在 threadPoolExecutor 中取消任务

Question

I instantiated threadPoolExecutor and submitted asynchronous task to pool and got Future object and then i tried to cancel the task, cancel operation is not working...

The task is related to database calls.

Code is as below,

private ThreadPoolExecutor _scheduledThreadPool;
_scheduledThreadPool = (ThreadPoolExecutor) Executors.newFixedThreadPool(_poolSize,   factory);
Future<Object> task = _scheduledThreadPool.submit(task);
tasks.cancel(true);

After the above code, I observed the _scheduledThreadPool.getQueue() is giving empty.

I am working on this from few days but not worked.

Please help me how to cancel a task individually in ThreadPoolExecutor.

thanks

Answer 1

If the task is already running at the time that cancel() is called-- which is what appears to be happening in your case-- then this won't "magically" guarantee that the task will stop running in the way that you expect. The thread running the task will be interrupted , and the code performed by your task has to handle this with your desired behaviour.

Answer 2

你可以试试：

_scheduledThreadPool.getQueue().clear();

Answer 3

We have a Future object here:

Future<Object> task = _scheduledThreadPool.submit(task);

ThreadPoolExecutor uses FutureTask as the default Future implementation. And FutureTask uses thread interruption to cancel the task . So it invokes Thread.intterrupt on the worker thread.

However, for the cancellation to happen, the task must be responsive to interruptions. In other words, it must check whether the current thread is interrupted by invoking Thread.interrupted / Thread.currentThread().isInterrupted . As an example, we're checking the interruption status in a loop:

final Runnable task = () -> {
    while (!Thread.currentThread().isInterrupted()) {
        // Do work.
    }
}

Alternatively, the task must be blocked on code that respects interruption, like LinkedBlockingQueue.take . The standard libraries generally throw an InterruptedException when they detect an interruption.