How to connect to multiple databases in Hibernate

Ideally you should move to Distributed transaction type of system[using Java Transaction Analyzer org.hibernate.transaction.JTATransactionFactory] in this case. If you are running in JBoss App Server, you can do it by using "Distributed Transaction Managers". You can learn more about it here.

Using annotation mappings as an example:

Configuration cfg1 = new AnnotationConfiguration();
cfg1.configure("/hibernate-oracle.cfg.xml");
cfg1.addAnnotatedClass(SomeClass.class); // mapped classes
cfg1.addAnnotatedClass(SomeOtherClass.class);
SessionFactory sf1 = cfg1.buildSessionFactory();

Configuration cfg2 = new AnnotationConfiguration();
cfg2.configure("/hibernate-mysql.cfg.xml");
cfg2.addAnnotatedClass(SomeClass.class); // could be the same or different than above
cfg2.addAnnotatedClass(SomeOtherClass.class);
SessionFactory sf2 = cfg2.buildSessionFactory();

Then use sf1 and sf2 to get the sessions for each database. For mapping files, you just use cfg.addClass instead of addAnnotatedClass. Put the cfg.xml files in the root package in this case. Those will have the Oracle or MySQL dialect and connection information.

You can also use a catalog with the value of the other database

@Table(name = "foo", schema = "bar", catalog = "OtherDatabase")

It cannot be done using one hibernate configuration file. You need to have two configurations files for it.

To configure mysql database

hibernate-mysql.cfg.xml

To configure oracle database

hibernate-oracle.cfg.xml

In Details, mysql configuration file be like this.

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <property name="hibernate.bytecode.use_reflection_optimizer">false</property>
        <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="hibernate.connection.password">PASSWORD</property>
        <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/UR_DB_NAME</property>
        <property name="hibernate.connection.username">USERNAME</property>
        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
        <property name="show_sql">true</property>
        <mapping class="domain.EmployeeMysql"></mapping>
    </session-factory>
</hibernate-configuration>

In Details, oracle configuration file be like this.

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <property name="hibernate.bytecode.use_reflection_optimizer">false</property>
        <property name="hibernate.connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
        <property name="hibernate.connection.password">PASSWORD</property>
        <property name="hibernate.connection.url">jdbc:oracle:thin:UR DB NAME</property>
        <property name="hibernate.connection.username">USERNAME</property>
        <property name="hibernate.dialect">org.hibernate.dialect.Oracle10gDialect</property>
        <property name="show_sql">true</property>
        <mapping class="domain.EmployeeOracleSql"></mapping>
    </session-factory>
</hibernate-configuration>

And code should be like this.

mysql configuration

private static SessionFactory sessionAnnotationFactory; 

sessionAnnotationFactory = new Configuration().configure("hibernate-mysql.cfg.xml").buildSessionFactory();

Session session = sessionAnnotationFactory.openSession();

oracle sql configuration

sessionAnnotationFactory = new Configuration().configure("hibernate-oracle.cfg.xml").buildSessionFactory();

Session session = sessionAnnotationFactory.openSession()