Solution:
JBoss AS 5.1 executes the JMS onMessage() operation using the classloader of the message sender, not the web context classloader of the receiver. This results in the exception above because the org.activiti.engine.impl.javax.el.ExpressionFactory.newInstance() method does a type check using Class.isAssignableFrom(), and the 2 types it is comparing come from 2 different class loaders, so the comparison fails.
Bottom Line: make sure the classloader used to execute Activiti inside a JBoss WAR does not get switched when responding to JMS messages.
My workaround was to assign the classloader manually in my JMS listener:
@Override
public void onMessage(Message msg)
{
if (msg instanceof TextMessage)
{
ClassLoader originalCL = Thread.currentThread().getContextClassLoader();
try
{
String xml = ((TextMessage) msg).getText();
Thread.currentThread().setContextClassLoader( this.getClass().getClassLoader() );
doActivitiStuff();
}
catch (JMSException e)
{
e.printStackTrace();
}
finally
{
Thread.currentThread().setContextClassLoader( originalCL );
}
}
}
