpot memory spaces utilization such as your Java Heap
(YoungGen, OldGen) & PermGen space. This is quite useful when excessive GC is suspected as a
possible root cause so you can do out-of-the-box correlation with Thread data / patterns found

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Heap
PSYoungGen total 466944K, used 178734K [0xffffffff45c00000, 0xffffffff70800000,
0xffffffff70800000)
eden space 233472K, 76% used
[0xffffffff45c00000,0xffffffff50ab7c50,0xffffffff54000000)
from space 233472K, 0% used
[0xffffffff62400000,0xffffffff62400000,0xffffffff70800000)
to space 233472K, 0% used
[0xffffffff54000000,0xffffffff54000000,0xffffffff62400000)
PSOldGen
total 1400832K, used 1400831K [0xfffffffef0400000,
0xffffffff45c00000, 0xffffffff45c00000)
object space 1400832K, 99% used
[0xfffffffef0400000,0xffffffff45bfffb8,0xffffffff45c00000)
PSPermGen
total 262144K, used 248475K [0xfffffffed0400000,
0xfffffffee0400000, 0xfffffffef0400000)
object space 262144K, 94% used
[0xfffffffed0400000,0xfffffffedf6a6f08,0xfffffffee0400000)

Thread Dump breakdown overview
In order for you to better understand, find below a diagram showing you a visual breakdown of a
HotSpot VM Thread Dump and its common Thread Pools found:

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As you can there are several pieces of information that you can find from a HotSpot VM Thread Dump.
Some of these pieces will be more important than others depending of your problem pattern.
For now, find below a detailed explanation for each Thread Dump section as per our sample HotSpot
Thread Dump:
# Full thread dump identifier
This is basically the unique keyword that you will find in your middleware / standalong Java standard
output log once you generate a Thread Dump (ex: via kill -3  for UNIX). This is the beginning of
the Thread Dump snapshot data.
Full thread dump Java HotSpot(TM) 64-Bit Server VM (20.0-b11 mixed mode):

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# Java EE middleware, third party & custom application Threads
This portion is the core of the Thread Dump and where you will typically spend most of your analysis
time. The number of Threads found will depend on your middleware software that you use, third party
libraries (that might have its own Threads) and your application (if creating any custom Thread, which
is generally not a best practice).
In our sample Thread Dump, Weblogic is the middleware used. Starting with Weblogic 9.2, a selftuning Thread Pool is used with unique identifier "'weblogic.kernel.Default (self-tuning)"
"[STANDBY] ExecuteThread: '414' for queue: 'weblogic.kernel.Default (selftuning)'" daemon prio=3 tid=0x000000010916a800 nid=0x2613 in Object.wait()
[0xfffffffe9edff000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0xffffffff27d44de0> (a weblogic.work.ExecuteThread)
at java.lang.Object.wait(Object.java:485)
at weblogic.work.ExecuteThread.waitForRequest(ExecuteThread.java:160)
- locked <0xffffffff27d44de0> (a weblogic.work.ExecuteThread)
at weblogic.work.ExecuteThread.run(ExecuteThread.java:181)

# HotSpot VM Thread
This is an internal Thread managed by the HotSpot VM in order to perform internal native operations.
Typically you should not worry about this one unless you see high CPU (via Thread Dump & prstat /
native Thread id correlation).
"VM Periodic Task Thread" prio=3 tid=0x0000000101238800 nid=0x19 waiting on
condition

# HotSpot GC Thread
When using HotSpot parallel GC (quite common these days when using multi physical cores
hardware), the HotSpot VM create by default or as per your JVM tuning a certain # of GC Threads.
These GC Threads allow the VM to perform its periodic GC cleanups in a parallel manner, leading to
an overall reduction of the GC time; at the expense of increased CPU utilization.
"GC task thread#0 (ParallelGC)" prio=3 tid=0x0000000100120000 nid=0x3 runnable
"GC task thread#1 (ParallelGC)" prio=3 tid=0x0000000100131000 nid=0x4 runnable

This is crucial data as well since when facing GC related problems such as excessive GC, memory
leaks etc, you will be able to correlate any high CPU observed from the OS / Java process(es) with
these Threads using their native id value (nid=0x3).

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# JNI global references count
JNI (Java Native Interface) global references are basically Object references from the native code to a
Java object managed by the Java garbage collector. Its role is to prevent collection of an object that is
still in use by native code but technically with no "live" references in the Java code.
It is also important to keep an eye on JNI references in order to detect JNI related leaks. This can
happen if you program use JNI directly or using third party tools like monitoring tools which are prone
to native memory leaks.
JNI global references: 1925

# Java Heap utilization view
This data was added back to JDK 1 .6 and provides you with a short and fast view of your HotSpot
Heap. I find it quite useful when troubleshooting GC related problems along with HIGH CPU since you
get both Thread Dump & Java Heap in a single snapshot allowing you to determine (or to rule out) any
pressure point in a particular Java Heap memory space along with current Thread computing currently
being done at that time. As you can see in our sample Thread Dump, the Java Heap OldGen is maxed
out!
Heap
PSYoungGen
total 466944K, used 178734K [0xffffffff45c00000,
0xffffffff70800000, 0xffffffff70800000)
eden space 233472K, 76% used
[0xffffffff45c00000,0xffffffff50ab7c50,0xffffffff54000000)
from space 233472K, 0% used
[0xffffffff62400000,0xffffffff62400000,0xffffffff70800000)
to
space 233472K, 0% used
[0xffffffff54000000,0xffffffff54000000,0xffffffff62400000)
PSOldGen
total 1400832K, used 1400831K [0xfffffffef0400000,
0xffffffff45c00000, 0xffffffff45c00000)
object space 1400832K, 99% used
[0xfffffffef0400000,0xffffffff45bfffb8,0xffffffff45c00000)
PSPermGen
total 262144K, used 248475K [0xfffffffed0400000,
0xfffffffee0400000, 0xfffffffef0400000)
object space 262144K, 94% used
[0xfffffffed0400000,0xfffffffedf6a6f08,0xfffffffee0400000)

In order for you to quickly identify a problem pattern from a Thread Dump, you first need to understand
how to read a Thread Stack Trace and how to get the “story” right. This means that if I ask you to tell
me what the Thread #38 is doing; you should be able to precisely answer; including if Thread Stack
Trace is showing a healthy (normal) vs. hang condition.
Java Stack Trace revisited
Most of you are familiar with Java stack traces. This is typical data that we find from server and
application log files when a Java Exception is thrown. In this context, a Java stack trace is giving us
the code execution path of the Thread that triggered the Java Exception such as a

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java.lang.NoClassDefFoundError, java.lang.NullPpointerException etc. Such code execution path
allows us to see the different layers of code that ultimately lead to the Java Exception.
Java stack traces must always be read from bottom-up:
• The line at the bottom will expose the originator of the request such as a Java / Java EE
container Thread.
• The first line at the top of the stack trace will show you the Java class where that last Exception
got triggered.
Let's go through this process via a simple example. We created a sample Java program simply
executing some Class methods calls and throwing an Exception. The program output generated is as
per below:
JavaStrackTraceSimulator
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com
Exception in thread "main" java.lang.IllegalArgumentException:
at org.ph.javaee.training.td.Class2.call(Class2.java:12)
at org.ph.javaee.training.td.Class1.call(Class1.java:14)
at org.ph.javaee.training.td.JavaSTSimulator.main(JavaSTSimulator.java:20)

•
•
•
•
•

Java program JavaSTSimulator is invoked (via the "main" Thread)
The simulator then invokes method call() from Class1
Class1 method call() then invokes Class2 method call()
Class2 method call()throws a Java Exception: java.lang.IllegalArgumentException
The Java Exception is then displayed in the log / standard output

As you can see, the code execution path that lead to this Exception is always displayed from bottomup.
The above analysis process should be well known for any Java programmer. What you will see next is
that the Thread Dump Thread stack trace analysis process is very similar to above Java stack trace
analysis.

Thread Dump: Thread Stack Trace analysis
Thread Dump generated from the JVM provides you with a code level execution snapshot of all the
"created" Threads of the entire JVM process. Created Threads does not mean that all these Threads
are actually doing something. In a typical Thread Dump snapshot generated from a Java EE container
JVM:
•
•

Some Threads could be performing raw computing tasks such as XML parsing, IO / disk
access etc.
Some Threads could be waiting for some blocking IO calls such as a remote Web Service call,

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a DB / JDBC query etc.
Some Threads could be involved in garbage collection at that time e.g. GC Threads
Some Threads will be waiting for some work to do (Threads not doing any work typically go in
wait() state)
Some Threads could be waiting for some other Threads work to complete e.g. Threads waiting
to acquire a monitor lock (synchronized block{}) on some objects

A Thread stack trace provides you with a snapshot of its current execution. The first line typically
includes native information of the Thread such as its name, state, address etc. The current execution
stack trace has to be read from bottom-up. Please follow the analysis process below. The more
experience you get with Thread Dump analysis, the faster you will able to read and identify very
quickly the work performed by each Thread:
•
•
•
•

•

•
•

Start to read the Thread stack trace from the bottom.
First, identify the originator (Java EE container Thread, custom Thread, GC Thread, JVM
internal Thread, standalone Java program "main" Thread etc.).
The next step is to identify the type of request the Thread is executing (WebApp, Web Service,
JMS, Remote EJB (RMI), internal Java EE container etc.).
The next step is to identify from the execution stack trace your application module(s) involved
the actual core work the Thread is trying to perform. The complexity of analysis will depend of
the layers of abstraction of your middleware environment and application.
The next step is to look at the last ~10-20 lines prior to the first line. Identify the protocol or
work the Thread is involved with e.g. HTTP call, Socket communication, JDBC or raw
computing tasks such as disk access, class loading etc.
The next step is to look at the first line. The first line usually tells a LOT on the Thread state
since it is the current piece of code executed at the time you took the snapshot.
The combination of the last 2 steps is what will give you the core of information to conclude of
what work and / or hanging condition the Thread is involved with.

Now find below a visual breakdown of the above steps using a real example of a Thread Dump Thread
stack trace captured from a JBoss 5 production environment. In this example, many Threads were
showing a similar problem pattern of excessive IO when creating new instances of JAX-WS Service
instances.

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As you can see, the last 10 lines along with the first line will tell us what hanging or slow condition the
Thread is involved with, if any. The lines from the bottom will give us detail of the originator and type of
request.

Java Thread CPU analysis on Windows
This section will provide you with a tutorial on how you can quickly pinpoint the Java thread

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contributors to a high CPU problem on the Windows OS. Windows, like other OS such as Linux,
Solaris & AIX allow you to monitor the CPU utilization at the process level but also for individual
Thread executing a task within a process.
For this tutorial, we created a simple Java program that will allow you to learn this technique in a step
by step manner.
Troubleshooting tools
The following tools will be used below for this tutorial:
•
•

Windows Process Explorer (to pinpoint high CPU Thread contributors)
JVM Thread Dump (for Thread correlation and root cause analysis at code level)

High CPU simulator Java program
The simple program below is simply looping and creating new String objects. It will allow us to perform
this CPU per Thread analysis. I recommend that you import it in an IDE of your choice e.g. Eclipse
and run it from there. You should observe an increase of CPU on your Windows machine as soon as
you execute it.

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package org.ph.javaee.tool.cpu;
/**
* HighCPUSimulator
* @author Pierre-Hugues Charbonneau
* http://javaeesupportpatterns.blogspot.com
*
*/
public class HighCPUSimulator {
private final static int NB_ITERATIONS = 500000000;
// ~1 KB data footprint
private final static String DATA_PREFIX =
"datadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadata
datadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatad
atadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatada
tadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadat
adatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadata
datadatadatadatadatadatadatadatadatadatadatadatadata";
public static void main(String[] args) {
System.out.println("HIGH CPU Simulator 1.0");
System.out.println("Author: Pierre-Hugues Charbonneau");
System.out.println("http://javaeesupportpatterns.blogspot.com/");
try {
for (int i = 0; i < NB_ITERATIONS; i++) {
// Perform some String manipulations
to slowdown and expose looping process...
String data = DATA_PREFIX + i;
}
} catch (Throwable any) {
System.out.println("Unexpected Exception! " +
any.getMessage()
+ " [" + any +
"]");
}
System.out.println("HighCPUSimulator done!");
}
}

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Step #1 - Launch Process Explorer
The Process Explorer tool visually shows the CPU usage dynamically. It is good for live analysis. If
you need historical data on CPU per Thread then you can also use Windows perfmon with %
Processor Time & Thread Id data counters. You can download Process Explorer from the link here.
In our example, you can see that the Eclipse javaw.exe process is now using ~25% of total CPU
utilization following the execution of our sample program.

Step #2 - Launch Process Explorer Threads view
The next step is to display the Threads view of the javaw.exe process. Simply right click on the
javaw.exe process and select Properties. The Threads view will be opened as per below snapshot:

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The first column is the Thread Id (decimal format)
The second column is the CPU utilization % used by each Thread
The third column is also another counter indicating if Thread is running on the CPU

In our example, we can see our primary culprit is Thread Id #5996 using ~ 25% of CPU.
Step #3 - Generate a JVM Thread Dump
At this point, Process Explorer will no longer be useful. The goal was to pinpoint one or multiple Java
Threads consuming most of the Java process CPU utilization which is what we achieved. In order to
go the next level in your analysis you will need to capture a JVM Thread Dump. This will allow you to
correlate the Thread Id with the Thread Stack Trace so you can pinpoint that type of processing is
consuming such high CPU.
JVM Thread Dump generation can be done in a few manners. If you are using JRockit VM you can
simply use the jrcmd tool as per below example:

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Once you have the Thread Dump data, simply search for the Thread Id and locate the Thread Stack
Trace that you are interested in.
For our example, the Thread "Main Thread" which was fired from Eclipse got exposed as the primary
culprit which is exactly what we wanted to demonstrate.
"Main Thread" id=1 idx=0x4 tid=5996 prio=5 alive, native_blocked
at org/ph/javaee/tool/cpu/HighCPUSimulator.main
(HighCPUSimulator.java:31)
at jrockit/vm/RNI.c2java(IIIII)V(Native Method)
-- end of trace

Step #4 - Analyze the culprit Thread(s) Stack Trace and determine root cause
At this point you should have everything that you need to move forward with the root cause analysis.
You will need to review each Thread Stack Trace and determine what type of problem you are dealing
with. That final step is typically where you will spend most of your time and problem can be simple
such as infinite looping or complex such as garbage collection related problems.
In our example, the Thread Dump did reveal the high CPU originates from our sample Java program
around line 31. As expected, it did reveal the looping condition that we engineered on purpose for this
tutorial.

Case Study - Too many open files
This case study describes the complete root cause analysis and resolution of a File Descriptor (Too

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many open files) related problem that we faced following a migration from Oracle ALSB 2.6 running on
Solaris OS to Oracle OSB 11g running on AIX.
This section will also provide you with proper AIX OS commands you can use to troubleshoot and
validate the File Descriptor configuration of your Java VM process.
Environment specifications
•
•
•
•

Java EE server: Oracle Service Bus 11g
Middleware OS: IBM AIX 6.1
Java VM: IBM JRE 1.6.0 SR9 - 64 bit
Platform type: Service Bus - Middle Tier

Problem overview
Problem type: java.net.SocketException: Too many open files error was observed under heavy load
causing our Oracle OSB managed servers to suddenly hang.
Such problem was observed only during high load and did require our support team to take corrective
action e.g. shutdown and restart the affected Weblogic OSB managed servers.
Gathering and validation of facts
As usual, a Java EE problem investigation requires gathering of technical and non technical facts so
we can either derive other facts and/or conclude on the root cause. Before applying a corrective
measure, the facts below were verified in order to conclude on the root cause:
•
•
•
•
•

What is the client impact? HIGH; Full JVM hang
Recent change of the affected platform? Yes, recent migration from ALSB 2.6 (Solaris OS) to
Oracle OSB 11g (AIX OS)
Any recent traffic increase to the affected platform? No
What is the health of the Weblogic server? Affected managed servers were no longer
responsive along with closure of the Weblogic HTTP (Server Socket) port
Did a restart of the Weblogic Integration server resolve the problem? Yes but temporarily only

Conclusion #1: The problem appears to be load related
Weblogic server log files review
A quick review of the affected managed servers log did reveal the error below:
java.net.SocketException: Too many open files

This error indicates that our Java VM process was running out of File Descriptor. This is a severe
condition that will affect the whole Java VM process and cause Weblogic to close its internal Server
Socket port (HTTP/HTTPS port) preventing any further inbound & outbound communication to the
affected managed server(s).

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File Descriptor - Why so important for an Oracle OSB environment?
The File Descriptor capacity is quite important for your Java VM process. The key concept you must
understand is that File Descriptors are not only required for pure File Handles but also for inbound and
outbound Socket communication. Each new Java Socket created to (inbound) or from (outound) your
Java VM by Weblogic kernel Socket Muxer requires a File Descriptor allocation at the OS level.
An Oracle OSB environment can require a significant number of Sockets depending how much
inbound load it receives and how much outbound connections (Java Sockets) it has to create in order
to send and receive data from external / downstream systems (System End Points).
For that reason, you must ensure that you allocate enough File Descriptors / Sockets to your Java VM
process in order to support your daily load; including problematic scenarios such as sudden slowdown
of external systems which typically increase the demand on the File Descriptor allocation.
Runtime File Descriptor capacity check for Java VM and AIX OS
Following the discovery of this error, our technical team did perform a quick review of the current
observed runtime File Descriptor capacity & utilization of our OSB Java VM processes. This can be
done easily via the following AIX command:
procfiles  | grep rlimit & lsof -p  | wc -l

## Java VM process File Descriptor total capacity
>> procfiles 5425732 | grep rlimit
Current rlimit: 2000 file descriptors

## Java VM process File Descriptor current utilization
>> lsof -p  | wc -l
1920

As you can see, the current capacity was found at 2000; which is quite low for a medium size Oracle
OSB environment. The average utilization under heavy load was also found to be quite close to the
upper limit of 2000.
The next step was to verify the default AIX OS File Descriptor limit via the command:
>> ulimit -S -n
2000

Conclusion #2: The current File Descriptor limit for both OS and OSB Java VM appears to be quite low
and setup at 2000. The File Descriptor utilization was also found to be quite close to the upper limit

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which explains why so many JVM failures were observed at peak load.
Weblogic File Descriptor configuration review
The File Descriptor limit can typically be overwritten when you start your Weblogic Java VM. Such
configuration is managed by the WLS core layer and script can be found at the following location:
/wlserver_10.3/common/bin/commEnv.sh

Root cause: File Descriptor override only working for Solaris OS!
As you can see with the script screenshot below, the override of the File Descriptor limit via ulimit is
only applicable for Solaris OS (SunOS) which explains why our current OSB Java VM running on AIX
OS did end up with the default value of 2000 vs. our older ALSB 2.6 environment running on Solaris
OS which had a File Descriptor limit of 65536.

Solution: script tweaking for AIX OS
The resolution of this problem was done by modifying the Weblogic commEnv script as per below.
This change did ensure a configuration of 65536 File Descriptor (from 2000); including for the AIX OS:

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** Please note that the activation of any change to the Weblogic File Descriptor configuration requires
a restart of both the Node Manager (if used) along with the managed servers. **
A runtime validation was also performed following the activation of the new configuration which did
confirm the new active File Descriptor limit:
>> procfiles 6416839 | grep rlimit
Current rlimit: 65536 file descriptors

No failure has been observed since then.
Conclusion and recommendations
When upgrading your Weblogic Java EE container to a new version, please ensure that you verify
your current File Descriptor limit as per the above case study. From a capacity planning perspective,
please ensure that you monitor your File Descriptor utilizaiton on a regular basis in order to identify
any potential capacity problem, Socket leak etc.

GC overhead limit exceeded – Analysis and Patterns
This section will provide you with a description of this new JVM 1.6 HotSpot OutOfMemoryError error
message and how you should attack this problem until its resolution.

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You can also refer to this post for a real case study on a Java Heap problem (OutOfMemoryError: GC
overhead limit exceeded) affecting a JBoss production system.
java.lang.OutOfMemoryError: GC overhead limit exceeded - what is it?
Everyone involved in Java EE production support is familiar with OutOfMemoryError problems since
they are one of the most common problem type you can face. However, if your environment recently
upgraded to Java HotSpot 1.6 VM, you may have observed this error message in the logs:
java.lang.OutOfMemoryError: GC overhead limit exceeded.
GC overhead limit exceeded is a new policy that was added by default for the Java HotSpot VM 1.6
only. It basically allows the VM to detect potential OutOfMemoryError conditions earlier and before it
runs out of Java Heap space; allowing the JVM to abort the current Thread(s) processing with this
OOM error.
The official Sun statement is provided below:
The parallel / concurrent collector will throw an OutOfMemoryError if too much time is being spent in
garbage collection: if more than 98% of the total time is spent in garbage collection and less than 2%
of the heap is recovered, an OutOfMemoryError will be thrown. This feature is designed to prevent
applications from running for an extended period of time while making little or no progress because
the heap is too small. If necessary, this feature can be disabled by adding the option -XX:UseGCOverheadLimit to the command line.
Is it useful for Java EE production systems?
I have found on most of my problem cases that this new policy is useful at some level since it is
preventing a full JVM hang and allowing you to take some actions such as data collection, JVM Heap
Dump, JVM Thread Dump etc. before the whole JVM becomes unresponsive.
But don't expect this new feature to fix your Java Heap problem, it is meant to prevent a full JVM hang
and to abort some big memory allocation etc. you must still perform your own analysis and due
diligence.
Is there any scenario where it can cause more harm than good?
Yes, Java applications dealing with large memory allocations / chunks could see much more frequent
OOM due to GC overhead limit exceeded. Some applications dealing with a long GC elapsed time but
healthy overall memory usage could also be affected.
In the above scenarios, you may want to consider turning OFF this policy and see if it's helping your
environment stability.
java.lang.OutOfMemoryError: GC overhead limit exceeded - can I disable it?
Yes, you can disable this default policy by simply adding this parameter at your JVM start-up:

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-XX:-UseGCOverheadLimit
Please keep in mind that this error is very likely to be a symptom of a JVM Heap / tuning problem so
my recommendation to you is always to focus on the root cause as opposed to the symptom.
java.lang.OutOfMemoryError: GC overhead limit exceeded - how can I fix it?
You should not worry too much about the GC overhead limit error itself since it's very likely just a
symptom / hint. What you must focus on is on your potential Java Heap problem(s) such as Java
Heap leak, improper Java Heap tuning etc. Find below a list of high level steps to troubleshoot further:
•
•

•
•

•

If not done already, enabled verbose GC >> -verbose:gc
Analyze the verbose GC output and determine the memory footprint of the Java Heap;
including the ratio of Young Gen vs. Old Gen. Having an old gen footprint too high will lead to
too many frequent Full GC and ultimately to the OOM: GC overhead limit exceeded.
Analyze the verbose GC output or use a tool like JConsole to determine if your Java Heap is
leaking over time. This can be observed via monitoring of the HotSpot old gen space.
Look at your young Gen requirement as well, if you application generates a lot of short live
objects then your Java Heap space must be big enough in order for the VM to allocate a bigger
Young Gen space.
If facing a Java Heap leak and/or if you have concern on your Old Gen footprint then add the
following parameter to your start-up JVM arguments: -XX:+HeapDumpOnOutOfMemoryError.
This will generate a Heap Dump (hprof format) on OOM event that you can analyze using a
tool like Memory Analyzer or Jhat.

Heap Analysis
Now we will provide you with a sample program and a tutorial on how to analyze your Java HotSpot
Heap footprint using Memory Analyzer following an OutOfMemoryError. I highly recommend that you
execute and analyse the Heap Dump yourself using this tutorial in order to better understand these
principles.
Troubleshooting tools
** all these tools can be downloaded for free **
•
•
•

Eclipse Indigo Release
Memory Analyzer via IBM Support Assistant 4.1 (HotSpot Heap Dump analysis)
Java VM: Windows HotSpot JRE 1.6.0_24 64-bit

Sample Java program
The simple sample Java program below will be used to triggered an OutOfMemoryError; allowing us to
analyze the generated HotSpot Heap Dump file. Simply create a new Java class :
JVMOutOfMemoryErrorSimulator.java to the Eclipse project of your choice and either rename or keep
the current package as is.

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This program is basically creating multiple String instances within a Map data structure until the Java
Heap depletion.

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package org.ph.javaee.javaheap;
import java.util.Map;
import java.util.HashMap;
/**
* JVMOutOfMemoryErrorSimulator
*
* @author PH
*
*/
public class JVMOutOfMemoryErrorSimulator {
private final static int NB_ITERATIONS = 500000;
// ~1 KB data footprint
private final static String LEAKING_DATA_PREFIX =
"datadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadat
adatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadatadat
adatadatadatadatadatadatadatadatadatadatadatadatadatadatadatada";
// Map used to stored our leaking String instances
private static Map leakingMap;
static {
leakingMap = new HashMap();
}
public static void main(String[] args) {
System.out.println("JVM OutOfMemoryError Simulator 1.0");
System.out.println("Author: Pierre-Hugues Charbonneau");
System.out.println("http://javaeesupportpatterns.blogspot.com/");
try {
for (int i = 0; i < NB_ITERATIONS; i++) {
String data = LEAKING_DATA_PREFIX + i;
// Add data to our leaking Map data structure...
leakingMap.put(data, data);
}
} catch (Throwable any) {
if (any instanceof java.lang.OutOfMemoryError) {
System.out.println("OutOfMemoryError triggered! "
+ any.getMessage() + " [" + any + "]");
} else {
System.out.println("Unexpected Exception! " + any.getMessage()
+ " [" + any + "]");
}
}
System.out.println("simulator done!");
}
}

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Step #1 - Setup your JVM start-up arguments
First, setup your Eclipse Java runtime arguments as per below. For our example, we used an external
JRE 1.6 outside the Eclipse IDE with a Java Heap maximum capacity of 512 MB.
The key JVM argument allowing us to generate a Heap Dump is -XX:
+HeapDumpOnOutOfMemoryError which tells the JVM to generate a Heap Dump following an
OutOfMemoryError condition.

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Step #2 - Run the sample Java program
The next step is to run our Java program. Depending on your computer specs, this program will run
between 5-30 seconds before exiting with an OutOfMemoryError.

As you can see, the JVM generated a Heap Dump file java_pid3880.hprof. It is now time to fire the
Memory Analyzer tool and analyze the JVM Heap Dump.
Step #3 - Load the Heap Dump
Analyzing a Heap Dump is an analysis activity that can be simple or very complex. The goal of this
tutorial is to give you the basics of Heap Dump analysis. For more Heap Dump analysis, please refer
to the other case studies presented in this handbook.

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Step #4 - Analyze Heap Dump
Below are the snapshots and analysis steps that you can follow to understand the memory leak that
we simulated in our sample Java program.

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As you can see, the Heap Dump analysis using the Memory Analyzer tool was able to easily identify
our primary leaking Java class and data structure.
Conclusion
I hope this simple Java program and Heap Dump analysis tutorial has helped you understand the
basic principles of Java Heap analysis using the raw Heap Dump data. This analysis is critical when
dealing with OutOfMemoryError: GC overhead problems since those are symptoms of either Java
Heap leak of Java Heap footprint / tuning problem.

Java deadlock troubleshooting and analysis
This section will revisit the classic thread problem of deadlocks and summarize some key
troubleshooting and resolution techniques that can be used.
Java deadlock: what is it?
A true Java deadlock can essentially be described as a situation where two or more threads are
blocked forever, waiting for each other. This situation is very different from other more commons "dayto-day" thread problem patterns such as lock contention & thread races, threads waiting on blocking
IO calls etc. Such lock-ordering deadlock situation can be visualized as presented below:

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In the above visual example, the attempt by Thread A & Thread B to acquire 2 locks in different orders
is fatal. Once threads reached the deadlocked state, they can never recover, forcing you to restart the
affected JVM process.
There is also another type of deadlock: resource deadlock. This is by far the most common thread
problem pattern I have seen in my experience with Java EE enterprise system troubleshooting. A
resource deadlock is essentially a scenario where one or multiple threads are waiting to acquire a
resource which will never be available such as JDBC Pool depletions.
Lock-ordering deadlocks
You should know by now that I am a big fan of JVM thread dump analysis; crucial skill to acquire for
individuals either involved in Java/Java EE development or production support. The good news is that
Java-level deadlocks can be easily identified out-of-the-box by most JVM thread dump formats
(HotSpot, IBM VM...) since they contain a native deadlock detection mechanism which will actually
show you the threads involved in a true Java-level deadlock scenario along with the execution stack
trace. JVM thread dump can be captured via the tool of your choice such as JVisualVM, jstack or
natively such as kill -3  on Unix-based OS. Find below the JVM Java-level deadlock detection
section after running lab 1:

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Now this is the easy part…The core of the root cause analysis effort is to understand why such
threads are involved in a deadlock situation at the first place. Lock-ordering deadlocks could be
triggered from your application code but unless you are involved in high concurrency programming,
chances are that the culprit code is a third part API or framework that you are using or the actual Java
EE container itself, when applicable.
Now let’s review below the lock-ordering deadlock resolution strategies presented by Heinz in his
presentation “HOL6500 - Finding And Solving Java Deadlocks”:
# Deadlock resolution by global ordering (see lab1 solution)
Essentially involves the definition of a global ordering for the locks that would always prevent deadlock
(please see lab1 solution)
# Deadlock resolution by TryLock (see lab2 solution)
•
•
•
•

Lock the first lock
Then try to lock the second lock
If you can lock it, you are good to go
If you cannot, wait and try again

The above strategy can be implemented using Java Lock & ReantrantLock which also gives you also
flexibility to setup a wait timeout in order to prevent thread starvation in the event the first lock is
acquired for too long.

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public interface Lock {
void lock();
void lockInterruptibly() throws InterruptedException;
boolean tryLock();
boolean tryLock(long timeout, TimeUnit unit)
throws InterruptedException;
void unlock();
Condition newCondition();
}

If you look at the JBoss AS7 implementation, you will notice that Lock & ReantrantLock are widely
used from core implementation layers such as:
•
•
•
•

Deployment service
EJB3 implementation (widely used)
Clustering and session management
Internal cache & data structures (LRU, ConcurrentReferenceHashMap...)

Now and as per Heinz's point, the deadlock resolution strategy #2 can be quite efficient but proper
care is also required such as releasing all held lock via a finally{} block otherwise you can transform
your deadlock scenario into a livelock.
Resource deadlocks
Now let's move to resource deadlock scenarios. I'm glad that Heinz's lab #3 covered this since from
my experience this is by far the most common "deadlock" scenario that you will see, especially if you
are developing and supporting large distributed Java EE production systems.
Now let's get the facts right.
•
•
•

•

•

Resource deadlocks are not true Java-level deadlocks.
The JVM Thread Dump will not magically should you these types of deadlocks. This means
more work for you to analyze and understand this problem as a starting point.
Thread dump analysis can be especially confusing when you are just starting to learn how to
read Thread Dump since threads will often show up as RUNNING state vs. BLOCKED state for
Java-level deadlocks. For now, it is important to keep in mind that thread state is not that
important for this type of problem e.g. RUNNING state != healthy state.
The analysis approach is very different than Java-level deadlocks. You must take multiple
thread dump snapshots and identify thread problem/wait patterns between each snapshot. You
will be able to see threads not moving e.g. threads waiting to acquire a resource from a pool
and other threads that already acquired such resource and hanging.
Thread Dump analysis is not the only data point/fact important here. You will need to collect
other facts such statistics on the resource(s) the threads are waiting for, overall middleware or
environment health etc. The combination of all these facts will allow you to conclude on the
root cause along with a resolution strategy which may or may not involve code change.

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Java Thread deadlock - Case Study
This section will describe the complete root cause analysis of a recent Java deadlock problem
observed from a Weblogic 11g production system running on the IBM JVM 1.6. It will also demonstrate
the importance of mastering Thread Dump analysis skills; including for the IBM JVM Thread Dump
format.
Environment specifications
•
•
•
•

Java EE server: Oracle Weblogic Server 11g & Spring 2.0.5
OS: AIX 5.3
Java VM: IBM JRE 1.6.0
Platform type: Portal & ordering application

Monitoring and troubleshooting tools
•
•

JVM Thread Dump (IBM JVM format)
Compuware Server Vantage (Weblogic JMX monitoring & alerting)

Problem overview
A major stuck Threads problem was observed & reported from Compuware Server Vantage and
affecting 2 of our Weblogic 11g production managed servers causing application impact and timeout
conditions from our end users.
Gathering and validation of facts
As usual, a Java EE problem investigation requires gathering of technical and non-technical facts so
we can either derived other facts and/or conclude on the root cause. Before applying a corrective
measure, the facts below were verified in order to conclude on the root cause:
•
•
•
•
•

What is the client impact? MEDIUM (only 2 managed servers / JVM affected out of 16)
Recent change of the affected platform? Yes (new JMS related asynchronous component)
Any recent traffic increase to the affected platform? No
How does this problem manifest itself? A sudden increase of Threads was observed leading to
rapid Thread depletion
Did a Weblogic managed server restart resolve the problem? Yes, but problem is returning
after few hours (unpredictable & intermittent pattern)

Conclusion #1: The problem is related to an intermittent stuck Threads behaviour affecting only a few
Weblogic managed servers at the time.
Conclusion #2: Since problem is intermittent, a global root cause such as a non-responsive
downstream system is not likely.
Thread Dump analysis - first pass
The first thing to do when dealing with stuck Thread problems is to generate a JVM Thread Dump.

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This is a golden rule regardless of your environment specifications & problem context. A JVM Thread
Dump snapshot provides you with crucial information about the active Threads and what type of
processing / tasks they are performing at that time.
Now back to our case study, an IBM JVM Thread Dump (javacore.xyz format) was generated which
did reveal the following Java Thread deadlock condition below:
1LKDEADLOCK
Deadlock detected !!!
NULL
--------------------NULL
2LKDEADLOCKTHR Thread "[STUCK] ExecuteThread: '8' for queue:
'weblogic.kernel.Default (self-tuning)'" (0x000000012CC08B00)
3LKDEADLOCKWTR
is waiting for:
4LKDEADLOCKMON
sys_mon_t:0x0000000126171DF8 infl_mon_t:
0x0000000126171E38:
4LKDEADLOCKOBJ
weblogic/jms/frontend/FESession@0x07000000198048C0/0x07000000198048D8:
3LKDEADLOCKOWN
which is owned by:
2LKDEADLOCKTHR Thread "[STUCK] ExecuteThread: '10' for queue:
'weblogic.kernel.Default (self-tuning)'" (0x000000012E560500)
3LKDEADLOCKWTR
which is waiting for:
4LKDEADLOCKMON
sys_mon_t:0x000000012884CD60 infl_mon_t:
0x000000012884CDA0:
4LKDEADLOCKOBJ
weblogic/jms/frontend/FEConnection@0x0700000019822F08/0x0700000019822F20:
3LKDEADLOCKOWN
which is owned by:
2LKDEADLOCKTHR Thread "[STUCK] ExecuteThread: '8' for queue:
'weblogic.kernel.Default (self-tuning)'" (0x000000012CC08B00)

This deadlock situation can be translated as per below:
•
•

Weblogic Thread #8 is waiting to acquire an Object monitor lock owned by Weblogic Thread
#10
Weblogic Thread #10 is waiting to acquire an Object monitor lock owned by Weblogic Thread
#8

Conclusion: Both Weblogic Threads #8 & #10 are waiting on each other; forever!
Now before going any deeper in this root cause analysis, let me provide you a high level overview on
Java Thread deadlocks.
Java Thread deadlock overview
Most of you are probably familiar with Java Thread deadlock principles but did you really experience a
true deadlock problem?
From my experience, true Java deadlocks are rare and I have only seen ~5 occurrences over the last
10 years. The reason is that most stuck Threads related problems are due to Thread hanging
conditions (waiting on remote IO call etc.) but not involved in a true deadlock condition with other
Thread(s).

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A Java Thread deadlock is a situation for example where Thread A is waiting to acquire an Object
monitor lock held by Thread B which is itself waiting to acquire an Object monitor lock held by Thread
A. Both these Threads will wait for each other forever. This situation can be visualized as per below
diagram:

Thread deadlock is confirmed...now what can you do?
Once the deadlock is confirmed (most JVM Thread Dump implementations will highlight it for you), the
next step is to perform a deeper dive analysis by reviewing each Thread involved in the deadlock
situation along with their current task & wait condition.
Find below the partial Thread Stack Trace from our problem case for each Thread involved in the
deadlock condition:
** Please note that the real application Java package name was renamed for confidentiality purposes
**
Weblogic Thread #8

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"[STUCK] ExecuteThread: '8' for queue: 'weblogic.kernel.Default (self-tuning)'"
J9VMThread:0x000000012CC08B00, j9thread_t:0x00000001299E5100,
java/lang/Thread:0x070000001D72EE00, state:B, prio=1
(native thread ID:0x111200F, native priority:0x1, native policy:UNKNOWN)
Java callstack:
at weblogic/jms/frontend/FEConnection.stop(FEConnection.java:671(Compiled
Code))
at weblogic/jms/frontend/FEConnection.invoke(FEConnection.java:1685(Compiled
Code))
at
weblogic/messaging/dispatcher/Request.wrappedFiniteStateMachine(Request.java:96
1(Compiled Code))
at
weblogic/messaging/dispatcher/DispatcherImpl.syncRequest(DispatcherImpl.java:18
4(Compiled Code))
at
weblogic/messaging/dispatcher/DispatcherImpl.dispatchSync(DispatcherImpl.java:2
12(Compiled Code))
at
weblogic/jms/dispatcher/DispatcherAdapter.dispatchSync(DispatcherAdapter.java:4
3(Compiled Code))
at weblogic/jms/client/JMSConnection.stop(JMSConnection.java:863(Compiled
Code))
at weblogic/jms/client/WLConnectionImpl.stop(WLConnectionImpl.java:843)
at
org/springframework/jms/connection/SingleConnectionFactory.closeConnection(Sing
leConnectionFactory.java:342)
at
org/springframework/jms/connection/SingleConnectionFactory.resetConnection(Sing
leConnectionFactory.java:296)
at org/app/JMSReceiver.receive()
……………………………………………………………………

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Weblogic Thread #10
"[STUCK] ExecuteThread: '10' for queue: 'weblogic.kernel.Default (self-tuning)'"
J9VMThread:0x000000012E560500, j9thread_t:0x000000012E35BCE0,
java/lang/Thread:0x070000001ECA9200, state:B, prio=1
(native thread ID:0x4FA027, native priority:0x1, native policy:UNKNOWN)
Java callstack:
at weblogic/jms/frontend/FEConnection.getPeerVersion(FEConnection.java:1381(Compiled
Code))
at weblogic/jms/frontend/FESession.setUpBackEndSession(FESession.java:755(Compiled
Code))
at weblogic/jms/frontend/FESession.consumerCreate(FESession.java:1025(Compiled Code))
at weblogic/jms/frontend/FESession.invoke(FESession.java:2995(Compiled Code))
at
weblogic/messaging/dispatcher/Request.wrappedFiniteStateMachine(Request.java:961(Compile
d Code))
at
weblogic/messaging/dispatcher/DispatcherImpl.syncRequest(DispatcherImpl.java:184(Compile
d Code))
at
weblogic/messaging/dispatcher/DispatcherImpl.dispatchSync(DispatcherImpl.java:212(Compil
ed Code))
at
weblogic/jms/dispatcher/DispatcherAdapter.dispatchSync(DispatcherAdapter.java:43(Compile
d Code))
at weblogic/jms/client/JMSSession.consumerCreate(JMSSession.java:2982(Compiled Code))
at weblogic/jms/client/JMSSession.setupConsumer(JMSSession.java:2749(Compiled Code))
at weblogic/jms/client/JMSSession.createConsumer(JMSSession.java:2691(Compiled Code))
at weblogic/jms/client/JMSSession.createReceiver(JMSSession.java:2596(Compiled Code))
at weblogic/jms/client/WLSessionImpl.createReceiver(WLSessionImpl.java:991(Compiled
Code))
at
org/springframework/jms/core/JmsTemplate102.createConsumer(JmsTemplate102.java:204(Compi
led Code))
at org/springframework/jms/core/JmsTemplate.doReceive(JmsTemplate.java:676(Compiled
Code))
at org/springframework/jms/core/JmsTemplate$10.doInJms(JmsTemplate.java:652(Compiled
Code))
at org/springframework/jms/core/JmsTemplate.execute(JmsTemplate.java:412(Compiled
Code))
at
org/springframework/jms/core/JmsTemplate.receiveSelected(JmsTemplate.java:650(Compiled
Code))
at
org/springframework/jms/core/JmsTemplate.receiveSelected(JmsTemplate.java:641(Compiled
Code))
at org/app/JMSReceiver.receive()

……………………………………………………………

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As you can see in the above Thread Strack Traces, such deadlock did originate from our application
code which is using the Spring framework API for the JMS consumer implementation (very useful
when not using MDB's). The Stack Traces are quite interesting and revealing that both Threads are in
a race condition against the same Weblogic JMS consumer session / connection and leading to a
deadlock situation:
•
•
•

Weblogic Thread #8 is attempting to reset and close the current JMS connection
Weblogic Thread #10 is attempting to use the same JMS Connection / Session in order to
create a new JMS consumer
Thread deadlock is triggered!

Root cause: non Thread safe Spring JMS SingleConnectionFactory implementation
A code review and a quick research from Spring JIRA bug database did reveal the following Thread
safe defect below with a perfect correlation with the above analysis:
# SingleConnectionFactory's resetConnection is causing deadlocks with underlying OracleAQ's JMS
connection – Bug Link
A patch for Spring SingleConnectionFactory was released back in 2009 which did involve adding
proper synchronized{} block in order to prevent Thread deadlock in the event of a JMS Connection
reset operation:
synchronized (connectionMonitor) {
//if condition added to avoid possible deadlocks when trying to reset the
target connection
if (!started) {
this.target.start();
started = true;
}
}

Solution
Our team is currently planning to integrate this Spring patch in to our production environment shortly.
The initial tests performed in our test environment are positive.
Conclusion
I hope this case study has helped understand a real-life Java Thread deadlock problem and how
proper Thread Dump analysis skills can allow you to quickly pinpoint the root cause of stuck Thread
related problems at the code level.

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Java concurrency: the hidden thread deadlocks
Most Java programmers are familiar with the Java thread deadlock concept. It essentially involves 2
threads waiting forever for each other. This condition is often the result of flat (synchronized) or
ReentrantLock (read or write) lock-ordering problems.
Found one Java-level deadlock:
=============================
"pool-1-thread-2":
waiting to lock monitor 0x0237ada4 (object 0x272200e8, a java.lang.Object),
which is held by "pool-1-thread-1"
"pool-1-thread-1":
waiting to lock monitor 0x0237aa64 (object 0x272200f0, a java.lang.Object),
which is held by "pool-1-thread-2"

The good news is that the HotSpot JVM is always able to detect this condition for you...or is it?
A recent thread deadlock problem affecting an Oracle Service Bus production environment has forced
us to revisit this classic problem and identify the existence of "hidden" deadlock situations.
This section will demonstrate and replicate via a simple Java program a very special lock-ordering
deadlock condition which is not detected by the latest HotSpot JVM 1.7. You will also find a video here
explaining you the Java sample program and the troubleshooting approach used.
The crime scene
I usually like to compare major Java concurrency problems to a crime scene where you play the lead
investigator role. In this context, the "crime" is an actual production outage of your client IT
environment. Your job is to:
•
•

Collect all the evidences, hints & facts (thread dump, logs, business impact, load figures...)
Interrogate the witnesses & domain experts (support team, delivery team, vendor, client...)

The next step of your investigation is to analyze the collected information and establish a potential list
of one or many "suspects" along with clear proofs. Eventually, you want to narrow it down to a primary
suspect or root cause. Obviously the law "innocent until proven guilty" does not apply here, exactly the
opposite.
Lack of evidence can prevent you to achieve the above goal. What you will see next is that the lack of
deadlock detection by the Hotspot JVM does not necessary prove that you are not dealing with this
problem.
The suspect
In this troubleshooting context, the "suspect" is defined as the application or middleware code with the
following problematic execution pattern.

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Usage of FLAT lock followed by the usage of ReentrantLock WRITE lock (execution path #1)
Usage of ReentrantLock READ lock followed by the usage of FLAT lock (execution path #2)
Concurrent execution performed by 2 Java threads but via a reversed execution order

The above lock-ordering deadlock criteria's can be visualized as per below:

Now let's replicate this problem via our sample Java program and look at the JVM thread dump output.
Sample Java program
This above deadlock conditions was first identified from our Oracle OSB problem case. We then recreated it via a simple Java program. You can download the entire source code of our program here.
The program is simply creating and firing 2 worker threads. Each of them execute a different execution
path and attempt to acquire locks on shared objects but in different orders. We also created a
deadlock detector thread for monitoring and logging purposes.
For now, find below the Java class implementing the 2 different execution paths.

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package org.ph.javaee.training8;
import java.util.concurrent.locks.ReentrantReadWriteLock;
public class Task {
// Object used for FLAT lock
private final Object sharedObject = new Object();
// ReentrantReadWriteLock used for WRITE & READ locks
private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
public void executeTask1() {
// 1. Attempt to acquire a ReentrantReadWriteLock READ lock
lock.readLock().lock();
// Wait 2 seconds to simulate some work...
try { Thread.sleep(2000);}catch (Throwable any) {}
try {
// 2. Attempt to acquire a Flat lock...
synchronized (sharedObject) {}
}
// Remove the READ lock
finally {
lock.readLock().unlock();
}
System.out.println("executeTask1() :: Work Done!");
}
public void executeTask2() {
// 1. Attempt to acquire a Flat lock
synchronized (sharedObject) {
// Wait 2 seconds to simulate some work...
try { Thread.sleep(2000);}catch (Throwable any) {}
// 2. Attempt to acquire a WRITE lock
lock.writeLock().lock();
try {
// Do nothing
}
// Remove the WRITE lock
finally {
lock.writeLock().unlock();
}
}
System.out.println("executeTask2() :: Work Done!");
}
public ReentrantReadWriteLock getReentrantReadWriteLock() {
return lock;
}
}

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As soon ad the deadlock situation was triggered, a JVM thread dump was generated using JvisualVM.

As you can see from the Java thread dump sample. The JVM did not detect this deadlock condition
(e.g. no presence of Found one Java-level deadlock) but it is clear these 2 threads are in deadlock

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state.
Root cause: ReetrantLock READ lock behavior
The main explanation we found at this point is associated with the usage of the ReetrantLock READ
lock. The read locks are normally not designed to have a notion of ownership. Since there is not a
record of which thread holds a read lock, this appears to prevent the HotSpot JVM deadlock detector
logic to detect deadlock involving read locks.
Some improvements were implemented since then but we can see that the JVM still cannot detect this
special deadlock scenario.
Now if we replace the read lock (execution pattern #1) in our program by a write lock, the JVM will
finally detect the deadlock condition but why?
Found one Java-level deadlock:
=============================
"pool-1-thread-2":
waiting for ownable synchronizer 0x272239c0, (a
java.util.concurrent.locks.ReentrantReadWriteLock$NonfairSync),
which is held by "pool-1-thread-1"
"pool-1-thread-1":
waiting to lock monitor 0x025cad3c (object 0x272236d0, a java.lang.Object),
which is held by "pool-1-thread-2"

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Java stack information for the threads listed above:
===================================================
"pool-1-thread-2":
at sun.misc.Unsafe.park(Native Method)
- parking to wait for <0x272239c0> (a
java.util.concurrent.locks.ReentrantReadWriteLock$NonfairSync)
at java.util.concurrent.locks.LockSupport.park(LockSupport.java:186)
at
java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(Abs
tractQueuedSynchronizer.java:834)
at
java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQue
uedSynchronizer.java:867)
at
java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSyn
chronizer.java:1197)
at
java.util.concurrent.locks.ReentrantReadWriteLock$WriteLock.lock(ReentrantReadW
riteLock.java:945)
at org.ph.javaee.training8.Task.executeTask2(Task.java:54)
- locked <0x272236d0> (a java.lang.Object)
at org.ph.javaee.training8.WorkerThread2.run(WorkerThread2.java:29)
at
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:722)
"pool-1-thread-1":
at org.ph.javaee.training8.Task.executeTask1(Task.java:31)
- waiting to lock <0x272236d0> (a java.lang.Object)
at org.ph.javaee.training8.WorkerThread1.run(WorkerThread1.java:29)
at
java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at
java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:722)

This is because write locks are tracked by the JVM similar to flat locks. This means the HotSpot JVM
deadlock detector appears to be currently designed to detect:
•
•

Deadlock on Object monitors involving FLAT locks
Deadlock involving Locked ownable synchronizers associated with WRITE locks

The lack of read lock per-thread tracking appears to prevent deadlock detection for this scenario and
significantly increase the troubleshooting complexity.
I suggest that you read Doug Lea’s comments on this whole issue since concerns were raised back in
2005 regarding the possibility to add per-thread read-hold tracking due to some potential lock
overhead.

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Find below my troubleshooting recommendations if you suspect a hidden deadlock condition involving
read locks:
•
•

Analyze closely the thread call stack trace, it may reveal some code potentially acquiring read
locks and preventing other threads to acquire write locks.
If you are the owner of the code, keep track of the read lock count via the usage of the
lock.getReadLockCount() method

OutOfMemoryError patterns
An OutOfMemoryError problem is one of the most frequent and complex problems a Java EE
application support person can face with a production system. This section will focus on a particular
OOM flavour: PermGen space depletion of a Java HotSpot VM.
Find below some of the most common patterns of OutOfMemoryError due to the depletion of the
PermGen space.
Pattern

Symptoms

Possible root cause
scenarios

Resolution

OOM observed during
or after a migration of a
Java EE server to
newer version

- OOM may be
observed on server
start-up at deployment
time
- OOM may be
observed very shortly
after server start-up and
after 1 or 2+ hours of
production traffic

- Higher PermGen
capacity is often
required due to
increased Java EE
server vendor code and
libraries

- Increase your
PermGen space
capacity via
-XX:MaxPermSize

OOM observed after a
certain period of time

- OOM observed after a
longer but consistent
period of time (days)
- PermGen space
monitoring will show
hourly or daily increase
during your application
business hours

- There are many
possible causes of
PermGen space
memory leak. The most
common is a class
loader leak: increasing
number of Class objects
overtime
- Improper JVM
arguments like usage of
the Xnoclassgc flag
(turn OFF Class
garbage collection)

- Review your JVM
HotSpot start-up
arguments for any
obvious problem like
Xnoclassgc flag
- Analyse the JVM
HotSpot Heap Dump as
it can provides some
hints on the source of a
class loader leak
- Investigate any third
party API you are using
for any potential class
loader leak defect
- Investigate your
application code for any
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Reflection API and / or
dynamic class loading

OOM observed
following a redeploy of
your application code
(EAR, WAR files...)

- OOM may be
observed during or
shortly after your
application redeploy
process

- Unloading and
reloading of your
application code can
lead to PermGen leak
(class loader leak) and
deplete your PermGen
space fairly quickly

- Open a ticket with your
Java EE vendor for any
known class loader leak
issue
- Shutdown and restart
your server (JVM) post
deployment to cleanup
any class loader leak

OutOfMemoryError: Java heap space - what is it?
This error message is typically what you will see your middleware server logs (Weblogic, WAS, JBoss
etc.) following a JVM OutOfMemoryError condition:
•
•

It is generated from the actual Java HotSpot VM native code
It is triggered as a result of Java Heap (Young Gen / Old Gen spaces) memory allocation
failure (due to Java Heap exhaustion)

Ok, so my application Java Heap is exhausted...how can I monitor and track my application Java
Heap?
The simplest way to properly monitor and track the memory footprint of your Java Heap spaces
(Young Gen & Old Gen spaces) is to enable verbose GC from your HotSpot VM. Please simply add
the following parameters within your JVM start-up arguments:
-verbose:gc -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -Xloggc:/gc.log

You can then follow my tutorial here in order to understand how to read and analyze your HotSpot
Java Heap footprint.
Ok thanks, now I can see that I have a big Java Heap memory problem...but how can I fix it?
There are multiple scenarios which can lead to Java Heap depletion such as:
•
•
•

Java Heap space too small vs. your application traffic & footprint
Java Heap memory leak (OldGen space slowly growing over time...)
Sudden and / or rogue Thread(s) consuming large amount of memory in short amount of time
etc.

Find below a list of high level steps you can follow in order to further troubleshoot:

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If not done already, enabled verbose GC >> -verbose:gc
Analyze the verbose GC output and determine the memory footprint of the Java Heap for each
of the Java Heap spaces (YoungGen & OldGen)
Analyze the verbose GC output or use a tool like JConsole to determine if your Java Heap is
leaking over time. This can be observed via monitoring of the HotSpot old gen space.
Monitor your middleware Threads and generate JVM Thread Dumps on a regular basis,
especially when a sudden increase of Java Heap utilization is observed. Thread Dump
analysis will allow you to pinpoint potential long running Thread(s) allocating a lot of objects on
your Java Heap in a short amount of time; if any
Add the following parameter within your JVM start-up arguments:
-XX:HeapDumpOnOutOfMemoryError This will allow your HotSpot VM to generate a binary
Heap Dump (HPROF) format. A binary Heap Dump is a critical data allowing to analyze your
application memory footprint and / or source(s) of Java Heap memory leak

From a resolution perspective, my recommendation to you is to analyze your Java Heap memory
footprint using the generated Heap Dump. The binary Heap Dump (HPROF format) can be analyzed
using the free Memory Analyzer tool (MAT). This will allow you to understand your java application
footprint and / or pinpoint source(s) of possible memory leak. Once you have a clear picture of the
situation, you will be able to resolve your problem by increasing your Java Heap capacity (via -Xms &
Xmx arguments) or reducing your application memory footprint and / or eliminating the memory leaks
from your application code. Please note that memory leaks are often found in middleware server code
and JDK as well.

OutOfMemoryError: Out of swap space - Problem Patterns
In this section we will revisit a common Java HotSpot VM problem that you probably already
experienced at some point in your JVM troubleshooting experience on Solaris OS; especially on a 32bit JVM.
We will provide you with a description of this particular type of OutOfMemoryError, the common
problem patterns and the recommended resolution approach.
java.lang.OutOfMemoryError: Out of swap space? - what is it?
This error message is thrown by the Java HotSpot VM (native code) following a failure to allocate
native memory from the OS to the HotSpot C-Heap or dynamically expand the Java Heap etc... This
problem is very different than a standard OutOfMemoryError (normally due to an exhaustion of the
Java Heap or PermGen space).
A typically error found in your application / server logs is:
Exception in thread "main" java.lang.OutOfMemoryError: requested 53459 bytes
for ChunkPool::allocate. Out of swap space?

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Also, please note that depending of the OS that you use (Windows, AIX, Solaris etc.) some
OutOfMemoryError due to C-Heap exhaustion may not give you detail such as "Out of swap space". In
this case, you will need to review the OOM error Stack Trace and determine if the computing task that
triggered the OOM and determine which OutOfMemoryError problem pattern your problem is related
to (Java Heap, PermGen or Native Heap exhaustion).
Ok so can I increase the Java Heap via -Xms & -Xmx to fix it?
Definitely not! This is the last thing you want to do as it will make the problem worse. As you learned,
the Java HotSpot VM is split between 3 memory spaces (Java Heap, PermGen, C-Heap). For a 32-bit
VM, all these memory spaces compete between each other for memory. Increasing the Java Heap
space will further reduce capacity of the C-Heap and reserve more memory from the OS.
Your first task is to determine if you are dealing with a C-Heap depletion or OS physical / virtual
memory depletion.
Now let's see the most common patterns of this problem.
Common problem patterns
There are multiple scenarios which can lead to a native OutOfMemoryError. I will share with you what I
have seen in my past experience as the most common patterns.
•
•
•
•
•
•

Native Heap (C-Heap) depletion due to too many Java EE applications deployed on a single
32-bit JVM (combined with large Java Heap e.g. 2 GB) * most common problem *
Native Heap (C-Heap) depletion due to a non-optimal Java Heap size e.g. Java Heap too large
for the application(s) needs on a single 32-bit JVM
Native Heap (C-Heap) depletion due to too many created Java Threads e.g. allowing the Java
EE container to create too many Threads on a single 32-bit JVM
OS physical / virtual memory depletion preventing the HotSpot VM to allocate native memory
to the C-Heap (32-bit or 64-bit VM)
OS physical / virtual memory depletion preventing the HotSpot VM to expand its Java Heap or
PermGen space at runtime (32-bit or 64-bit VM)
C-Heap / native memory leak (third party monitoring agent / library, JVM bug etc.)

Troubleshooting and resolution approach
Please keep in mind that each HotSpot native memory problem can be unique and requires its own
troubleshooting & resolution approach.
Find below a list of high level steps you can follow in order to further troubleshoot:
•

First, determine if the OOM is due to C-Heap exhaustion or OS physical / virtual memory. For
this task, you will need to perform close monitoring of your OS memory utilization and Java
process size. For example on Solaris, a 32-bit JVM process size can go to about 3.5 GB
(technically 4 GB limit) then you can expect some native memory allocation failures. The Java
process size monitoring will also allow you to determine if you are dealing with a native
memory leak (growing overtime / several days...).

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The OS vendor and version that you use is important as well. For example, some versions of
Windows (32-bit) by default support a process size up to 2 GB only (leaving you with minimal
flexibility for Java Heap and Native Heap allocations). Please review your OS and determine
what is the maximum process size e.g. 2 GB, 3 GB or 4 GB or more (64-bit OS).
Like the OS, it is also important that you review and determine if you are using a 32-bit VM or
64-bit VM. Native memory depletion for a 64-bit VM typically means that your OS is running out
of physical / virtual memory.
Review your JVM memory settings. For a 32-bit VM, a Java Heap of 2 GB+ can really start to
add pressure point on the C-Heap; depending how many applications you have deployed, Java
Threads etc. In that case, please determine if you can safely reduce your Java Heap by about
256 MB (as a starting point) and see if it helps improve your JVM memory "balance".
Analyze the verbose GC output or use a tool like JConsole to determine your Java Heap
footprint. This will allow you to determine if you can reduce your Java Heap in a safe manner
or not.
When OutOfMemoryError is observed. Generate a JVM Thread Dump and determine how
many Threads are active in your JVM; the more Threads, the more native memory your JVM
will use. You will then be able to combine this data with OS, Java process size and verbose
GC; allowing to determine where the problem is.

Once you have a clear view of the situation in your environment and root cause, you will be in a better
position to explore potential solutions as per below:
•
•
•

•

Reduce the Java Heap (if possible / after close monitoring of the Java Heap) in order to give
that memory back to the C-Heap / OS.
Increase the physical RAM / virtual memory of your OS (only applicable if depletion of the OS
memory is observed; especially for a 64-bit OS & VM).
Upgrade your HotSpot VM to 64-bit (for some Java EE applications, a 64-bit VM is more
appropriate) or segregate your applications to different JVM's (increase demand on your
hardware but reduce utilization of C-Heap per JVM).
Native memory leak are trickier and requires deeper dive analysis such as analysis of the
Solaris pmap / AIX svmon data and review of any third party library (e.g. monitoring agents).
Please also review the Oracle Sun Bug database and determine if your HotSpot version you
use is exposed to known native memory problems.

OutOfMemoryError: unable to create new native thread
One of the common problems I have observed from Java EE production systems is
OutOfMemoryError: unable to create new native thread; error thrown when the HotSpot JVM is unable
to further create a new Java thread.
This section will revisit this HotSpot VM error and provide you with recommendations and resolution
strategies.
OutOfMemoryError: unable to create new native thread - what is it?
Let's start with a basic explanation. This HotSpot JVM error is thrown when the internal JVM native

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code is unable to create a new Java thread. More precisely, it means that the JVM native code was
unable to create a new "native" thread from the OS (Solaris, Linux, MAC, Windows...). Unfortunately
at this point you won't get more detail than this error, with no indication of why the JVM is unable to
create a new thread from the OS.
HotSpot JVM: 32-bit or 64-bit?
Before you go any further in the analysis, one fundamental fact that you must determine from your
Java or Java EE environment is which version of HotSpot VM you are using e.g. 32-bit or 64-bit.
Why is it so important? What you will learn shortly is that this JVM problem is very often related to
native memory depletion; either at the JVM process or OS level. For now please keep in mind that:
•
•

•

A 32-bit JVM process is in theory allowed to grow up to 4 GB (even much lower on some older
32-bit Windows versions).
For a 32-bit JVM process, the C-Heap is in a race with the Java Heap and PermGen space
e.g. C-Heap capacity = 2-4 GB - Java Heap size (-Xms, -Xmx) - PermGen size (XX:MaxPermSize)
A 64-bit JVM process is in theory allowed to use most of the OS virtual memory available or up
to 16 EB (16 million TB)

As you can see, if you allocate a large Java Heap (2 GB+) for a 32-bit JVM process, the native
memory space capacity will be reduced automatically, opening the door for JVM native memory
allocation failures.
For a 64-bit JVM process, your main concern, from a JVM C-Heap perspective, is the capacity and
availability of the OS physical, virtual and swap memory.
OK great but how does native memory affect Java threads creation?
Now back to our primary problem. Another fundamental JVM aspect to understand is that Java
threads created from the JVM requires native memory from the OS. You should now start to
understand the source of your problem.
The high level thread creation process is as per below:
•
•
•

•
•

A new Java thread is requested from the Java program & JDK.
The JVM native code then attempt to create a new native thread from the OS.
The OS then attempts to create a new native thread as per attributes which include the thread
stack size. Native memory is then allocated (reserved) from the OS to the Java process native
memory space; assuming the process has enough address space (e.g. 32-bit process) to
honour the request.
The OS will refuse any further native thread & memory allocation if the 32-bit Java process
size has depleted its memory address space e.g. 2 GB, 3 GB or 4 GB process size limit.
The OS will also refuse any further Thread & native memory allocation if the virtual memory of
the OS is depleted (including Solaris swap space depletion since thread access to the stack
can generate a SIGBUS error, crashing the JVM (also see here).

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In summary:
•
•

Java threads creation require native memory available from the OS; for both 32-bit & 64-bit
JVM processes
For a 32-bit JVM, Java thread creation also requires memory available from the C-Heap or
process address space

Problem diagnostic
Now that you understand native memory and JVM thread creation a little better, is it now time to look
at your problem. As a starting point, I suggest that your follow the analysis approach below:
•
•
•
•

Determine if you are using HotSpot 32-bit or 64-bit JVM
When problem is observed, take a JVM Thread Dump and determine how many Threads are
active
Monitor closely the Java process size utilization before and during the OOM problem
replication
Monitor closely the OS virtual memory utilization before and during the OOM problem
replication; including the swap memory space utilization if using Solaris OS

Proper data gathering as per above will allow you to collect the proper data points, allowing you to
perform the first level of investigation. The next step will be to look at the possible problem patterns
and determine which one is applicable for your problem case.
Problem pattern #1 - C-Heap depletion (32-bit JVM)
From my experience, OutOfMemoryError: unable to create new native thread is quite common for 32bit JVM processes. This problem is often observed when too many threads are created vs. C-Heap
capacity.
JVM Thread Dump analysis and Java process size monitoring will allow you to determine if this is the
cause.
Problem pattern #2 - OS virtual memory depletion (64-bit JVM)
In this scenario, the OS virtual memory is fully depleted. This could be due to a few 64-bit JVM
processes taking lot memory e.g. 10 GB+ and / or other high memory footprint rogue processes.
Again, Java process size & OS virtual memory monitoring will allow you to determine if this is the
cause.
Problem pattern #3 - OS virtual memory depletion (32-bit JVM)
The third scenario is less frequent but can still be observed. The diagnostic can be a bit more complex
but the key analysis point will be to determine which processes are causing a full OS virtual memory
depletion. Your 32-bit JVM processes could be either the source or the victim such as rogue
processes using most of the OS virtual memory and preventing your 32-bit JVM processes to reserve
more native memory for its thread creation process.
Please note that this problem can also manifest itself as a full JVM crash (as per below sample) when

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running out of OS virtual memory or swap space on Solaris.
#
# A fatal error has been detected by the Java Runtime Environment:
#
# java.lang.OutOfMemoryError: requested 32756 bytes for ChunkPool::allocate.
Out of swap space?
#
# Internal Error (allocation.cpp:166), pid=2290, tid=27
# Error: ChunkPool::allocate
#
# JRE version: 6.0_24-b07
# Java VM: Java HotSpot(TM) Server VM (19.1-b02 mixed mode solaris-sparc )
# If you would like to submit a bug report, please visit:
#
http://java.sun.com/webapps/bugreport/crash.jsp
#
---------------

T H R E A D

---------------

Current thread (0x003fa800): JavaThread "CompilerThread1" daemon
[_thread_in_native, id=27, stack(0x65380000,0x65400000)]
Stack: [0x65380000,0x65400000], sp=0x653fd758, free space=501k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)

Native memory depletion: symptom or root cause?
You now understand your problem and know which problem pattern you are dealing with. You are now
ready to provide recommendations to address the problem... are you?
Your work is not done yet, please keep in mind that this JVM OOM event is often just a "symptom" of
the actual root cause of the problem. The root cause is typically much deeper so before providing
recommendations to your client I recommend that you really perform deeper analysis. The last thing
you want to do is to simply address and mask the symptoms. Solutions such as increasing OS
physical / virtual memory or upgrading all your JVM processes to 64-bit should only be considered
once you have a good view on the root cause and production environment capacity requirements.
The next fundamental question to answer is how many threads were active at the time of the
OutOfMemoryError? In my experience with Java EE production systems, the most common root
cause is actually the application and / or Java EE container attempting to create too many threads at a
given time when facing non happy paths such as thread stuck in a remote IO call, thread race
conditions etc. In this scenario, the Java EE container can start creating too many threads when
attempting to honour incoming client requests, leading to increase pressure point on the C-Heap and
native memory allocation. Bottom line, before blaming the JVM, please perform your due diligence and
determine if you are dealing with an application or Java EE container thread tuning problem as the
root cause.
Once you understand and address the root cause (source of thread creations), you can then work on
tuning your JVM and OS memory capacity in order to make it more fault tolerant and better "survive"

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these sudden thread surge scenarios.
Recommendations:
First, quickly rule out any obvious OS memory (physical & virtual memory) & process capacity
(e.g. ulimit) problem.
Perform a JVM Thread Dump analysis and determine the source of all the active threads vs.
an established baseline. Determine what is causing your Java application or Java EE container
to create so many threads at the time of the failure.
Please ensure that your monitoring tools closely monitor both your Java VM processes size &
OS virtual memory. This crucial data will be required in order to perform a full root cause
analysis. Please remember that a 32-bit Java process size is limited between 2 GB - 4 GB
depending of your OS.
Look at all running processes and determine if your JVM processes are actually the source of
the problem or victim of other processes consuming all the virtual memory.
Revisit your Java EE container thread configuration & JVM thread stack size. Determine if the
Java EE container is allowed to create more threads than your JVM process and / or OS can
handle.
Determine if the Java Heap size of your 32-bit JVM is too large, preventing the JVM to create
enough threads to fulfill your client requests. In this scenario, you will have to consider
reducing your Java Heap size (if possible), vertical scaling or upgrade to a 64-bit JVM.

•
•

•

•
•

•

Capacity planning analysis to the rescue
Lack of capacity planning analysis is often the source of the problem. Any comprehensive load and
performance testing exercise should also properly determine the Java EE container threads, JVM &
OS native memory requirement for your production environment; including impact measurements of
"non-happy" paths. This approach will allow your production environment to stay away from this type
of problem and lead to better system scalability and stability in the long run.

ClassNotFoundException: How to resolve
This section will provide you with an overview of this common Java exception, a sample Java program
to support your learning process and resolution strategies.
As per the Oracle documentation, ClassNotFoundException is thrown following the failure of a class
loading call, using its string name, as per below:
•
•
•

The Class.forName method
The ClassLoader.findSystemClass method
The ClassLoader.loadClass method

In other words, it means that one particular Java class was not found or could not be loaded at
"runtime" from your application current context class loader.
This problem can be particularly confusing for Java beginners. This is why I always recommend to

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Java developers to learn and refine their knowledge on Java class loaders. Unless you are involved in
dynamic class loading and using the Java Reflection API, chances are that the
ClassNotFoundException error you are getting is not from your application code but from a referencing
API. Another common problem pattern is a wrong packaging of your application code. We will get back
to the resolution strategies at the end of the section.
java.lang.ClassNotFoundException: Sample Java program
Now find below a very simple Java program which simulates the 2 most common
ClassNotFoundException scenarios via Class.forName() & ClassLoader.loadClass(). Please simply
copy/paste and run the program with the IDE of your choice (Eclipse IDE was used for this example).
The Java program allows you to choose between problem scenario #1 or problem scenario #2 as per
below. Simply change to 1 or 2 depending of the scenario you want to study.
# Class.forName()
private static final int PROBLEM_SCENARIO = 1;
# ClassLoader.loadClass()
private static final int PROBLEM_SCENARIO = 2;

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package org.ph.javaee.training5;
public class ClassNotFoundExceptionSimulator {
private static final String CLASS_TO_LOAD = "org.ph.javaee.training5.ClassA";
private static final int PROBLEM_SCENARIO = 1;
public static void main(String[] args) {
System.out.println("java.lang.ClassNotFoundException Simulator - Training 5");
System.out.println("Author: Pierre-Hugues Charbonneau");
System.out.println("http://javaeesupportpatterns.blogspot.com");
switch(PROBLEM_SCENARIO) {
// Scenario #1 - Class.forName()
case 1:
System.out.println("\n** Problem scenario #1: Class.forName() **\n");
try {
Class newClass = Class.forName(CLASS_TO_LOAD);
System.out.println("Class "+newClass+" found successfully!");
} catch (ClassNotFoundException ex) {
ex.printStackTrace();
System.out.println("Class "+CLASS_TO_LOAD+" not found!");
} catch (Throwable any) {
System.out.println("Unexpected error! "+any);
}
break;
// Scenario #2 - ClassLoader.loadClass()
case 2:
System.out.println("\n** Problem scenario #2: ClassLoader.loadClass() **\n");
try {
ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
Class callerClass = classLoader.loadClass(CLASS_TO_LOAD);
Object newClassAInstance = callerClass.newInstance();
System.out.println("SUCCESS!: "+newClassAInstance);
} catch (ClassNotFoundException ex) {
ex.printStackTrace();
System.out.println("Class "+CLASS_TO_LOAD+" not found!");
} catch (Throwable any) {
System.out.println("Unexpected error! "+any);
}
break;
}
System.out.println("\nSimulator done!");
}
}

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package org.ph.javaee.training5;
/**
* ClassA
* @author Pierre-Hugues Charbonneau
*
*/
public class ClassA {
private final static Class CLAZZ = ClassA.class;
static {
System.out.println("Class loading of "+CLAZZ+" from ClassLoader
'"+CLAZZ.getClassLoader()+"' in progress...");
}
public ClassA() {
System.out.println("Creating a new instance of "+ClassA.class.getName()+"...");
doSomething();
}
private void doSomething() {
// Nothing to do...
}
}

If you run the program as is, you will see the output as per below for each scenario:
#Scenario 1 output (baseline)
java.lang.ClassNotFoundException Simulator - Training 5
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com
** Problem scenario #1: Class.forName() **
Class loading of class org.ph.javaee.training5.ClassA from ClassLoader
'sun.misc.Launcher$AppClassLoader@bfbdb0' in progress...
Class class org.ph.javaee.training5.ClassA found successfully!
Simulator done!

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#Scenario 2 output (baseline)
java.lang.ClassNotFoundException Simulator - Training 5
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com
** Problem scenario #2: ClassLoader.loadClass() **
Class loading of class org.ph.javaee.training5.ClassA from ClassLoader
'sun.misc.Launcher$AppClassLoader@2a340e' in progress...
Creating a new instance of org.ph.javaee.training5.ClassA...
SUCCESS!: org.ph.javaee.training5.ClassA@6eb38a
Simulator done!

For the “baseline” run, the Java program is able to load ClassA successfully.
Now let’s voluntary change the full name of ClassA and re-run the program for each scenario. The
following output can be observed:
#ClassA changed to ClassB
#Scenario 1 output (problem replication)
java.lang.ClassNotFoundException Simulator - Training 5
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com
** Problem scenario #1: Class.forName() **
java.lang.ClassNotFoundException: org.ph.javaee.training5.ClassB
at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:186)
at
org.ph.javaee.training5.ClassNotFoundExceptionSimulator.main(ClassNotFoundExcep
tionSimulator.java:29)
Class org.ph.javaee.training5.ClassB not found!
Simulator done!

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#Scenario 2 output (problem replication)
java.lang.ClassNotFoundException Simulator - Training 5
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com
** Problem scenario #2: ClassLoader.loadClass() **
java.lang.ClassNotFoundException: org.ph.javaee.training5.ClassB
at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
at
org.ph.javaee.training5.ClassNotFoundExceptionSimulator.main(ClassNotFoundExcep
tionSimulator.java:51)
Class org.ph.javaee.training5.ClassB not found!
Simulator done!

What happened? Well since we changed the full class name to org.ph.javaee.training5.ClassB, such
class was not found at runtime (does not exist), causing both Class.forName() and
ClassLoader.loadClass() calls to fail.
You can also replicate this problem by packaging each class of this program to its own JAR file and
then omit ting the jar file containing ClassA.class from the main class path Please try this and see the
results for yourself... (hint: NoClassDefFoundError)
Now let's jump to the resolution strategies.
java.lang.ClassNotFoundException: Resolution strategies
Now that you understand this problem, it is now time to resolve it. Resolution can be fairly simple or
very complex depending of the root cause.
•
•

•

•
•

Don't jump on complex root causes too quickly, rule out the simplest causes first.
First review the java.lang.ClassNotFoundException stack trace as per the above and
determine which Java class was not loaded properly at runtime e.g. application code, third
party API, Java EE container itself etc.
Identify the caller e.g. Java class you see from the stack trace just before the Class.forName()
or ClassLoader.loadClass() calls. This will help you understand if your application code is at
fault vs. a third party API.
Determine if your application code is not packaged properly e.g. missing JAR file(s) from your
classpath.
If the missing Java class is not from your application code, then identify if it belongs to a third
party API you are using as per of your Java application. Once you identify it, you will need to

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add the missing JAR file(s) to your runtime classpath or web application WAR/EAR file.
If still struggling after multiple resolution attempts, this could means a more complex class
loader hierarchy problem. In this case, please review the NoClassDefFoundError section below
for more examples and resolution strategies.

NoClassDefFoundError Problem patterns
Getting a java.lang.NoClassDefFoundError when supporting a Java EE application is quite common
and at the same time complicated to resolve.
The section will provide you with the common problem patterns responsible for
java.lang.NoClassDefFoundError problems.
java.lang.NoClassDefFoundError- what is it?
This runtime error is thrown by the JVM when it tries to load the definition of a Class and when such
Class definition could not be found in the current Class loader tree.
This normally means that the compiled version of the reference to this Class was done successfully
but that such reference at runtime can not be found.
Sound confusing? Let's have a look at the visual diagram below so you can better understand this
fundamental problem.

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Now if you are interested, find below the source code of our sample program along with
java.lang.NoClassDefFoundError error.
package com.cgi.tools.java;
public class ClassA {
private ClassB instanceB = null;
private ClassC instanceC = null;
public ClassA() {
instanceB = new ClassB();
instanceC = new ClassC();
}
}

// ClassB.java
package com.cgi.tools.java;
public class ClassB {
}

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// ClassC.java
package com.cgi.tools.java;
public class ClassC {
}

package com.cgi.tools.java;
public class ProgramA {
/**
* @param args
*/
public static void main(String[] args) {
try {
ClassA instanceA = new ClassA();
System.out.println("ClassA instance created properly!");
}
catch (Throwable any) {
System.out.println("Unexpected problem! "+any.getMessage()+" ["+any+"]");
}
}
}

## ProgramA runtime classpath and output – with ClassC.jar
java -classpath ClassA.jar;ClassB.jar;ClassC.jar;ProgramA.jar
com.cgi.tools.java.ProgramA
ClassA instance created properly!

## ProgramA runtime classpath and output – without ClassC.jar
// We voluntarily omitted to add ClassC.jar in the System classpath
java -classpath ClassA.jar;ClassB.jar;ProgramA.jar
com.cgi.tools.java.ProgramA
Unexpected problem! com/cgi/tools/java/ClassC
[java.lang.NoClassDefFoundError: com/cgi/tools/java/ClassC]

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What are the most common scenarios causing NoClassDefFoundError?
There are a few common scenarios which can lead to NoClassDefFoundError in your Java EE
environment or standalone Java program.
# Problem pattern #1 - Missing vendor or third party library in System classpath or Java EE App
classloader
A missing Java library of your Java EE server itself (Weblogic, WAS, JBoss etc.) or third party
(Apache, Spring, Hibernate etc.) is the most common program; exactly like our above sample
program.
# Solution
Resolution requires proper root cause analysis as per below recommended steps:
1. Review the NoClassDefFoundError error and identify the missing Java Class
2. Search through your local development and / or build environment and identify which Jar file
contains the missing Java Class
3. Once jar file(s) is / are identified, compare your local / build classpath with your production /
problematic environment
4. Resolution may include adding the missing JAR file(s) to the System class path or to your
application EAR file for example
# Problem pattern #2 - Vendor or third party library version mismatch in System classpath or Java EE
App classloader
This problem pattern is less common but trickier to pinpoint the root cause. This is a normally caused
by using wrong version of a shared third party library like Apache commons logging etc.
# Solution
The resolution is quite similar to pattern #1:
1. Review the NoClassDefFoundError error and identify the missing Java Class along with the
referrer (very important)
2. Search through your local development and / or build environment and identify which Jar file
contains the missing Java Class
3. Search through your local development and / or build environment and identify which Jar file
contains the referrer Java Class
4. Once jar file(s) is / are identified, compare your local / build classpath with your production /
problematic environment
5. Resolution may include replacing the problematic JAR file(s) with the right version as per the
third party API documentation; this might include replacement of the JAR file referrer
depending on your root cause analysis results
# Problem pattern #3 - static{} block code failure

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This problem pattern is also quite common and can take some time to pinpoint. Java offers the
capability to write some code to be executed once in life time of the JVM / Class loader. This is
achieved via a static{} block, called static initializer, normally located right after the class instance
variables.
Unfortunately, proper error handling and "non happy paths" for static initializer code blocks are often
overlooked which opens the door for problems.
Any failure such as an uncaught Exception will prevent such Java class to be loaded to its class
loader. The pattern is as per below:
•
•

the first attempt to load the class will generate a java.lang.ExceptionInInitializerError;
preventing the class loader to load the referenced class
subsequent calls will then generate a java.lang.NoClassDefFoundError from any other
referencing classes in a consistent manner until the problem is resolved and the JVM restarted
(or live redeploy via your Java EE server redeploy task)

# Solution
Resolution requires proper root cause analysis as per below recommended steps:
1. Review the NoClassDefFoundError error and identify the affected Java Class
2. Perform a code review of the affected Java class and see if any static{} initializer block can be
found
3. If found, review the error handling and add proper try{} catch{} along with proper logging in
order to understand the root cause of the static block code failure
4. Compile, redeploy, retest and confirm problem resolution

NoClassDefFoundError – How to resolve
Exception in thread "main" java.lang.NoClassDefFoundError is one of the most common and difficult
problems that you can face when developing Java EE enterprise or standalone Java applications. The
complexity of the root cause analysis and resolution process mainly depend of the size of your Java
EE middleware environment, especially given the high number of class loaders present across the
various Java EE applications.
As mentioned before, this runtime error is thrown by the JVM when there is an attempt by a
ClassLoader to load the definition of a Class (Class referenced in your application code etc.) and
when such Class definition could not be found within the current ClassLoader tree.
Basically, this means that such Class definition was found at compiled time but is not found at runtime.
Java ClassLoader overview
Before going any further, it is very important that you have a high level of understanding of the Java
ClassLoader principles. Quite often individuals debugging NoClassDefFoundError problems are
struggling because they are lacking proper knowledge and understanding of Java ClassLoader

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principles; preventing them to pinpoint the root cause.
A class loader is a Java object responsible for loading classes. Basically a class loader attempts to
locate or generate data that constitutes a definition for the class. One of the key points to understand
is that Java class loaders by default use a delegation model to search for classes. Each instance of
ClassLoader has an associated parent class loader. So let's say that your application class loader
needs to load class A. The first thing that it will attempt to do is to delegate the search for Class A to its
parent class loader before attempting to find the Class A itself. You can end up with a large class
loader chain with many parent class loaders up to the JVM system classpath bootstrap class loader.
What is the problem? Well if Class A is found from a particular parent class loader then it will be
loaded by such parent which open the doors for NoClassDefFoundError if you are expecting Class A
to be loaded by your application (child) class loader. For example, third part JAR file dependencies
could only be present to your application child class loader.
Now let’s visualize this whole process in the context of a Java EE enterprise environment so you can
better understand.

As you can see, any code loaded by the child class loader (Web application) will first delegate to the
parent class loader (Java EE App). Such parent class loader will then delegate to the JVM system
class path class loader. If no such class is found from any parent class loader then the Class will be

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loaded by the child class loader (assuming that the class was found). Please note that Java EE
containers such as Oracle Weblogic have mechanisms to override this default class loader delegation
behavior.

NoClassDefFoundError problem case 1 - missing JAR file
The first problem case we will cover is related to a Java program packaging and / or classpath
problem. A typical Java program can include one or many JAR files created at compile time.
NoClassDefFoundError can often be observed when you forget to add JAR file(s) containing Java
classes referenced by your Java or Java EE application.
This type of problem is normally not hard to resolve once you analyze the Java Exception and missing
Java class name.
Sample Java program
The following simple Java program is split as per below:

•
•
•
•

The main Java program NoClassDefFoundErrorSimulator
The caller Java class CallerClassA
The referencing Java class ReferencingClassA
A util class for ClassLoader and logging related facilities JavaEETrainingUtil

This program is simple attempting to create a new instance and execute a method of the Java class
CallerClassA which is referencing the class ReferencingClassA.It will demonstrate how a
simple classpath problem can trigger NoClassDefFoundError. The program is also displaying detail on
the current class loader chain at class loading time in order to help you keep track of this process. This
will be especially useful for future and more complex problem cases when dealing with larger class
loader chains.

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#### NoClassDefFoundErrorSimulator.java
package org.ph.javaee.training1;
import org.ph.javaee.training.util.JavaEETrainingUtil;
/**
* NoClassDefFoundErrorTraining1
* @author Pierre-Hugues Charbonneau
*
*/
public class NoClassDefFoundErrorSimulator {
/**
* @param args
*/
public static void main(String[] args) {
System.out.println("java.lang.NoClassDefFoundError Simulator - Training
1");
System.out.println("Author: Pierre-Hugues Charbonneau");
System.out.println("http://javaeesupportpatterns.blogspot.com");
// Print current Classloader context
System.out.println("\nCurrent ClassLoader chain:
"+JavaEETrainingUtil.getCurrentClassloaderDetail());
// 1. Create a new instance of CallerClassA
CallerClassA caller = new CallerClassA();
// 2. Execute method of the caller
caller.doSomething();
System.out.println("done!");
}
}

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#### CallerClassA.java
package org.ph.javaee.training1;
import org.ph.javaee.training.util.JavaEETrainingUtil;
/**
* CallerClassA
* @author Pierre-Hugues Charbonneau
*
*/
public class CallerClassA {
private final static String CLAZZ = CallerClassA.class.getName();
static {
System.out.println("Classloading of "+CLAZZ+" in
progress..."+JavaEETrainingUtil.getCurrentClassloaderDetail());
}
public CallerClassA() {
System.out.println("Creating a new instance of
"+CallerClassA.class.getName()+"...");
}
public void doSomething() {
// Create a new instance of ReferencingClassA
ReferencingClassA referencingClass = new ReferencingClassA();
}
}

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#### ReferencingClassA.java
package org.ph.javaee.training1;
import org.ph.javaee.training.util.JavaEETrainingUtil;
/**
* ReferencingClassA
* @author Pierre-Hugues Charbonneau
*
*/
public class ReferencingClassA {
private final static String CLAZZ = ReferencingClassA.class.getName();
static {
System.out.println("Classloading of "+CLAZZ+" in
progress..."+JavaEETrainingUtil.getCurrentClassloaderDetail());
}
public ReferencingClassA() {
System.out.println("Creating a new instance of
"+ReferencingClassA.class.getName()+"...");
}
public void doSomething() {
//nothing to do...
}
}

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#### JavaEETrainingUtil.java
package org.ph.javaee.training.util;
import java.util.Stack;
import java.lang.ClassLoader;
public class JavaEETrainingUtil {
public static String getCurrentClassloaderDetail() {
StringBuffer classLoaderDetail = new StringBuffer();
Stack classLoaderStack = new Stack();
ClassLoader currentClassLoader =
Thread.currentThread().getContextClassLoader();
classLoaderDetail.append("\n----------------------------------------------------------------\n");
// Build a Stack of the current ClassLoader chain
while (currentClassLoader != null) {
classLoaderStack.push(currentClassLoader);
currentClassLoader = currentClassLoader.getParent();
}
// Print ClassLoader parent chain
while(classLoaderStack.size() > 0) {
ClassLoader classLoader = classLoaderStack.pop();
// Print current
classLoaderDetail.append(classLoader);

}

if (classLoaderStack.size() > 0) {
classLoaderDetail.append("\n--- delegation ---\n");
} else {
classLoaderDetail.append(" **Current ClassLoader**");
}

classLoaderDetail.append("\n----------------------------------------------------------------\n");
return classLoaderDetail.toString();
}

}

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Problem reproduction
In order to replicate the problem, we will simply “voluntary” omit one of the JAR files from the
classpath that contains the referencing Java class ReferencingClassA.
The Java program is packaged as per below:

•
•
•

MainProgram.jar (contains NoClassDefFoundErrorSimulator.class and
JavaEETrainingUtil.class)
CallerClassA.jar (contains CallerClassA.class)
ReferencingClassA.jar (contains ReferencingClassA.class)

Now, let’s run the program as is:
## Baseline (normal execution)
..\bin>java -classpath CallerClassA.jar;ReferencingClassA.jar;MainProgram.jar
org.ph.javaee.training1.NoClassDefFoundErrorSimulator
java.lang.NoClassDefFoundError Simulator - Training 1
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com
Current ClassLoader chain:
----------------------------------------------------------------sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation --sun.misc.Launcher$AppClassLoader@214c4ac9 **Current ClassLoader**
----------------------------------------------------------------Classloading of org.ph.javaee.training1.CallerClassA in progress...
----------------------------------------------------------------sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation --sun.misc.Launcher$AppClassLoader@214c4ac9 **Current ClassLoader**
----------------------------------------------------------------Creating a new instance of org.ph.javaee.training1.CallerClassA...
Classloading of org.ph.javaee.training1.ReferencingClassA in progress...
----------------------------------------------------------------sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation --sun.misc.Launcher$AppClassLoader@214c4ac9 **Current ClassLoader**
----------------------------------------------------------------Creating a new instance of org.ph.javaee.training1.ReferencingClassA...
done!

For the initial run (baseline), the main program was able to create a new instance of CallerClassA

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and execute its method successfully; including successful class loading of the referencing class
ReferencingClassA.
## Problem reproduction run (with removal of ReferencingClassA.jar)
../bin>java -classpath CallerClassA.jar;MainProgram.jar
org.ph.javaee.training1.NoClassDefFoundErrorSimulator
java.lang.NoClassDefFoundError Simulator - Training 1
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com
Current ClassLoader chain:
----------------------------------------------------------------sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation --sun.misc.Launcher$AppClassLoader@214c4ac9 **Current ClassLoader**
----------------------------------------------------------------Classloading of org.ph.javaee.training1.CallerClassA in progress...
----------------------------------------------------------------sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation --sun.misc.Launcher$AppClassLoader@214c4ac9 **Current ClassLoader**
----------------------------------------------------------------Creating a new instance of org.ph.javaee.training1.CallerClassA...
Exception in thread "main" java.lang.NoClassDefFoundError:
org/ph/javaee/training1/ReferencingClassA
at
org.ph.javaee.training1.CallerClassA.doSomething(CallerClassA.java:25)
at
org.ph.javaee.training1.NoClassDefFoundErrorSimulator.main(NoClassDefFoundError
Simulator.java:28)
Caused by: java.lang.ClassNotFoundException:
org.ph.javaee.training1.ReferencingClassA
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 2 more

What happened? The removal of the ReferencingClassA.jar, containing ReferencingClassA, did
prevent the current class loader to locate this referencing Java class at runtime leading to
ClassNotFoundException and NoClassDefFoundError.
This is the typical Exception that you will get if you omit JAR file(s) from your Java start-up classpath
or within an EAR / WAR for Java EE related applications.

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ClassLoader view
Now let’s review the ClassLoader chain so you can properly understand this problem case. As you
saw from the Java program output logging, the following Java ClassLoaders were found:
Classloading of org.ph.javaee.training1.CallerClassA in progress...
----------------------------------------------------------------sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation --sun.misc.Launcher$AppClassLoader@214c4ac9 **Current ClassLoader**
-----------------------------------------------------------------

** Please note that the Java bootstrap class loader is responsible to load the core JDK classes and is
written in native code **
## sun.misc.Launcher$AppClassLoader
This is the system class loader responsible to load our application code found from the Java classpath
specified at start-up.
##sun.misc.Launcher$ExtClassLoader
This is the extension class loader responsible to load code in the extensions directories
(/lib/ext, or any other directory specified by the java.ext.dirs system property).
As you can see from the Java program logging output, the extension class loader is the actual super
parent of the system class loader. Our sample Java program was loaded at the system class loader
level. Please note that this class loader chain is very simple for this problem case since we did not
create child class loaders at this point.
Recommendations and resolution strategies
Now find below my recommendations and resolution strategies for NoClassDefFoundError problem
case 1:
•
•
•
•
•

Review the java.lang.NoClassDefFoundError error and identify the missing Java class
Verify and locate the missing Java class from your compile / build environment
Determine if the missing Java class is from your application code, third part API or even the
Java EE container itself. Verify where the missing JAR file(s) is / are expected to be found
Once found, verify your runtime environment Java classpath for any typo or missing JAR file(s)
If the problem is triggered from a Java EE application, perform the same above steps but verify
the packaging of your EAR / WAR file for missing JAR and other library file dependencies such
as MANIFEST

Java static initializer revisited
The Java programming language provides you with the capability to "statically" initialize variables or a
block of code. This is achieved via the "static" variable identifier or the usage of a static {} block at the
header of a Java class. Static initializers are guaranteed to be executed only once in the JVM life cycle

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and are Thread safe by design which make their usage quite appealing for static data initialization
such as internal object caches, loggers etc.
What is the problem? I will repeat again, static initializers are guaranteed to be executed only once in
the JVM life cycle...This means that such code is executed at the class loading time and never
executed again until you restart your JVM. Now what happens if the code executed at that time
(@Class loading time) terminates with an unhandled Exception?
Welcome to the java.lang.NoClassDefFoundError problem case #2!

NoClassDefFoundError problem case 2 - static initializer failure
This type of problem is occurring following the failure of static initializer code combined with
successive attempts to create a new instance of the affected (non-loaded) class.
Sample Java program
The following simple Java program is split as per below:

•
•
•

The main Java program NoClassDefFoundErrorSimulator
The affected Java class ClassA
ClassA provides you with a ON/OFF switch allowing you the replicate the type of problem
that you want to study

This program is simply attempting to create a new instance of ClassA 3 times (one after each
other). It will demonstrate that an initial failure of either a static variable or static block initializer
combined with successive attempt to create a new instance of the affected class triggers
java.lang.NoClassDefFoundError.

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#### NoClassDefFoundErrorSimulator.java
package org.ph.javaee.tools.jdk7.training2;
public class NoClassDefFoundErrorSimulator {
/**
* @param args
*/
public static void main(String[] args) {
System.out.println("java.lang.NoClassDefFoundError Simulator - Training
2");
System.out.println("Author: Pierre-Hugues Charbonneau");
System.out.println("http://javaeesupportpatterns.blogspot.com\n\n");
try {
// Create a new instance of ClassA (attempt #1)
System.out.println("FIRST attempt to create a new instance of
ClassA...\n");
ClassA classA = new ClassA();
} catch (Throwable any) {
any.printStackTrace();
}
try {
// Create a new instance of ClassA (attempt #2)
System.out.println("\nSECOND attempt to create a new instance
of ClassA...\n");
ClassA classA = new ClassA();
} catch (Throwable any) {
any.printStackTrace();
}
try {
// Create a new instance of ClassA (attempt #3)
System.out.println("\nTHIRD attempt to create a new instance of
ClassA...\n");
ClassA classA = new ClassA();
} catch (Throwable any) {
any.printStackTrace();
}
System.out.println("\n\ndone!");
}
}

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#### ClassA.java
package org.ph.javaee.tools.jdk7.training2;
/**
* ClassA
* @author Pierre-Hugues Charbonneau
*
*/
public class ClassA {
private final static String CLAZZ = ClassA.class.getName();
// Problem replication switch ON/OFF
private final static boolean REPLICATE_PROBLEM1 = true; // static variable
initializer
private final static boolean REPLICATE_PROBLEM2 = false; // static block{}
initializer
// Static variable executed at Class loading time
private static String staticVariable = initStaticVariable();
// Static initializer block executed at Class loading time
static {
// Static block code execution...
if (REPLICATE_PROBLEM2) throw new
IllegalStateException("ClassA.static{}: Internal Error!");
}
public ClassA() {
System.out.println("Creating a new instance of "+ClassA.class.getName()
+"...");
}
/**
*
* @return
*/
private static String initStaticVariable() {
String stringData = "";
if (REPLICATE_PROBLEM1) throw new
IllegalStateException("ClassA.initStaticVariable(): Internal Error!");
return stringData;
}
}

Problem reproduction

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In order to replicate the problem, we will simply "voluntary" trigger a failure of the static initializer code.
Please simply enable the problem type that you want to study e.g. either static variable or static block
initializer failure:

// Problem replication switch ON (true) / OFF (false)
private final static boolean REPLICATE_PROBLEM1 = true; // static variable initializer
private final static boolean REPLICATE_PROBLEM2 = false; // static block{} initializer

Now, let’s run the program with both switch at OFF (both boolean values at false)
## Baseline (normal execution)
java.lang.NoClassDefFoundError Simulator - Training 2
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com

FIRST attempt to create a new instance of ClassA...
Creating a new instance of org.ph.javaee.tools.jdk7.training2.ClassA...
SECOND attempt to create a new instance of ClassA...
Creating a new instance of org.ph.javaee.tools.jdk7.training2.ClassA...
THIRD attempt to create a new instance of ClassA...
Creating a new instance of org.ph.javaee.tools.jdk7.training2.ClassA...
done!

For the initial run (baseline), the main program was able to create 3 instances of ClassA successfully
with no problem.
## Problem reproduction run (static variable initializer failure)

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java.lang.NoClassDefFoundError Simulator - Training 2
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com

FIRST attempt to create a new instance of ClassA...
java.lang.ExceptionInInitializerError
at
org.ph.javaee.tools.jdk7.training2.NoClassDefFoundErrorSimulator.main(NoClassDe
fFoundErrorSimulator.java:21)
Caused by: java.lang.IllegalStateException: ClassA.initStaticVariable():
Internal Error!
at
org.ph.javaee.tools.jdk7.training2.ClassA.initStaticVariable(ClassA.java:37)
at org.ph.javaee.tools.jdk7.training2.ClassA.(ClassA.java:16)
... 1 more
SECOND attempt to create a new instance of ClassA...
java.lang.NoClassDefFoundError: Could not initialize class
org.ph.javaee.tools.jdk7.training2.ClassA
at
org.ph.javaee.tools.jdk7.training2.NoClassDefFoundErrorSimulator.main(NoClassDe
fFoundErrorSimulator.java:30)
THIRD attempt to create a new instance of ClassA...
java.lang.NoClassDefFoundError: Could not initialize class
org.ph.javaee.tools.jdk7.training2.ClassA
at
org.ph.javaee.tools.jdk7.training2.NoClassDefFoundErrorSimulator.main(NoClassDe
fFoundErrorSimulator.java:39)

done!

## Problem reproduction run (static block initializer failure)

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java.lang.NoClassDefFoundError Simulator - Training 2
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com

FIRST attempt to create a new instance of ClassA...

java.lang.ExceptionInInitializerError
at
org.ph.javaee.tools.jdk7.training2.NoClassDefFoundErrorSimulator.main(NoClassDe
fFoundErrorSimulator.java:21)
Caused by: java.lang.IllegalStateException: ClassA.static{}: Internal Error!
at org.ph.javaee.tools.jdk7.training2.ClassA.(ClassA.java:22)
... 1 more

SECOND attempt to create a new instance of ClassA...

java.lang.NoClassDefFoundError: Could not initialize class
org.ph.javaee.tools.jdk7.training2.ClassA
at
org.ph.javaee.tools.jdk7.training2.NoClassDefFoundErrorSimulator.main(NoClassDe
fFoundErrorSimulator.java:30)

THIRD attempt to create a new instance of ClassA...

java.lang.NoClassDefFoundError: Could not initialize class
org.ph.javaee.tools.jdk7.training2.ClassA
at
org.ph.javaee.tools.jdk7.training2.NoClassDefFoundErrorSimulator.main(NoClassDe
fFoundErrorSimulator.java:39)

done!

What happened? As you can see, the first attempt to create a new instance of ClassA did trigger a
java.lang.ExceptionInInitializerError. This exception indicates the failure of our static
initializer for our static variable & bloc which is exactly what we wanted to achieve.
The key point to understand at this point is that this failure did prevent the whole class loading of
ClassA. As you can see, attempt #2 and attempt #3 both generated a

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java.lang.NoClassDefFoundError, why? Well since the first attempt failed, class loading of ClassA
was prevented. Successive attempts to create a new instance of ClassA within the current
ClassLoader did generate java.lang.NoClassDefFoundError over and over since ClassA was not
found within current ClassLoader.
As you can see, in this problem context, the NoClassDefFoundError is just a symptom or
consequence of another problem. The original problem is the ExceptionInInitializerError triggered
following the failure of the static initializer code. This clearly demonstrates the importance of proper
error handling and logging when using Java static initializers.
Recommendations and resolution strategies
Now find below my recommendations and resolution strategies for NoClassDefFoundError problem
case 2:
•
•
•
•

Review the java.lang.NoClassDefFoundError error and identify the missing Java class
Perform a code walkthrough of the affected class and determine if it contains static initializer
code (variables & static block)
Review your server and application logs and determine if any error (e.g.
ExceptionInInitializerError) originates from the static initializer code
Once confirmed, analyze the code further and determine the root cause of the initializer code
failure. You may need to add some extra logging along with proper error handling to prevent
and better handle future failures of your static initializer code going forward

Parent first Classloader
The following section will describe one common problem pattern when using the default class loader
delegation model.
A simple Java program will again be provided in order to help you understand this problem pattern.
Default JVM Classloader delegation model
As we saw before, the default class loader delegation model is from bottom-up e.g. parent first. This
means that the JVM is going up to the class loader chain in order to find and load each of your
application Java classes. If the class is not found from the parent class loaders, the JVM will then
attempt to load it from the current Thread context class loader; typically a child class loader.
NoClassDefFoundError problems can occur, for example, when you wrongly package your application
(or third part API's) between the parent and child class loaders. Another example is code / JAR files
injection by the container itself or third party API's deployed at a higher level in the class loader chain.
In the above scenarios:
•
•

The JVM loads one part of the affected code to a parent class loader (SYSTEM or parent class
loaders)
The JVM loads the other parts of the affected code to a child class loader (Java EE container

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or application defined class loader)
Now what happens when Java classes loaded from the parent attempt to load reference classes
deployed only to the child classloader? NoClassDefFoundError!
Please remember that a parent class loader has no visibility or access to child class loaders. This
means that any referencing code must be found either from the parent class loader chain (bottom-up)
or at the current Thread context class loader level; otherwise java.lang.NoClassDefFoundError is
thrown by the JVM.
This is exactly what the following Java program will demonstrate.
Sample Java program
The following simple Java program is split as per below:

•
•
•
•

The main Java program NoClassDefFoundErrorSimulator is packaged in
MainProgram.jar
A logging utility class JavaEETrainingUtil is packaged in MainProgram.jar
The Java class caller CallerClassA is packaged in caller.jar
The referencing Java class ReferencingClassA is packaged in referencer.jar

These following tasks are performed:
•
•
•
•
•

Create a child class loader (java.net.URLClassLoader)
Assign the caller and referencing Java class jar files to the child class loader
Change the current Thread context ClassLoader to the child ClassLoader
Attempt to load and create a new instance of CallerClassA from the current Thread context
class loader e.g. child
Proper logging was added in order to help you understand the class loader tree and Thread
context class loader state

It will demonstrate that a wrong packaging of the application code is leading to a
NoClassDefFoundError as per the default class loader delegation model.

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#### NoClassDefFoundErrorSimulator.java
package org.ph.javaee.training3;
import java.net.URL;
import java.net.URLClassLoader;
import org.ph.javaee.training.util.JavaEETrainingUtil;
public class NoClassDefFoundErrorSimulator {
/**
* @param args
*/
public static void main(String[] args) {
System.out.println("java.lang.NoClassDefFoundError Simulator - Training 3");
System.out.println("Author: Pierre-Hugues Charbonneau");
System.out.println("http://javaeesupportpatterns.blogspot.com");
// Local variables
String currentThreadName = Thread.currentThread().getName();
String callerFullClassName = "org.ph.javaee.training3.CallerClassA";
// Print current ClassLoader context & Thread
System.out.println("\nCurrent Thread name: '"+currentThreadName+"'");
System.out.println("Initial ClassLoader chain:
"+JavaEETrainingUtil.getCurrentClassloaderDetail());
try {

// Location of the application code for our child ClassLoader
URL[] webAppLibURL = new URL[] {new URL("file:caller.jar"),new
URL("file:referencer.jar")};
// Child ClassLoader instance creation
URLClassLoader childClassLoader = new URLClassLoader(webAppLibURL);
/*** Application code execution... ***/
// 1. Change the current Thread ClassLoader to the child ClassLoader
Thread.currentThread().setContextClassLoader(childClassLoader);
System.out.println(">> Thread '"+currentThreadName+"' Context ClassLoader now
changed to '"+childClassLoader+"'");
System.out.println("\nNew ClassLoader chain:
"+JavaEETrainingUtil.getCurrentClassloaderDetail());
// 2. Load the caller Class within the child ClassLoader...
System.out.println(">> Loading '"+callerFullClassName+"' to child ClassLoader
'"+childClassLoader+"'...");
Class callerClass = childClassLoader.loadClass(callerFullClassName);
// 3. Create a new instance of CallerClassA
Object callerClassInstance = callerClass.newInstance();
} catch (Throwable any) {
System.out.println("Throwable: "+any);
any.printStackTrace();
}
}

System.out.println("\nSimulator completed!");

}

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#### CallerClassA.java
package org.ph.javaee.training3;
import org.ph.javaee.training3.ReferencingClassA;
/**
* CallerClassA
* @author Pierre-Hugues Charbonneau
*
*/
public class CallerClassA {
private final static Class CLAZZ = CallerClassA.class;
static {
System.out.println("Class loading of "+CLAZZ+" from ClassLoader
'"+CLAZZ.getClassLoader()+"' in progress...");
}
public CallerClassA() {
System.out.println("Creating a new instance of
"+CallerClassA.class.getName()+"...");
doSomething();
}
private void doSomething() {
// Create a new instance of ReferencingClassA
ReferencingClassA referencingClass = new ReferencingClassA();
}
}

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#### ReferencingClassA.java
package org.ph.javaee.training3;
/**
* ReferencingClassA
* @author Pierre-Hugues Charbonneau
*
*/
public class ReferencingClassA {
private final static Class CLAZZ = ReferencingClassA.class;
static {
System.out.println("Class loading of "+CLAZZ+" from ClassLoader
'"+CLAZZ.getClassLoader()+"' in progress...");
}
public ReferencingClassA() {
System.out.println("Creating a new instance of
"+ReferencingClassA.class.getName()+"...");
}
public void doSomething() {
//nothing to do...
}
}

Problem reproduction
In order to replicate the problem, we will simply “voluntary” split the packaging of the application code
(caller & referencing class) between the parent and child class loader.
For now, let’s run the program with the right JAR files deployment and class loader chain:

•
•

The main program and utility class are deployed at the parent class loader (SYSTEM
classpath)
CallerClassA and ReferencingClassA and both deployed at the child class loader level

## Baseline (normal execution)

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\bin>java -classpath MainProgram.jar
org.ph.javaee.training3.NoClassDefFoundErrorSimulator
java.lang.NoClassDefFoundError Simulator - Training 3
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com
Current Thread name: 'main'
Initial ClassLoader chain:
----------------------------------------------------------------sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation --sun.misc.Launcher$AppClassLoader@214c4ac9 **Current Thread 'main' Context
ClassLoader**
---------------------------------------------------------------->> Thread 'main' Context ClassLoader now changed to
'java.net.URLClassLoader@6a4d37e5'
New ClassLoader chain:
----------------------------------------------------------------sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation --sun.misc.Launcher$AppClassLoader@214c4ac9
--- delegation --java.net.URLClassLoader@6a4d37e5 **Current Thread 'main' Context ClassLoader**
---------------------------------------------------------------->> Loading 'org.ph.javaee.training3.CallerClassA' to child ClassLoader
'java.net.URLClassLoader@6a4d37e5'...
Class loading of class org.ph.javaee.training3.CallerClassA from ClassLoader
'java.net.URLClassLoader@6a4d37e5' in progress...
Creating a new instance of org.ph.javaee.training3.CallerClassA...
Class loading of class org.ph.javaee.training3.ReferencingClassA from
ClassLoader 'java.net.URLClassLoader@6a4d37e5' in progress...
Creating a new instance of org.ph.javaee.training3.ReferencingClassA...
Simulator completed!

For the initial run (baseline), the main program was able to create successfully a new instance of
CallerClassA from the child class loader (java.net.URLClassLoader) along with its referencing
class with no problem.
Now let’s run the program with the wrong application packaging and class loader chain:

•
•
•

The main program and utility class are deployed at the parent class loader (SYSTEM
classpath)
CallerClassA and ReferencingClassA and both deployed at the child class loader level
CallerClassA (caller.jar) is also deployed at the parent class loader level

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## Problem reproduction run (static variable initializer failure)
\bin>java -classpath MainProgram.jar;caller.jar
org.ph.javaee.training3.NoClassDefFoundErrorSimulator
java.lang.NoClassDefFoundError Simulator - Training 3
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com
Current Thread name: 'main'
Initial ClassLoader chain:
----------------------------------------------------------------sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation --sun.misc.Launcher$AppClassLoader@214c4ac9 **Current Thread 'main' Context ClassLoader**
---------------------------------------------------------------->> Thread 'main' Context ClassLoader now changed to 'java.net.URLClassLoader@6a4d37e5'
New ClassLoader chain:
----------------------------------------------------------------sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation --sun.misc.Launcher$AppClassLoader@214c4ac9
--- delegation --java.net.URLClassLoader@6a4d37e5 **Current Thread 'main' Context ClassLoader**
---------------------------------------------------------------->> Loading 'org.ph.javaee.training3.CallerClassA' to child ClassLoader
'java.net.URLClassLoader@6a4d37e5'...
Class loading of class org.ph.javaee.training3.CallerClassA from ClassLoader
'sun.misc.Launcher$AppClassLoader@214c4ac9' in progress...// Caller is loaded from the
parent class loader, why???
Creating a new instance of org.ph.javaee.training3.CallerClassA...
Throwable: java.lang.NoClassDefFoundError: org/ph/javaee/training3/ReferencingClassA
java.lang.NoClassDefFoundError: org/ph/javaee/training3/ReferencingClassA
at org.ph.javaee.training3.CallerClassA.doSomething(CallerClassA.java:27)
at org.ph.javaee.training3.CallerClassA.(CallerClassA.java:21)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
at java.lang.reflect.Constructor.newInstance(Unknown Source)
at java.lang.Class.newInstance0(Unknown Source)
at java.lang.Class.newInstance(Unknown Source)
at
org.ph.javaee.training3.NoClassDefFoundErrorSimulator.main(NoClassDefFoundErrorSimulator
.java:51)
Caused by: java.lang.ClassNotFoundException: org.ph.javaee.training3.ReferencingClassA
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 9 more
Simulator completed!

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What happened?

•
•
•
•

The main program and utility classes were loaded as expected from the parent class loader
(sun.misc.Launcher$AppClassLoader)
The Thread context class loader was changed to child class loader as expected which includes
both caller and referencing jar files
However, we can see that CallerClassA was actually loaded by the parent class loader
(sun.misc.Launcher$AppClassLoader) instead of the child class loader
Since ReferencingClassA was not deployed to the parent class loader, the class cannot be
found from the current class loader chain since the parent class loader has no visibility on the
child class loader, NoClassDefFoundError is thrown

The key point to understand at this point is why CallerClassA was loaded by the parent class
loader. The answer is with the default class loader delegation model. Both child and parent class
loaders contain the caller JAR files. However, the default delegation model is always parent first which
is why it was loaded at that level. The problem is that the caller contains a class reference to
ReferencingClassA which is only deployed to the child class loader; java.lang.NoClassDefFoundError
condition is met.
As you can see, a packaging problem of your code or third part API can easily lead to this problem
due to the default class loader delegation behaviour. It is very important that you review your class
loader chain and determine if you are at risk of duplicate code or libraries across your parent and child
class loaders.
Recommendations and resolution strategies
Now find below my recommendations and resolution strategies for this problem pattern:

•
•

•
•

Review the java.lang.NoClassDefFoundError error and identify the Java class that the JVM is
complaining about
Review the packaging of the affected application(s), including your Java EE container and third
part API’s used. The goal is to identify duplicate or wrong deployments of the affected Java
class at runtime (SYSTEM class path, EAR file, Java EE container itself etc.).
Once identified, you will need to remove and / or move the affected library/libraries from the
affected class loader (complexity of resolution will depend of the root cause).
Enable JVM class verbose e.g. –verbose:class. This JVM debug flag is very useful to monitor
the loading of the Java classes and libraries from the Java EE container your applications. It
can really help you pinpoint duplicate Java class loading across various applications and class
loaders at runtime

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ABOUT THE AUTHOR
Pierre-Hugues Charbonneau (nickname P-H) is working for CGI Inc. Canada for the last
10 years as a senior IT consultant. His primary area of expertise is Java EE,
middleware & JVM technologies. He is a specialist in production system
troubleshooting, root cause analysis, middleware, JVM tuning, scalability and capacity
improvement; including internal processes improvement for IT support teams. P-H is the
principal author at Java EE Support Patterns.

ABOUT THE EDITOR
Ilias is a senior software engineer working in the telecom domain. He is an applications
developer in a wide variety of applications/services, currently the technical lead in a inhouse PCRF solution. Particularly interested in multi-tier architecture, middleware
services and mobile development (contact). Ilias Tsagklis is co-founder and Executive
Editor at Java Code Geeks.

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