h1.  Via annotation

Both lookup and injection of an EntityManager can be configured via the @PersistenceContext annotation.

{code:title=Usable by EJB, Interceptor, Servlet, Filter, or Listener}
package org.superbiz;

import javax.persistence.PersistenceContext;
import javax.persistence.EntityManager;
import javax.ejb.Stateless;
import javax.naming.InitialContext;

@Stateless
@PersistenceContext(name = "myFooEntityManager", unitName = "foo-unit")
public class MyBean implements MyInterface {

    @PersistenceContext(unitName = "bar-unit")
    private EntityManager myBarEntityManager;

    public void someBusinessMethod() throws Exception {
        if (myBarEntityManager == null) throw new NullPointerException("myBarEntityManager not injected");

        // Both can be looked up from JNDI as well
        InitialContext context = new InitialContext();
        EntityManager fooEntityManager = (EntityManager) context.lookup("java:comp/env/myFooEntityManager");
        EntityManager barEntityManager = (EntityManager) context.lookup("java:comp/env/org.superbiz.MyBean/myBarEntityManager");
    }
}
{code}

h1. Via xml

The above @PersistenceContext annotation usage is 100% equivalent to the following xml.

{code:xml|title=ejb-jar.xml or web.xml}
<persistence-context-ref>
    <persistence-context-ref-name>myFooEntityManager</persistence-context-ref-name>
    <persistence-unit-name>foo-unit</persistence-unit-name>
    <persistence-context-type>Transaction</persistence-context-type>
</persistence-context-ref>
<persistence-context-ref>
    <persistence-context-ref-name>org.superbiz.calculator.MyBean/myBarEntityManager</persistence-context-ref-name>
    <persistence-unit-name>bar-unit</persistence-unit-name>
    <persistence-context-type>Transaction</persistence-context-type>
    <injection-target>
        <injection-target-class>org.superbiz.calculator.MyBean</injection-target-class>
        <injection-target-name>myBarEntityManager</injection-target-name>
    </injection-target>
</persistence-context-ref>
{code}