Access multiple Databases in JPA

According to JPA specification we can define multiple "persistence-unit" elements (i.e. like below) in persistence.xml file and can easily refer them inside Dao layers as this.

public class PolarDaoImpl {

    @PersistenceContext(unitName="PolarPU")
    protected EntityManager entityManager;

--
}

public class BearDaoImpl {

    @PersistenceContext(unitName="BearPU")

    protected EntityManager entityManager;

--

}

Checkout sample persistence.xml



<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.0" xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">
    <!-- Database 1 -->
    <persistence-unit name="PolarPU" transaction-type="RESOURCE_LOCAL">
        <provider>org.hibernate.ejb.HibernatePersistence</provider>
        <non-jta-data-source/>
        <class>com.cisco.polar.entity.ManagedAcademy</class>
        <class>com.cisco.polar.entity.TeachAcademy</class>
        <exclude-unlisted-classes>true</exclude-unlisted-classes>
        <properties>
            <property name="hibernate.connection.driver_class" value="com.mysql.jdbc.Driver"/>
            <property name="hibernate.connection.url" value="jdbc:mysql://localhost:3306/polar"/>
            <property name="hibernate.connection.username" value="root"/>
            <property name="hibernate.connection.password" value=""/>
        </properties>
    </persistence-unit>
    <!-- Database 2 -->
    <persistence-unit name="BearPU" transaction-type="RESOURCE_LOCAL">
        <provider>org.hibernate.ejb.HibernatePersistence</provider>
        <non-jta-data-source/>
        <class>com.cisco.polar.entity.AcademyReportDate</class>
        <exclude-unlisted-classes>true</exclude-unlisted-classes>
        <properties>
            <property name="hibernate.connection.driver_class" value="com.mysql.jdbc.Driver"/>
            <property name="hibernate.connection.url" value="jdbc:mysql://localhost:3306/bear"/>
            <property name="hibernate.connection.username" value="root"/>
            <property name="hibernate.connection.password" value=""/>
        </properties>
    </persistence-unit>
</persistence>