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Difference between activiti:delegateExpression and activiti:class. Bug?

richardmward

Hi,

I have come across what I think may be a bug with the use of

activiti:delegateExpression‍

as an alternative to

activiti:class‍

on both

serviceTask‍

's and

taskListener‍

's.

When using

activiti:class‍

, one instance of a

JavaDelegate‍

is created for each

serviceTask‍

in a

ProcessDefinition‍

. When using

activiti:delegateExpression‍

, one instance of a

JavaDelegate‍

, peroid. This is as expected.

The issue comes when you have two

serviceTask‍

's which use different

Expression‍

's. For example:

In the

JavaDelegate‍

@Component("processDefinitionBean")
public class ProcessDefinitionBean {
    private Expression requiredVariable;
    private Expression optionalVariable;
   …
}
‍‍‍‍‍‍‍‍

bpmn1.bpmn20.xml‍

…
<serviceTask id="serviceTask" name="Service task" activiti:delegateExpression="${processDefinitionBean}">
  <extensionElements>
    <activiti:field name="requiredVariable">
      <activiti:expression>${requiredVariableValue}</activiti:expression>
    </activiti:field>
    <activiti:field name="optionalVariable">
      <activiti:expression>${optionalVariableValue}</activiti:expression>
    </activiti:field>
</serviceTask>
…
‍‍‍‍‍‍‍‍‍‍‍‍‍

bpmn2.bpmn20.xml‍

…
<serviceTask id="serviceTask" name="Service task" activiti:delegateExpression="${processDefinitionBean}">
  <extensionElements>
    <activiti:field name="requiredVariable">
      <activiti:expression>${requiredVariableValue}</activiti:expression>
    </activiti:field>
</serviceTask>
…
‍‍‍‍‍‍‍‍‍‍

Now, if an instance of

bpmn1‍

is started, when it reaches the

serviceTask‍

it will initialise all three of the

Expression‍

's in the

JavaDelegate‍

. If an instance of

bpmn2‍

is then started, when it reaches the

serviceTask‍

, the

requiredVariable‍

Expression‍

is being set to the value in

bpmn2‍

, but the

optionalVariable‍

Expression‍

is still set to the value from

bpmn1‍

. This is a pretty significant issue as it means that the

processDefinitionBean‍

is not in the state it expects to be in, and will either end up with a value it is not meant to have in it (should

bpmn2‍

happen to set

${optionalVariableValue}‍

somewhere along the way, or it will fall over because

optionalVariableValue‍

does not exist.

This isn't a problem when using

activiti:class‍

becuase there is one instance for each of the two

serviceTask‍

's, so there is no possibility that different combinations of

activiti:field‍

's can be set on the same isntance of a

JavaDelegate‍

.

For the time being, we are nullifying each of the

Expression‍

's at the end of the

exectute‍

method in each of our

JavaDelegate‍

's - but I wonder if an official fix should be investigated (should this be considered a bug).

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Comments

richardmward

I seem to be unable to edit the above post in which I have spotted a few errors - they are as follows:

"When using activiti:delegateExpression, one instance of a JavaDelegate, peroid." should read: "When using activiti:delegateExpression, one instance of a JavaDelegate is created, peroid."
The <code>JavaDelegate</code> code snippet should have <code>implements JavaDelegate</code>
"Now, if an instance of bpmn1 is started, when it reaches the serviceTask it will initialise all three of the Expression's in the JavaDelegate." should read: "Now, if an instance of bpmn1 is started, when it reaches the serviceTask it will initialise both of the Expression's in the JavaDelegate."
"For the time being, we are nullifying each of the Expression's at the end of the exectute method in each of our JavaDelegate's" - I am aware this may cause concurrency issues, and will be testing to prove that shortly. If anyone can suggest an alternative approach, it would be much appreciated!

richardmward

Current resolution

We have removed out nullifiying code in order to not cause concurrency issues and instead have used <code>@Scope(BeanDefinition.SCOPE_PROTOTYPE)</code> on the beans used as <code>JavaDelegate</code>'s. This allows a new bean to be created each time it is accessed - i.e. a new bean will be created for each and every instance of a <code>serviceTask</code>. Whilst this is slightly inefficient, we don't anticipate issues as looking through the Activiti code we see:

in <code>ServiceTaskDelegateExpressionActivityBehavior.execute(ActivitiExecution)</code>
<java>
// Note: we can't cache the result of the expression, because the
// execution can change: eg.
// delegateExpression='${mySpringBeanFactory.randomSpringBean()}'
Object delegate = expression.getValue(execution);
</java>

Following the references to <code>delegate</code> from this point onwards, shows it being used in the following line (when the <code>delegate</code> is an instance of <code>JavaDelegate</code>):
<java>
Context.getProcessEngineConfiguration().getDelegateInterceptor().handleInvocation(new JavaDelegateInvocation((JavaDelegate) delegate, execution));
</java>

And checking <code>handleInvocation</code> I can see that the invocations are not held onto, so Activiti will release the delegates after they have executed, and then we can leave it to the garbage collector to clear the objects up.