jbpm is a workflow engine, the identity management features are very limited and not suited for production in my opinion.
You use the EnvironmentImpl.setAuthenticatedUserId() to tell the Engine which user is performing actions.
Getting the authenticated user, password management etc. is not in the scope of jbpm, this is the job of other frameworks like spring-security, seam or JAAS...
I am calling a Java Handler class when my task/decision is executed. Inside this Java code I need to know the user id of the user executing the task. Hoo can I achieve that?
you might want to try EnvironmentImpl.getAuthenticatedUserId() or task.getAssignee()