Hibernate Quick Guide

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HIBERNATE QUICK GUIDE

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HIBERNATE QUICK GUIDE

Hibernate is an Object-Relational Mapping (ORM) solution for JAVA and it raised as an open source persistent

framework created by Gavin King in 2001. It is a powerful, hig h performance Object-Relational Persistence and

Query service for any Java Application.

Hibernate maps Java classes to database tables and from Java data types to SQL data types and relieve the

developer from 95% of common data persistence related prog ramming tasks.

Hibernate sits between traditional Java objects and database server to handle all the work in persisting those

objects based on the appropriate O/R mechanisms and patterns.

Hibernate Advantag es:

Hibernate takes care of mapping Java classes to database tables using XML files and without writing any

line of code.

Provides simple APIs for storing and retrieving Java objects directly to and from the database.

If there is chang e in Database or in any table then the only need to chang e XML file properties.

Abstract away the unfamiliar SQL types and provide us to work around familiar Java Objects.

Hibernate does not require an application server to operate.

Manipulates Complex associations of objects of your database.

Minimize database access with smart fetching strateg ies.

Provides Simple querying of data.

Supported Databases:

Hibernate supports almost all the major RDBMS. Following is list of few of the database eng ines supported by

Hibernate.

HSQL Database Eng ine

DB2/NT

MySQL

Postg reSQL

FrontBase

Oracle

Microsoft SQL Server Database

Sybase SQL Server

Informix Dynamic Server

Hibernate Architecture:

The Hibernate architecture is layered to keep you isolated from having to know the underlying APIs. Hibernate

makes use of the database and config uration data to provide persistence services (and persistent objects) to the

application.

Following is a very hig h level view of the Hibernate Application Architecture.

Following is a detailed view of the Hibernate Application Architecture with few important core classes.

Hibernate uses various existing Java APIs, like JDBC, Java Transaction API(JTA), and Java Naming and

Directory Interface (JNDI). JDBC provides a rudimentary level of abstraction of functionality common to

relational databases, allowing almost any database with a JDBC driver to be supported by Hibernate. JNDI and

JTA allow Hibernate to be integ rated with J2EE application servers.

Following section g ives brief description of each of the class objects involved in Hibernate Application

Architecture.

Config uration Object:

The Config uration object is the first Hibernate object you create in any Hibernate application and usually created

only once during application initialization. It represents a config uration or properties file required by the

Hibernate. The Config uration object provides two keys components:

Database Connec tion: T his is handled throug h one or more config uration files supported by

Hibernate. These files are hibernate.properties and hibernate.c fg .xml.

Class Mapping Setup

This component creates the connection between the Java classes and database tables..

SessionFactory Object:

Config uration object is used to create a SessionFactory object which inturn config ures Hibernate for the

application using the supplied config uration file and allows for a Session object to be instantiated. The

SessionFactory is a thread safe object and used by all the threads of an application.

The SessionFactory is heavyweig ht object so usually it is created during application start up and kept for later

use. You would need one SessionFactory object per database using a separate config uration file. So if you are

using multiple databases then you would have to create multiple SessionFactory objects.

Session Object:

A Session is used to get a physical connection with a database. The Session object is lig htweig ht and desig ned to

be instantiated each time an interaction is needed with the database. Persistent objects are saved and retrieved

throug h a Session object.

The session objects should not be kept open for a long time because they are not usually thread safe and they

should be created and destroyed them as needed.

Transaction Object:

A T ransaction represents a unit of work with the database and most of the RDBMS supports transaction

functionality. Transactions in Hibernate are handled by an underlying transaction manag er and transaction (from

JDBC or JTA).

This is an optional object and Hibernate applications may choose not to use this interface, instead manag ing

transactions in their own application code.

Query Object:

Query objects use SQL or Hibernate Query Lang uag e (HQL) string to retrieve data from the database and

create objects. A Query instance is used to bind query parameters, limit the number of results returned by the

query, and finally to execute the query.

Criteria Object:

Criteria object are used to create and execute object oriented criteria queries to retrieve objects.

Hibernate Environment Setup

This chapter will explain how to install Hibernate and other associated packag es to prepare a develop

environment for the Hibernate applications. We will work with MySQL database to experiment with Hibernate

examples, so make sure you already have setup for MySQL database. For a more detail on MySQL you can

check our MySQL Tutorial.

Downloading Hibernate:

It is assumed that you already have latest version of Java is installed on your machine. Following are the simple

steps to download and install Hibernate on your machine.

Make a choice whether you want to install Hibernate on Windows, or Unix and then proceed to the next

step to download .zip file for windows and .tz file for Unix.

Download the latest version of Hibernate from http://www.hibernate.org /downloads.

At the time of writing this tutorial I downloaded hibernate-distribution-3.6.4.Final and when you

unzip the downloaded file it will give you directory structure as follows.

Installing Hibernate:

Once you downloaded and unzipped the latest version of the Hibernate Installation file, you need to perform

following two simple steps. Make sure you are setting your CLASSPATH variable properly otherwise you will

face problem while compiling your application.

Now copy all the library files from /lib into your CLASSPATH, and chang e your classpath variable to

include all the JARs:

Finally copy hibernate3.jar file into your CLASSPATH. This file lies in the root directory of the

installation and is the primary JAR that Hibernate needs to do its work.

Hibernate Prerequisites:

Following is the list of the packages/libraries required by Hibernate and you should install them before starting

with Hibernate. To install these packages you would have to copy library files from /lib into your CLASSPATH,

and chang e your CLASSPATH variable according ly.

S.N. Packag es/Libraries

1dom4j - XML parsing www.dom4j.org /

2Xalan - XSLT Processor http://xml.apache.org /xalan-j/

3Xerc es - The Xerces Java Parser http://xml.apache.org /xerces-j/

4cg lib - Appropriate chang es to Java classes at runtime http://cg lib.sourceforg e.net/

5log 4j - Log g ing Faremwork http://log g ing .apache.org /log 4j

6Commons - Log g ing , Email etc. http://jakarta.apache.org /commons

7SLF4J - Log g ing Facade for Java http://www.slf4j.org

Hibernate Config uration

Hibernate requires to know in advance where to find the mapping information that defines how your Java classes

relate to the database tables. Hibernate also requires a set of config uration setting s related to database and

other related parameters. All such information is usually supplied as a standard Java properties file called

hibernate.properties, or as an XML file named hibernate.c fg .xml.

I will consider XML formatted file hibernate.c fg .xml to specify required Hibernate properties in my

examples. Most of the properties take their default values and it is not required to specify them in the property

file unless it is really required. This file is kept in the root directory of your application's classpath.

Hibernate Properties:

Following is the list of important properties you would require to config ure for a databases in a standalone

situation:

S.N. Properties and Desc ription

1hibernate.dialec t

This property makes Hibernate g enerate the appropriate SQL for the chosen database.

2hibernate.c onnection.driver_c lass

The JDBC driver class.

3hibernate.c onnection.url

The JDBC URL to the database instance.

4hibernate.c onnection.username

The database username.

5hibernate.c onnection.password

The database password.

6hibernate.c onnection.pool_size

Limits the number of connections waiting in the Hibernate database connection pool.

7hibernate.c onnection.autocommit

Allows autocommit mode to be used for the JDBC connection.

If you are using a database along with an application server and JNDI then you would have to config ure the

following properties:

S.N. Properties and Desc ription

1hibernate.c onnection.datasourc e

The JNDI name defined in the application server context youre using for the application.

2hibernate.jndi.class

The InitialContext class for JNDI.

3hibernate.jndi.<J NDIpropertyname>

Passes any JNDI property you like to the JNDI InitialContext.

4hibernate.jndi.url

Provides the URL for JNDI.

5hibernate.c onnection.username

The database username.

6hibernate.c onnection.password

The database password.

Hibernate with MySQL Database:

MySQL is one of the most popular open-source database systems available today. Let us create

hibernate.c fg .xml config uration file and place it in the root of your application's classpath. You would have to

make sure that you have testdb database available in your MySQL database and you have a user test available

to access the database.

The XML config uration file must conform to the Hibernate 3 Config uration DTD, which is available from

http://www.hibernate.org /dtd/hibernate-config uration-3.0.dtd.

<?xml version="1.0" encoding="utf-8"?>

<!DOCTYPE hibernate-configuration SYSTEM

"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

<session-factory>

org.hibernate.dialect.MySQLDialect

</property>

com.mysql.jdbc.Driver

</property>

jdbc:mysql://localhost/test

</property>

root

</property>

root123

</property>

</session-factory>

</hibernate-configuration>

The above config uration file includes <mapping > tag s which are related to hibernate-mapping file and we will

see in next chapter what exactly is a hibernate mapping file and how and why do we use it. Following is the list of

various important databases dialect property type:

Database Dialec t Property

DB2 org .hibernate.dialect.DB2Dialect

HSQLDB org .hibernate.dialect.HSQLDialect

HypersonicSQL org .hibernate.dialect.HSQLDialect

Informix org .hibernate.dialect.InformixDialect

Ing res org .hibernate.dialect.Ing resDialect

Interbase org .hibernate.dialect.InterbaseDialect

Microsoft SQL Server 2000 org .hibernate.dialect.SQLServerDialect

Microsoft SQL Server 2005 org .hibernate.dialect.SQLServer2005Dialect

Microsoft SQL Server 2008 org .hibernate.dialect.SQLServer2008Dialect

MySQL org .hibernate.dialect.MySQLDialect

Oracle (any version) org .hibernate.dialect.OracleDialect

Oracle 11g org .hibernate.dialect.Oracle10g Dialect

Oracle 10g org .hibernate.dialect.Oracle10g Dialect

Oracle 9i org .hibernate.dialect.Oracle9iDialect

Postg reSQL org .hibernate.dialect.Postg reSQLDialect

Prog ress org .hibernate.dialect.Prog ressDialect

SAP DB org .hibernate.dialect.SAPDBDialect

Sybase org .hibernate.dialect.SybaseDialect

Sybase Anywhere org .hibernate.dialect.SybaseAnywhereDialect

Hibernate Examples

Let us try an example of using Hibernate to provide Java persistence in a standalone application. We will g o

throug h different steps involved in creating Java Application using Hibernate technolog y.

Create POJO Classes:

The first step in creating an application is to build the Java POJO class or

classes, depending on the application that will be persisted to the

database. Let us consider our Employee class with getXXX and setXXX

methods to make it JavaBeans compliant class.

A POJO (Plain Old Java Object) is a Java object that doesn't extend or

implement some specialized classes and interfaces respectively required

by the EJB framework. All normal Java objects are POJO.

When you desig n a classs to be persisted by Hibernate, it's important to

provide JavaBeans compliant code as well as one attribute which would

work as index like id attribute in the Employee class.

public class Employee {

private int id;

private String firstName;

private String lastName;

private int salary;

public Employee() {}

public Employee(String fname, String lname, int salary) {

this.firstName = fname;

this.lastName = lname;

this.salary = salary;

}

public int getId() {

return id;

}

public void setId( int id ) {

this.id = id;

}

public String getFirstName() {

return firstName;

}

public void setFirstName( String first_name ) {

this.firstName = first_name;

}

public String getLastName() {

return lastName;

}

public void setLastName( String last_name ) {

this.lastName = last_name;

}

public int getSalary() {

return salary;

}

public void setSalary( int salary ) {

this.salary = salary;

}

Create Database Tables:

Second step would be creating tables in your database. There would be one table corresponding to each object

you are willing to provide persistence. Consider above objects need to be stored and retrieved into the

following RDBMS table:

create table EMPLOYEE (

id INT NOT NULL auto_increment,

first_name VARCHAR(20) default NULL,

last_name VARCHAR(20) default NULL,

salary INT default NULL,

PRIMARY KEY (id)

);

Create Mapping Config uration File:

This step is to create a mapping file that instructs Hibernate how to map the defined class or classes to the

database tables.

<?xml version="1.0" encoding="utf-8"?>

<!DOCTYPE hibernate-mapping PUBLIC

"-//Hibernate/Hibernate Mapping DTD//EN"

"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping>

This class contains the employee detail.

</meta>

</id>

</class>

</hibernate-mapping>

You should save the mapping document in a file with the format <classname>.hbm.xml. We saved our mapping

document in the file Employee.hbm.xml. Let us see little detail about the mapping document:

The mapping document is an XML document having <hibernate-mapping > as the root element which

contains all the <class> elements.

The <class> elements are used to define specific mapping s from a Java classes to the database tables.

The Java class name is specified using the name attribute of the class element and the database table

name is specified using the table attribute.

The <meta> element is optional element and can be used to create the class description.

The <id> element maps the unique ID attribute in class to the primary key of the database table. The

name attribute of the id element refers to the property in the class and the column attribute refers to the

column in the database table. The type attribute holds the hibernate mapping type, this mapping types will

convert from Java to SQL data type.

The <g enerator> element within the id element is used to automatically g enerate the primary key

values. Set the class attribute of the g enerator element is set to native to let hibernate pick up either

identity, sequence or hilo alg orithm to create primary key depending upon the capabilities of the

underlying database.

The <property> element is used to map a Java class property to a column in the database table. The

name attribute of the element refers to the property in the class and the column attribute refers to the

column in the database table. The type attribute holds the hibernate mapping type, this mapping types will

convert from Java to SQL data type.

There are other attributes and elements available which will be used in a mapping document and I would try to

cover as many as possible while discussing other Hibernate related topics.

Create Application Class:

Finally, we will create our application class with the main() method to run the application. We will use this

application to save few Employee's records and then we will apply CRUD operations on those records.

import java.util.List;

import java.util.Date;

import java.util.Iterator;

import org.hibernate.HibernateException;

import org.hibernate.Session;

import org.hibernate.Transaction;

import org.hibernate.SessionFactory;

import org.hibernate.cfg.Configuration;

public class ManageEmployee {

private static SessionFactory factory;

public static void main(String[] args) {

try{

factory = new Configuration().configure().buildSessionFactory();

}catch (Throwable ex) {

System.err.println("Failed to create sessionFactory object." + ex);

throw new ExceptionInInitializerError(ex);

}

ManageEmployee ME = new ManageEmployee();

/* Add few employee records in database */

Integer empID1 = ME.addEmployee("Zara", "Ali", 1000);

Integer empID2 = ME.addEmployee("Daisy", "Das", 5000);

Integer empID3 = ME.addEmployee("John", "Paul", 10000);

/* List down all the employees */

ME.listEmployees();

/* Update employee's records */

ME.updateEmployee(empID1, 5000);

/* Delete an employee from the database */

ME.deleteEmployee(empID2);

/* List down new list of the employees */

ME.listEmployees();

}

/* Method to CREATE an employee in the database */

public Integer addEmployee(String fname, String lname, int salary){

Session session = factory.openSession();

Transaction tx = null;

Integer employeeID = null;

try{

tx = session.beginTransaction();

Employee employee = new Employee(fname, lname, salary);

employeeID = (Integer) session.save(employee);

tx.commit();

}catch (HibernateException e) {

if (tx!=null) tx.rollback();

e.printStackTrace();

}finally {

session.close();

}

return employeeID;

}

/* Method to READ all the employees */

public void listEmployees( ){

Session session = factory.openSession();

Transaction tx = null;

try{

tx = session.beginTransaction();

List employees = session.createQuery("FROM Employee").list();

for (Iterator iterator =

employees.iterator(); iterator.hasNext();){

Employee employee = (Employee) iterator.next();

System.out.print("First Name: " + employee.getFirstName());

System.out.print(" Last Name: " + employee.getLastName());

System.out.println(" Salary: " + employee.getSalary());

}

tx.commit();

}catch (HibernateException e) {

if (tx!=null) tx.rollback();

e.printStackTrace();

}finally {

session.close();

}

/* Method to UPDATE salary for an employee */

public void updateEmployee(Integer EmployeeID, int salary ){

Session session = factory.openSession();

Transaction tx = null;

try{

tx = session.beginTransaction();

Employee employee =

(Employee)session.get(Employee.class, EmployeeID);

employee.setSalary( salary );

session.update(employee);

tx.commit();

}catch (HibernateException e) {

if (tx!=null) tx.rollback();

e.printStackTrace();

}finally {

session.close();

}

/* Method to DELETE an employee from the records */

public void deleteEmployee(Integer EmployeeID){

Session session = factory.openSession();

Transaction tx = null;

try{

tx = session.beginTransaction();

Employee employee =

(Employee)session.get(Employee.class, EmployeeID);

session.delete(employee);

tx.commit();

}catch (HibernateException e) {

if (tx!=null) tx.rollback();

e.printStackTrace();

}finally {

session.close();

}

Compilation and Execution:

Here are the steps to compile and run the above mentioned application. Make sure you have set PATH and

CLASSPATH appropriately before proceeding for the compilation and execution.

Create hibernate.cfg .xml config uration file as explained in config uration chapter.

Create Employee.hbm.xml mapping file as shown above.

Create Employee.java source file as shown above and compile it.

Create Manag eEmployee.java source file as shown above and compile it.

Execute Manag eEmployee binary to run the prog ram.

You would g et following result, and records would be created in EMPLOYEE table.

$java ManageEmployee

.......VARIOUS LOG MESSAGES WILL DISPLAY HERE........

First Name: Zara Last Name: Ali Salary: 1000

First Name: Daisy Last Name: Das Salary: 5000

First Name: John Last Name: Paul Salary: 10000

First Name: Zara Last Name: Ali Salary: 5000

First Name: John Last Name: Paul Salary: 10000

If you check your EMPLOYEE table, it should have following records:

mysql> select * from EMPLOYEE;

+----+------------+-----------+--------+

+----+------------+-----------+--------+

| 29 | Zara | Ali | 5000 |

| 31 | John | Paul | 10000 |

+----+------------+-----------+--------+

2 rows in set (0.00 sec

mysql>

Hibernate Quick Guide

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