Question

我正在调查CompletionService类，我发现提交队列与completition队列的解耦非常有用。

但是我错过了一种方法来轮询/取消已取消的任务（可以认为这种方式已完成）。是否可以轻松完成？

Future<String> task1Future =  completionService.submit(myCallableTask1);
Future<String> task2Future =  completionService.submit(myCallableTask2);
task2Future.cancel();
Future<String> lastComplTaskFuture = completionService.take(); 
                        //Seems to return only the completed or 
                        //interrupted tasks, not those cancelled (task2)

编辑：在检查了一些答案后，我意识到发生了什么。 CompletitionService以与提交的作业相同的顺序返回。如果你经营工作a，b和c;在工作时取消b和c;最后轮询completitionService，b和c的取消不会被通知，直到taks终止。此外，我意识到如果您关闭执行程序而不是取消单个任务，那么仍然在队列中的那些任务不会达到completitionservice，即使它们的取消在活动任务尚未结束时完成。

EDIT2：好的，我添加了一个完整的测试用例

import java.util.concurrent.*;

public class CompletionTest {

    public static void main(String[] args){
        CompletionService<String> completion = 
                new ExecutorCompletionService<String>(Executors.newSingleThreadExecutor()); 
        Future<String> aFuture =
                 completion.submit(() -> {Thread.sleep(10000); return "A";});
        completion.submit(() -> {return "B";}).cancel(true);
        completion.submit(() -> {return "C";}).cancel(true);
        completion.submit(() -> {return "D";}).cancel(true);

        long arbitraryTime = System.currentTimeMillis();
        while (true){
            String result = getNextResult(completion);
            System.out.println(result);

            if (System.currentTimeMillis() > arbitraryTime+5000){
                aFuture.cancel(true);
            }
         }
    }

    public static String getNextResult(CompletionService<String> service){
        try {
            String taskId = service.take().get();
            return "'"+taskId+"' job completed successfully.";
        } 
        catch (InterruptedException e ) { return "Unknown job was interrupted/stopped while waiting (no thread.wait() or infinite loop in the job so I guess this should not be possible)."; } 
        catch (CancellationException e) { return "Unknown job was cancelled."; }  
        catch (ExecutionException e) { 
            return "Unknown job returned with the following internal exception while running: " + e.getCause().getMessage();
        }
    }
}

我期望输出如下：

Unknown job was cancelled.
Unknown job was cancelled.
Unknown job was cancelled.
'A' job completed successfully.

但它恰好是：

'A' job completed successfully.
Unknown job was cancelled.
Unknown job was cancelled.
Unknown job was cancelled.

我甚至尝试使用PriorityBlockingQueue作为CompletionService的队列，使用Comparator.<Future<String>,Boolean>comparing(Future::isCancelled).reversed()，但它既不起作用（我想如果元素改变状态，它也不会诉诸）

Answer 1

您正在使用单个线程向线程池提交任务。

您提交的第一个任务将使用该线程（短时间休眠）。

其他人将在ExecutorService内排队。取消相应的期货不会从该队列中删除相应的任务。它只是改变了Future的状态。

任务将保留在队列中，直到线程可以自由处理它们。线程将注意到相应的Future被取消并取消整个任务。当CompletionService知道他们已经完成＆＃34;时，就会发生这种情况。

考虑增加线程池的大小。

通过CompletionService检查已取消的任务

1 个答案: