I see it http://stackoverflow.com/questions/10714598/processing-a-task-in-an-asynchronous-workitemhandler
salaboy say:
"If you have everything set up correctly when you call the taskClient.complete() method the session will notified and the ksession.getWorkItemManager().completeWorkItem() will be called automatically for you. There is no need to call it by yourself. You only need to remember to register the WorkItemHandler to the session and keep the session alive so it can be notified. Cheers"