Disclaimer: jBPM 4.3 does not support historic on task reassignment. This means if task task1 is first assigned to user homer, then it is delegated manually or otherwise to user bart and finally to user marge, jBPM will only keep track of the last user, marge in this case, that was assigned to task1.
Having said that, if you are ok with getting the currently user assigned to a task you can use TaskService to create a TaskQuery filtering by process instance id and task name, as in the following example:
TaskService taskService = ...;
TaskQuery taskQuery = taskService.createTaskQuery().processInstanceId(processInstance.getId()).activityName("task1");Task task = taskQuery.uniqueResult(); // task.getAssignee() contains the login of the user, if any, to which the task is assigned to.