Guide To Thread Local Random In Java Baeldung

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3/30/2018

Guide to ThreadLocalRandom in Java | Baeldung

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Guide to
ThreadLocalRandom in
Java
Last modi ed: February 20, 2018

by baeldung (http://www.baeldung.com/author/baeldung/)
Java (http://www.baeldung.com/category/java/)

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1. Overview
Generating random values is a very common task. This is why Java provides the
java.util.Random class.
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However, this class doesn’t perform well in a multi-threaded environment.
In a simpli ed way, the reason for poor performance of Random in a multi-threaded
environment is due to contention – given that multiple threads share the same
Random instance.
To address that limitation, Java introduced
the java.util.concurrent.ThreadLocalRandom class in JDK 7 – for generating
random numbers in a multi-threaded environment.
Let’s see how ThreadLocalRandom performs and how to use it in real-world
applications.

2. ThreadLocalRandom over Random
ThreadLocalRandom is a combination of ThreadLocal
(http://www.baeldung.com/java-threadlocal) and Random classes, which is
isolated to the current thread. Thus, it achieves better performance in a
multithreaded environment by simply avoiding any concurrent access to the Random
objects.
The random number obtained by one thread is not a ected by the other thread,
whereas java.util.Random provides random numbers globally.
Also, unlike Random, ThreadLocalRandom doesn’t support setting the seed explicitly.
Instead, it overrides the setSeed(long seed) method inherited from Random to always
throw an UnsupportedOperationException if called.
Let’s now take a look at some of the ways to generate random int, long and double
values.

3. Generating Random Values Using
ThreadLocalRandom
As per the Oracle documentation, we just need to call
ThreadLocalRandom.current() method, and it will return the instance of
ThreadLocalRandom for the current thread. We can then generate random values
by invoking available instance methods of the class.
Let’s generate a random int value without any bounds:
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int unboundedRandomValue = ThreadLocalRandom.current().nextInt());

Next, let’s see how we can generate a random bounded int value, meaning a value
between a given lower and upper limit.
Here’s an example of generating a random int value between 0 and 100:
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int boundedRandomValue = ThreadLocalRandom.current().nextInt(0, 100);

Please note, 0 is the inclusive lower limit and 100 is the exclusive upper limit.
We can generate random values for long and double by invoking nextLong() and
nextDouble() methods in a similar way as shown in the examples above.
Java 8 also adds the nextGaussian() method to generate the next normally-distributed
value with a 0.0 mean and 1.0 standard deviation from the generator’s sequence.
As with the Random class, we can also use the doubles(), ints() and longs() methods
to generate streams of random values.

4. Comparing ThreadLocalRandom and Random
Using JMH
Let’s see how we can generate random values in a multi-threaded environment, by
using the two classes, then compare their performance using JMH.
First, let’s create an example where all the threads are sharing a single instance of
Random. Here, we’re submitting the task of generating a random value using the
Random instance to an ExecutorService:
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ExecutorService executor = Executors.newWorkStealingPool();
List> callables = new ArrayList<>();
Random random = new Random();
for (int i = 0; i < 1000; i++) {
callables.add(() -> {
return random.nextInt();
});
}
executor.invokeAll(callables);

Let’s check the performance of the code above using JMH benchmarking:

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# Run complete. Total time: 00:00:36
Benchmark
Mode Cnt Score
Error
ThreadLocalRandomBenchMarker.randomValuesUsingRandom avgt 20 771.613 ± 222.220

Similarly, let’s now use ThreadLocalRandom instead of the Random instance, which
uses one instance of ThreadLocalRandom for each thread in the pool:
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ExecutorService executor = Executors.newWorkStealingPool();
List> callables = new ArrayList<>();
for (int i = 0; i < 1000; i++) {
callables.add(() -> {
return ThreadLocalRandom.current().nextInt();
});
}
executor.invokeAll(callables);

Here’s the result of using ThreadLocalRandom:
omplete. Total time: 00:00:36
rk
Mode Cnt Score
Error
Un
ocalRandomBenchMarker.randomValuesUsingThreadLocalRandom avgt 20 624.911 ± 113.268 u

Finally, by comparing the JMH results above for both Random and
ThreadLocalRandom, we can clearly see that the average time taken to generate
1000 random values using Random is 772 microseconds, whereas using
ThreadLocalRandom it’s around 625 microseconds.
Thus, we can conclude that ThreadLocalRandom is more e cient in a highly
concurrent environment.
To learn more about JMH, check out our previous article here
(http://www.baeldung.com/java-microbenchmark-harness).

5. Conclusion
This article illustrated the di erence between java.util.Random and
java.util.concurrent.ThreadLocalRandom.
We also saw the advantage of ThreadLocalRandom over Random in a multithreaded
environment, as well as performance and how we can generate random values using
the class.

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