Java Tip 118: Utilize the EjbProxy

Invoke remote EJBs with a simple utility class

By Gorsen Huang, JavaWorld.com, 10/12/01

You must follow four basic steps to invoke an EJB:

1. Create a JNDI initial context
2. Look up the EJB name in the context
3. Get the home object
4. Create the EJB object's instance via the home object's create() method

The implementation for Steps 1 through 3 is trivial:

    InitialContext ctx = new InitialContext (prop);
    Object home = ctx.lookup(beanName);
    EJBHome obHome =
      (EJBHome) PortableRemoteObject.narrow (home, EJBHome.class);

As you can see, you can obtain the EJB home object without knowing the specific implementation of that particular EJB, except the JNDI name lookup string.

Step 4 is not so straightforward. When you try to create the EJB object using the home object, you can't invoke the create() method from the home object, since the EJBHome interface doesn't define the create() method. Only the actual EJB implementations define and implement this method.

As one possible solution, you could define an interface that extends EJBHome and declares the create() method. Then you could simply cast the home object to this interface and call the create() method. However, this approach has one drawback: it requires all EJBs to implement this particular interface. You'd want to invoke the create() method of an arbitrary home interface.

A better solution would use Java's Reflection API capabilities. (You can refer to Resources for detailed information on the Reflection API.) By using reflection, you can load the specified class using Class.forName, and then call getDeclaredMethod() to retrieve the method the class declares. Once you obtain the Method object, you can execute the method with Method.invoke.

The following code uses reflection to create the EJB object's instance from its home object:

Method m = obHome.getClass().getDeclaredMethod("create", new Class[0]);
Object objRemote = m.invoke (obHome, new Object[0]);
Object obj = PortableRemoteObject.narrow(objRemote, theClass);

That is, this code segment gets the class from the home object, and then calls getDeclaredMethod() with the create() method's name in order to retrieve create's Method object. It also uses the invoke() method to execute the create() method, which returns the EJB object's instance.

The EjbProxy class

The EjbProxy class, found in the source code's EjbProxy.java, manages these nasty details for you.