topic Re: What is the best way to call methods of a Stateless EJB? in Alfresco Archive

What is the best way to call methods of a Stateless EJB?

mskaesz — Mon, 13 Sep 2010 22:56:52 GMT

Hey guys,I found four ways to call a method of a Stateless EJB. Which one do you recommend?My environment: Activiti5.beta1; EJB3; JBoss AS 5.1.) In the first listing I have an EJB which is already injected via JBoss AS. I'm setting the EJB as variable when the process is started.…@EJB(name="ejb/frie

Re: What is the best way to call methods of a Stateless EJB?

tombaeyens — Tue, 14 Sep 2010 05:40:25 GMT

you should call your ejb just like you would do from a servlet.

in the activiti behavior, you should look up the local home in jndi and perform the invocation on that bean.

Re: What is the best way to call methods of a Stateless EJB?

bernd_ruecker — Tue, 14 Sep 2010 07:56:41 GMT

In my oppinion implementing the Activiti Behavior is just a temporary solution since EventListeners (like in jBPM) are not available. So I think it is best for now.

Having it as a process variable is too risky in sense of lifecycle I think (you have to make sure, the reference is not persisted, and that it is available all the time you need it).

For 3 I cannot say too much, since I don't know the latest status to be honest, 4 may be an option, but I think technically you should avoid the WebService overhead if you can, even if that maybe makes your BPMN 2.0 process better portable to other BPMN 2.0 engines (theoretically ;-)).

Cheers
Bernd

Re: What is the best way to call methods of a Stateless EJB?

falko_menge — Tue, 14 Sep 2010 10:26:11 GMT

I agree with Bernd in that a Java class attached to a BPMN Service Task should not be forced to call performDefaultOutgoingBehavior(execution) or execution.take(transition).

Essentially, a Java-based BPMN Service Task should work like a jBPM4 Java Activity and must not be able to influence the execution of the Sequence Flow, as that would violate BPMN's execution semantics. As a shortcut, one could have a ServiceTaskBehavior interface, that allows random access to data objects, but only read access to the execution state.

There might be also a need for custom BPMN Flow Node implementations, which do influence the execution of the Sequence Flow. However, such a custom Flow Node should be called neither Service Task nor Activity, when it can really behave like any Flow Node even Gateway or Event.

Re: What is the best way to call methods of a Stateless EJB?

tombaeyens — Wed, 15 Sep 2010 09:07:45 GMT

Would it be a solution to split the interface for the 2 different purposes:

1) One interface for service task implementations that does not expose control flow operations.
2) One interface for advanced use cases that does expose control flow operations.

First I want to hear if this is a good solution. And a subsequent aspect from this is the naming of those interfaces.

For (1) we could leverage the PVM EventListener interface. Is it necessary to create a BPMN duplicate for this? Last time we discussed this we said no i think.
For (2) we could just delegate to ActivityBehavior itself for this in similar manner.

Re: What is the best way to call methods of a Stateless EJB?

falko_menge — Wed, 15 Sep 2010 11:04:04 GMT

For (1) the PVM EventListener interface does not seem suitable, as it doesn't provide read-only access to the state of the process instance and is too much focused on event handling. I'd suggest something like:


public interface ServiceTaskBehavior {
  void execute(ServiceTaskExecution execution) throws Exception;
}

with ServiceTaskExecution being the following subset of ActivityExecution:


public interface ServiceTaskExecution extends DelegateExecution {
  PvmActivity getActivity();
  ActivityExecution getParent();
  List<? extends ActivityExecution> getExecutions();
  boolean isActive();
  boolean isConcurrent();
  boolean isProcessInstance();
  List<ActivityExecution> findInactiveConcurrentExecutions(PvmActivity activity);
}

However, one could argue, whether it is good to have Service Tasks the behave differently, based on their location in a process definition and the execution state of the process instance. If not, a DelegateExecution would be enough. Any opinions on that?

For (2) the ActivityBehavior interface is fine, although in terms of BPMN it is more a FlowNodeBehavior. But, I agree that it doesn't make sense to introduce another interface just for the sake of using BPMN terminology if the rest of the APIs isn't using it anyway.

As a third option I'd suggest the Service Task being able to call an arbitrary method of some Java class with explicit parameter passing as in the jBPM4 Java Activity.

Re: What is the best way to call methods of a Stateless EJB?

bernd_ruecker — Thu, 16 Sep 2010 07:37:26 GMT

wever, one could argue, whether it is good to have Service Tasks the behave differently, based on their location in a process definition and the execution state of the process instance. If not, a DelegateExecution would be enough. Any opinions on that?

I would like to have these information. If you for example just want to log the current node name or something like that, it make sense. The important thing is, that you cannot influence anything.

Re: What is the best way to call methods of a Stateless EJB?

falko_menge — Sat, 25 Sep 2010 09:05:10 GMT

The discussion about Java-based BPMN Service Tasks is continued in http://jira.codehaus.org/browse/ACT-156

Re: What is the best way to call methods of a Stateless EJB?

bernd_ruecker — Wed, 03 Nov 2010 12:49:37 GMT

Hey guys.
I wanted to ping that again. In the current implementation with the DelegateExecution you cannot access enough stuff, so you have to cast for a lot of requirements. We should improve that to avoid the necessity of casting for basic use cases (like seeing in with activity we are at the moment).

Re: What is the best way to call methods of a Stateless EJB?

jbarrez — Wed, 03 Nov 2010 14:05:21 GMT

Bernd, could you describe what you are missing ?

Currently its basically variables only, which is appropiate I think for the typical delegation use cases.
If you want the more powerful stuff, you need to implement the ActivityBehavior interface, which gives you access to the more powerful operations.

From the docs:

To implement a class that can be called during process execution, this class needs to implement the org.activiti.engine.delegate.JavaDelegation interface and provide the required logic in the execute method. When process execution arrives at this particular step, it will execute this logic defined in that method and leave the activity in the default BPMN 2.0 way.

[EXPERIMENTAL] It is also possible to provide a class that implements the org.activiti.engine.impl.pvm.delegate.ActivityBehavior interface. Implementations have then access to the more powerful ActivityExecution that for example also allows to influence the control flow of the process. Note however that this is not a very good practice, and should be avoided as much as possible. So, it is advised to use the ActivityBehavior interface only for advanced use cases and if you know exactly what you're doing.

Re: What is the best way to call methods of a Stateless EJB?

falko_menge — Mon, 08 Nov 2010 14:14:36 GMT

As Bernd said, the most important requirement would be access to the current activity. In case you still hesitate to expose the object graph of the parsed process definition for introspection, we should at least provide its name and ID.

Re: What is the best way to call methods of a Stateless EJB?

jbarrez — Tue, 09 Nov 2010 16:15:40 GMT

Which are available on the ActivityExecution (getActivity()).

What would you do with it in a regular service task invocation?
I still don't see the use case for exposing the current activity in the JavaDelegation.

Re: What is the best way to call methods of a Stateless EJB?

falko_menge — Wed, 10 Nov 2010 10:08:52 GMT

Does it hurt us so badly, if we just expose a String with id of the current activity through something like getCurrentActivityId(), so that the ActivityBehavior interface can "be avoided as much as possible."?

Re: What is the best way to call methods of a Stateless EJB?

tombaeyens — Wed, 10 Nov 2010 11:03:59 GMT

The JavaDelegation has as a main purpose to hide as much methods as possible so that users are encouraged to use the stable methods only.
There's always use cases to add more methods of ActivityExecution or ExecutionEntity. The more we add the less stable methods we'll expose.
So in conclusion, we want to limit the constraints we put on ourselves after 5.0. So I don't think it's good to add it. I don't see the problem with the cast or just implementing an ActivityBehavior instead.

Re: What is the best way to call methods of a Stateless EJB?

ryu — Thu, 27 Jan 2011 18:09:17 GMT

hi jbarrez, falko.menge, and tombaeyens,

Would you please explain me the downside of the following way to call EJB method (just another thought):

1. define a class A, which has method m1() invoking a stateless EJB.
2. instance an object from the class A and given it as a process variable a.
3. using expression call a.m1() in process

Is it really bad and why?

Waiting for your answer
Thank you very very much

Re: What is the best way to call methods of a Stateless EJB?

jbarrez — Fri, 28 Jan 2011 11:03:58 GMT

Nope, if I read it correctly there is nothing bad about that.