How to inject EntityManager dynamically

Question

I have a class in which I want to inject a EntityManager based on not compile time known persistence unit name.
In an EE environment you would normally do something like
@PersistenceContext(unitName="pu1") private EntityManager em;
Now the problem is that I want this "pu1" to be dynamic based based upon a parameter.
In an SE environment (or also EE) I could create a
emf = Persistence.createEntityManagerFactory(name);
manually and pass in a dynamic persistence unit name.
But creating the emf is exepensive.
What is best way to do this in an EE environment?

Answer 1

If there will only be one persistence unit in scope then you can just leave the unitName out altogether :
@PersistenceContext private EntityManager em;
In this case the container will just consider the one persistence unit defined at the module or application level as the default and inject it.
If there might be more than one, you can define the dependency in your web.xml or ejb-jar.xml file.
That will allow you to set the unit-name without having to recompile the code.
persistence-context-ref is the analog of @PersistenceContext within the deployment descriptor :
<persistence-context-ref>
<persistence-context-ref-name>persistence/myemref</persistence-context-ref-name>
<persistence-unit-name>pu1</persistence-unit-name>
</persistence-conetxt-ref>
Then you could look it up via java:comp/env :
(EntityManager) initialContext.lookup("java:comp/env/persistence/myemfref");
If you want to get a little fancier, you can use the .xml elements to cause the injection to happen.
<persistence-context-ref>
<persistence-context-ref-name>persistence/myemref</persistence-context-ref-name>
<persistence-unit-name>pu1</persistence-unit-name>
<injection-target>
<injection-target-class>com.acme.Foo</injection-target-class>
<injection-target-name>em</injection-target-name>
</injection-target>
</persistence-conetxt-ref>
Then your java code would look like :
// dependency and injection declared via deployment descriptor
private EntityManager em;
--ken