# Getting started with hibernate

## Remarks

The SessionFactory bean is responsible for creating, maintaining, closing and flushing all the database sessions that the TransactionManager asks it to create. That's why we autowire the SessionFactory into DAO's and make run all queries through it.

One of the biggest questions that new Hibernate users ask is "When do my changes get committed?" and the answer makes sense when you think how the TransactionManager works with the SesisonFactory. Your database changes will be flushed and committed when you exit the service method that was annotated with @Transactional. The reason for this is, that a transaction is supposed to represent a single 'unit' of unbroken work. If something goes wrong with the unit, then it is assumed that the unit failed and all changes should be rolled back. So the SessionFactory will flush and clear the session when you exit the service method that you called originally.

That's not to say that it won't also flush and clear the session while your transaction is going on. For example, if I call a service method to add a collection of 5 objects and return the total count of objects in the database, the SessionFactory would realise that the query (SELECT COUNT(*)) requires an updated state to be accurate, and so would flush the addition of the 5 objects before running the count query. The execution could look something like this:

## Versions

VersionDocumentation LinkRelease Date
4.2.0http://hibernate.org/orm/documentation/4.2/2013-03-01
4.3.0http://hibernate.org/orm/documentation/4.3/2013-12-01
5.0.0http://hibernate.org/orm/documentation/5.0/2015-09-01

## Simple hibernate example using XML

To set up a simple hibernate project using XML for the configurations you need 3 files, hibernate.cfg.xml, a POJO for each entity, and a EntityName.hbm.xml for each entity. Here is an example of each using MySQL:

hibernate.cfg.xml

DBSchemaName, testUserName, and testPassword would all be replaced. Make sure to use the full resource name if it is in a package.

Employee.java

Employee.hbm.xml

Again, if the class is in a package use the full class name packageName.className.

After you have these three files you are ready to use hibernate in your project.

## Using XML Configuration to set up Hibernate

I create a file called database-servlet.xml somewhere on the classpath.

Initially your config file will look like this:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:jdbc="http://www.springframework.org/schema/jdbc"
xmlns:tx="http://www.springframework.org/schema/tx"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.2.xsd
http://www.springframework.org/schema/jdbc http://www.springframework.org/schema/jdbc/spring-jdbc-3.2.xsd
http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-3.2.xsd">

</beans>


You'll notice I imported the tx and jdbc Spring namespaces. This is because we are going to use them quite heavily in this config file.

First thing you want to do is enable annotation based transaction management (@Transactional). The main reason that people use Hibernate in Spring is because Spring will manage all your transactions for you. Add the following line to your configuration file:

<tx:annotation-driven />


We need to create a data source. The data source is basically the database that Hibernate is going to use to persist your objects. Generally one transaction manager will have one data source. If you want Hibernate to talk to multiple data sources then you have multiple transaction managers.

<bean id="dataSource"
class="org.springframework.jdbc.datasource.DriverManagerDataSource">
<property name="driverClassName" value="" />
<property name="url" value="" />
<property name="username" value="" />
<property name="password" value="" />
</bean>


The class of this bean can be anything that implements javax.sql.DataSource so you could write your own. This example class is provided by Spring, but doesn't have its own thread pool. A popular alternative is the Apache Commons org.apache.commons.dbcp.BasicDataSource, but there are many others. I'll explain each of the properties below:

• driverClassName: The path to your JDBC driver. This is a database specific JAR that should be available on your classpath. Ensure that you have the most up to date version. If you are using an Oracle database, you'll need a OracleDriver. If you have a MySQL database, you'll need a MySQLDriver. See if you can find the driver you need here but a quick google should give you the correct driver.

• url: The URL to your database. Usually this will be something like jdbc\:oracle\:thin\:\path\to\your\database or jdbc:mysql://path/to/your/database. If you google around for the default location of the database you are using, you should be able to find out what this should be. If you are getting a HibernateException with the message org.hibernate.HibernateException: Connection cannot be null when 'hibernate.dialect' not set and you are following this guide, there is a 90% chance that your URL is wrong, a 5% chance that your database isn't started and a 5% chance that your username/password is wrong.

• username: The username to use when authenticating with the database.

• password: The password to use when authenticating with the database.

The next thing, is to set up the SessionFactory. This is the thing that Hibernate uses to create and manage your transactions, and actually talks to the database. It has quite a few configuration options that I will try to explain below.

<bean id="sessionFactory"
class="org.springframework.orm.hibernate4.LocalSessionFactoryBean">
<property name="dataSource" ref="dataSource" />
<property name="packagesToScan" value="au.com.project />
<property name="hibernateProperties">
<props>
<prop key="hibernate.use_sql_comments">true</prop>
<prop key="hibernate.hbm2ddl.auto">validate</prop>
</props>
</property>
</bean>

• dataSource: Your data source bean. If you changed the Id of the dataSource, set it here.

• packagesToScan: The packages to scan to find your JPA annotated objects. These are the objects that the session factory needs to manage, will generally be POJO's and annotated with @Entity. For more information on how to set up object relationships in Hibernate see here.

• annotatedClasses (not shown): You can also provide a list of classes for Hibernate to scan if they are not all in the same package. You should use either packagesToScan or annotatedClasses but not both. The declaration looks like this:

<property name="annotatedClasses">
<list>
<value>foo.bar.package.model.Person</value>
<value>foo.bar.package.model.Thing</value>
</list>
</property>

• hibernateProperties: There are a myriad of these all lovingly documented here. The main ones you will be using are as follows:
• hibernate.hbm2ddl.auto: One of the hottest Hibernate questions details this property. See it for more info. I generally use validate, and set up my database using either SQL scripts (for an in-memory), or create the database beforehand (existing database).
• hibernate.show_sql: Boolean flag, if true Hibernate will print all the SQL it generates to stdout. You can also configure your logger to show you the values that are being bound to the queries by setting log4j.logger.org.hibernate.type=TRACE log4j.logger.org.hibernate.SQL=DEBUG in your log manager (I use log4j).
• hibernate.format_sql: Boolean flag, will cause Hibernate to pretty print your SQL to stdout.
• hibernate.dialect (Not shown, for good reason): A lot of old tutorials out there show you how to set the Hibernate dialect that it will use to communicate to your database. Hibernate can auto-detect which dialect to use based on the JDBC driver that you are using. Since there are about 3 different Oracle dialects and 5 different MySQL dialects, I'd leave this decision up to Hibernate. For a full list of dialects Hibernate supports see here.

The last 2 beans you need to declare are:

<bean class="org.springframework.dao.annotation.PersistenceExceptionTranslationPostProcessor"
id="PersistenceExceptionTranslator" />

<bean id="transactionManager"
class="org.springframework.orm.hibernate4.HibernateTransactionManager">
<property name="sessionFactory" ref="sessionFactory" />
</bean>


The PersistenceExceptionTranslator translates database specific HibernateException or SQLExceptions into Spring exceptions that can be understood by the application context.

The TransactionManager bean is what controls the transactions as well as roll-backs.

Note: You should be autowiring your SessionFactory bean into your DAO's.

## XML-less Hibernate configuration

This example has been taken from here

Please note, that with latest Hibernate this approach doesn't work well (Hibernate 5.2 release still allow this configuration)