Lookup multiple datasource in ejb3.0 - persistence

Question

I am looking up multiple datasource, depending an value of x In EJB 3.0.
To do this I have written following code.
Session Bean
package entity.library;
import java.util.Collection;
import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import java.io.Serializable;
import javax.ejb.*;
#Remote(TestInterface.class)
#Stateless(mappedName="ejb3/TestBeans")
public class TestSessionBean implements Serializable, TestInterface {
/**
*
*/
private static final long serialVersionUID = 1L;
#PersistenceContext(unitName="EntityBeanDS1")
EntityManager emds1;
#PersistenceContext(unitName="EntityBeanDS2")
EntityManager emds2;
protected TestJPA test;
protected Collection <TestJPA> list;
public Collection <TestJPA> getAllList(int x) {
System.out.println("TestInterface.java:getAllPmns x "+x);
if(x==1)
{
System.out.println("going to lookup datasource1");
list=emds1.createQuery("SELECT test FROM TestJPA test").getResultList();
}
else if(x==2)
{
System.out.println("going to lookup datasource2");
list=emds2.createQuery("SELECT test FROM TestJPA test").getResultList();
}
return list;
}
}
Persitence.xml
<persistence version="1.0" 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_1_0.xsd">
<persistence-unit name="EntityBeanDS1" transaction-type="JTA">
<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
<jta-data-source>TESTDS</jta-data-source>
<non-jta-data-source>TESTDS</non-jta-data-source>
<properties>
<property name="eclipselink.target-server" value="WebLogic_10"/>
<property name="eclipselink.logging.level" value="FINEST"/>
</properties>
</persistence-unit>
<persistence-unit name="EntityBeanDS2" transaction-type="JTA">
<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
<jta-data-source>TESTDS1</jta-data-source>
<non-jta-data-source>TESTDS1</non-jta-data-source>
<properties>
<property name="eclipselink.target-server" value="WebLogic_10"/>
<property name="eclipselink.logging.level" value="FINEST"/>
</properties>
</persistence-unit>
</persistence>
Above code is working successfully.but i think this is not a good tech for following reasons.
1. There are 10-15 multiple session beans, in each bean i have to write if-else for lookup of datasource.
2. In future if new data source is added or any newvalue of x is added, then i have to modify all 10-15 files.
Can one give me code , for connecting to multiple data source such that i change a single file for any change in value of x.
How that single file will look like so that i can retrieve 'EntityManager' object. or there is any other method (like modification in persistence.xml) to do this?

Answer 1

You could inject both entity managers into another session bean (QueryBean) and inject QueryBean into your session beans instead of the entity managers themselves. Then delegate the query creation to QueryBean. This bean then decides which entity manager to use depending on the value of x.
When adding another entity manager or a new value of x you just have to adjust QueryBean.