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Invokers allow you to customise how a particular method is executed. This is particular useful if your underlying service objects are not plain javabeans and instead need to be created or looked up via a custom factory.

The following example illustrates how an invoker can be used to allow xfire to expose remote stateless session beans as a webservice. Given the method to invoke, this invoker will create a stateless session bean instance to invoke the method on. The same technique can be used to enable service calls to any object that requires custom creation/lookup.

The invoker implementation is as follows:

public class EJBInvoker implements Invoker
{
  private EJBHome home;
  private Method createMethod;
  private static final Object[] EMPTY_OBJECT = new Object[0];

  public EJBInvoker(EJBHome home)
  {
    this.home = home;
    try
    {
      if(!home.getEJBMetaData().isSession() || !home.getEJBMetaData().isStatelessSession())
      {
        throw new IllegalArgumentException("home must be for a stateless session bean");
      }
      createMethod = home.getClass().getMethod("create", new Class[0]);
    }
    catch(Exception ex)
    {
      throw new IllegalArgumentException("Unable to initialize invoker: " + ex);
    }
  }

  public Object invoke(Method m, Object[] params, MessageContext context) throws XFireFault
  {
    try
    {
      Object session = createMethod.invoke(home, EMPTY_OBJECT);
      return m.invoke(session, params);
    }
    catch(Exception ex)
    {
      throw new XFireFault("Error invoking ejb method " + m.getName(), ex, XFireFault.RECEIVER);
    }
  }
}

Invokers, once defined, need to be registered with the service binding. Once a handle onto a Service object has been obtained, the example invoker above can be registered on the binding like this:

Service ejbService = ....; //look up the service from XFire, or create it
ejbService.getBinding().setInvoker(new EJBInvoker(ejbHome));