Title: persistence-unit-ref
Both lookup and injection of an EntityManagerFactory can be configured via
the @PersistenceUnit annotation or <persistence-unit-ref> in xml. 
Annotations and xml have equal function in both lookup and injection.

<a name="persistence-unit-ref-InjectionandLookup"></a>
#  Injection and Lookup

<a name="persistence-unit-ref-Viaannotation"></a>
##  Via annotation

    package org.superbiz;

    import javax.persistence.PersistenceUnit;
    import javax.persistence.EntityManagerFactory;
    import javax.ejb.Stateless;
    import javax.naming.InitialUnit;

    @Stateless
    public class MyBean implements MyInterface {

        @PersistenceUnit(unitName = "bar-unit")
        private EntityManagerFactory myBarEntityManagerFactory;

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

            // Both can be looked up from JNDI as well
            InitialContext unit = new InitialContext();
            EntityManagerFactory barEntityManagerFactory = (EntityManagerFactory) context.lookup("java:comp/env/org.superbiz.MyBean/myBarEntityManagerFactory");
        }
    }


<a name="persistence-unit-ref-Viaxml"></a>
## Via xml

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

    <persistence-unit-ref>
        <persistence-unit-ref-name>org.superbiz.calculator.MyBean/myBarEntityManagerFactory</persistence-unit-ref-name>
        <persistence-unit-name>bar-unit</persistence-unit-name>
        <persistence-unit-type>Transaction</persistence-unit-type>
        <injection-target>
            <injection-target-class>org.superbiz.calculator.MyBean</injection-target-class>
            <injection-target-name>myBarEntityManagerFactory</injection-target-name>
        </injection-target>
    </persistence-unit-ref>

    
# Lookup only
    
##  Via annotation
    
    package org.superbiz;

    import javax.persistence.PersistenceUnit;
    import javax.persistence.EntityManagerFactory;
    import javax.ejb.Stateless;
    import javax.naming.InitialUnit;

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

        public void someBusinessMethod() throws Exception {
            InitialContext context = new InitialContext();
            EntityManagerFactory fooEntityManagerFactory = (EntityManagerFactory) context.lookup("java:comp/env/myFooEntityManagerFactory");
        }
    }


<a name="persistence-unit-ref-Viaxml"></a>
# Via xml

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

    <persistence-unit-ref>
        <persistence-unit-ref-name>myFooEntityManagerFactory</persistence-unit-ref-name>
        <persistence-unit-name>foo-unit</persistence-unit-name>
        <persistence-unit-type>Transaction</persistence-unit-type>
    </persistence-unit-ref>