Wednesday, January 22, 2014

Activiti/camunda BPM: custom behavior and BPMN extension elements using Blueprint



In the last few weeks I have been working on a problem regarding OSGi-Blueprint and Activiti. Because it wasn't as easy as I would have hoped I want to share my solution with you. I will start by explaining my environment, show you the problem and then I will explain my first attempt. After that I will present my solution. Finally I will show some ideas how to make it better and things that I did not test.

Just a short hint about the writing: when I reference a class or some XML it's written in italic, e.g. Object. When you see "process engine", I am talking about the whole thing, but when you see ProcessEngine it's the actual class.

My environment


I use the Activiti-framework in version 5.12.1, Apache Aries in version 1.0.0 with a little modification and my own ProSt bundles. ProSt can be found here. The explains why and what I changed in Aries.

I haven't tried, yet, but I am pretty sure that camunda BPM suffers the same problem because
both share the same MailActivityBehavior and BlueprintELResolver classes and use <extension-elements> for injection. So if you prefer camunda and see "activiti" somewhere you just have to replace it with "camunda" in your head ;-)

The problem 


In general, I just wanted to send an e-mail during my process-execution. Sounds pretty easy, right?

My SendMailWithAttachmentBehaviour class extends the previously mentioned MailActivitiBehavior class. The process definition contains all the necessary information to send the e-mail, e.g. from, to and subject. Only the attachment is missing, which I get from the execution environment.

Because I use Blueprint I cannot use the activit:class or type="mail" attributes in the process definition. I have to declare the class this way:

A little hint: the name in the braces has to match the one used as bean id in the blueprint.xml.

The other ways do not work with OSGi because of class visibility etc.

The easy part was to extend the BlueprintELResolver class (ProStBlueprintELResolver) and add a way to add custom behavior classes at the moment.

So, what happens when the process engine tries to resolve the expression?
When the bundle is loaded Blueprint creates a dynamic proxy and registers it at the ProStBlueprintELManager.
After the process reaches the ServiceTask which delegates to the ${sendMailWithAttachmentBehaviour} the process-engine asks its ExpressionLanguageResolvers if they know something with the name "sendMailWithAttachmentBehaviour". Logically the proxy is found.
After that the process engine tries to set the extension-elements at the class.
First it tries to find setter methods and if it cannot find setters it tries field injection. (see ClassDelegate.applyFieldDeclaration())
Both ways do not work.
But why?
Of course a proxy does not have any fields. But why is it not possible to just add the setters to the SendMailWithAttachmendBehaviour class?
The call is proxy.getClass().getMethods() and according to the documentation this will return all the methods of the interfaces that the proxy was created with. ActivityBehavior does not declare the setSubject() etc. methods because they are only needed for e-mails.

First attempt


At first I thought the solution was quite obvious. I would just export a second interface containing the setters like this:

<bean id="sendMailWithAttachment" class="de.blogspot.wrongtracks.prost.example.behaviour.SendMailWithAttachmentBehaviour" />
<service ref="sendMailWithAttachment">
But wait, if you take a look at the (old) context.xml you can see that my reference listener just listens for ActivityBehavior and not the other interface. That's why the created proxy won't contain the methods from the other interface. Too bad...

The solution


I found the solution accidentally while reading the Apache Aries Blueprint documentation. This chapter points out that you can also listen for service references.
I changed the methods to accept a ServiceReference instead of a ActivitiyBehavior and when the expression should be resolved I use the BundleContext to get the service. At that point it is not a proxy, it is the implementation.

Then everything works just fine. I don't even need the setter interface anymore.
You can see the solution when you look at the new context.xml and the previously mentioned ProStBlueprintELResolver. (the previously showed link pointed to an old version so nothing would be spoiled ;-) )

That's it, that is my solution to add custom behavior to the process engine and use extension elements in the BPMN XML.

How could we improve the whole thing?


Strangely, I have no idea how the whole thing could be improved. I would like to hear your ideas. Also, I would like to know if you think that the way presented here is good or bad or something in between.

What didn't I try?


You should note that I have not tried to find out how JavaDelegates behave in the same situation. I just did not have time and I wanted to show you my solution as soon as I finished it.


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