import org.apache.hadoop.mapreduce.v2.app.job.event.JobMapTaskRescheduledEvent; //导入依赖的package包/类
@Override
public TaskStateInternal transition(TaskImpl task, TaskEvent event) {
  TaskTAttemptEvent castEvent = (TaskTAttemptEvent) event;
  if (task.getInternalState() == TaskStateInternal.SUCCEEDED &&
      !castEvent.getTaskAttemptID().equals(task.successfulAttempt)) {
    // don't allow a different task attempt to override a previous
    // succeeded state
    task.finishedAttempts.add(castEvent.getTaskAttemptID());
    task.inProgressAttempts.remove(castEvent.getTaskAttemptID());
    return TaskStateInternal.SUCCEEDED;
  }

  // a successful REDUCE task should not be overridden
  //TODO: consider moving it to MapTaskImpl
  if (!TaskType.MAP.equals(task.getType())) {
    LOG.error("Unexpected event for REDUCE task " + event.getType());
    task.internalError(event.getType());
  }
  
  // tell the job about the rescheduling
  task.eventHandler.handle(
      new JobMapTaskRescheduledEvent(task.taskId));
  // super.transition is mostly coded for the case where an
  //  UNcompleted task failed.  When a COMPLETED task retroactively
  //  fails, we have to let AttemptFailedTransition.transition
  //  believe that there's no redundancy.
  unSucceed(task);
  // fake increase in Uncomplete attempts for super.transition
  task.inProgressAttempts.add(castEvent.getTaskAttemptID());
  return super.transition(task, event);
}