If you used candidate-users or candidate-groups to set the participants of a Task. You should use taskService.findGroupTask("John") to get the group task list.
You have defined the users as candidate-users="...", and that makes the tasks available on the group task list. For a user to list, you must call:
tasks = taskService.findGroupTasks("john");
Then if you want to work with it, you must pick it up, so the task is given a specific assignee, and no other person picks it up:
You must work out the tasks.get(0).getId() part
Edit: you can find more info on sub-section 22.214.171.124. task candidates here: http://docs.jboss.com/jbpm/v4/userguide/html_single/#task
Hope it helps.
Ok, I was melting candidate and assignee
I have also found a workaround with :
Thanks for your replies !