Question

我有棘手的情况，即使线程完成，future.isDone()也会返回false。

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class DataAccessor {
    private static ThreadPoolExecutor executor;
    private int timeout = 100000;
    static {
        executor = new ThreadPoolExecutor(10, 10, 1000, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(1000));
    }

    public static void main(String[] args) {
        List<String> requests = new ArrayList<String>();
        for(int i=0; i<20; i++){
            requests.add("request:"+i);
        }
        DataAccessor dataAccessor = new DataAccessor();

        List<ProcessedResponse> results = dataAccessor.getDataFromService(requests);
        for(ProcessedResponse response:results){
            System.out.println("response"+response.toString()+"\n");
        }
        executor.shutdown();
    }

    public List<ProcessedResponse> getDataFromService(List<String> requests) {
        final CountDownLatch latch = new CountDownLatch(requests.size());
        List<SubmittedJob> submittedJobs = new ArrayList<SubmittedJob>(requests.size());
        for (String request : requests) {
            Future<ProcessedResponse> future = executor.submit(new GetAndProcessResponse(request, latch));
            submittedJobs.add(new SubmittedJob(future, request));
        }
        try {
            if (!latch.await(timeout, TimeUnit.MILLISECONDS)) {
                // some of the jobs not done
                System.out.println("some jobs not done");
            }
        } catch (InterruptedException e1) {
            // take care, or cleanup
            for (SubmittedJob job : submittedJobs) {
                job.getFuture().cancel(true);
            }
        }
        List<ProcessedResponse> results = new LinkedList<DataAccessor.ProcessedResponse>();
        for (SubmittedJob job : submittedJobs) {
            try {
                // before doing a get you may check if it is done
                if (!job.getFuture().isDone()) {
                    // cancel job and continue with others
                    job.getFuture().cancel(true);
                    continue;
                }
                ProcessedResponse response = job.getFuture().get();
                results.add(response);
            } catch (ExecutionException cause) {
                // exceptions occurred during execution, in any
            } catch (InterruptedException e) {
                // take care
            }
        }
        return results;
    }

    private class SubmittedJob {
        final String request;
        final Future<ProcessedResponse> future;

        public Future<ProcessedResponse> getFuture() {
            return future;
        }

        public String getRequest() {
            return request;
        }

        SubmittedJob(final Future<ProcessedResponse> job, final String request) {
            this.future = job;
            this.request = request;
        }
    }

    private class ProcessedResponse {
        private final String request;
        private final String response;

        ProcessedResponse(final String request, final String response) {
            this.request = request;
            this.response = response;
        }

        public String getRequest() {
            return request;
        }

        public String getResponse() {
            return response;
        }

        public String toString(){
            return "[request:"+request+","+"response:"+ response+"]";
        }
    }

    private class GetAndProcessResponse implements Callable<ProcessedResponse> {
        private final String request;
        private final CountDownLatch countDownLatch;

        GetAndProcessResponse(final String request, final CountDownLatch countDownLatch) {
            this.request = request;
            this.countDownLatch = countDownLatch;
        }

        public ProcessedResponse call() {
            try {
                return getAndProcessResponse(this.request);
            } finally {
                countDownLatch.countDown();
            }
        }

        private ProcessedResponse getAndProcessResponse(final String request) {
            // do the service call
            // ........
            if("request:16".equals(request)){
                throw (new RuntimeException("runtime"));
            }
            return (new ProcessedResponse(request, "response.of." + request));
        }
    }
}

如果我致电future.isDone()，它会返回false，但coundownLatch.await()会返回true。任何的想法？还要注意，当发生这种情况时，countDownLatch.await立即出现。

如果您在此处找到格式不可读的视图，http://tinyurl.com/7j6cvep。

Answer 1

问题很可能是时机问题。在所有任务实际完成后关于Future（因为countDown()调用在call()方法中）之前，锁存器将被释放。

你基本上是在重建CompletionService的工作（实现是ExecutorCompletionService），我建议你改用它。您可以使用poll(timeout)方法获取结果。只需跟踪总时间，并确保将每次通话的超时时间减少到总剩余时间。

Answer 2

正如jtahlborn所说，这可能是一种竞争条件，其中CountdownLatch发出等待线程的信号，等待线程在FutureTask完成执行之前评估Future的取消条件（这将在countDown之后的某个时刻发生）。

您根本不能依赖CountdownLatch的同步机制与Future的同步机制同步。你应该做的是依靠未来告诉你何时完成。

您可以Future.get(long timeout, TimeUnit.MILLISECONDS)代替CountdownLatch.await(long timeout, TimeUnit.MILLISECONDS)。要获得与闩锁相同类型的效果，您可以将所有Future添加到List，迭代列表并获取每个Future。

Answer 3

以下是比赛条件的情景：

主线程在latch.await，它从Java调度程序接收没有CPU插槽毫秒
最后一个执行程序线程在countDownLatch.countDown()子句

finally

Java调度程序决定为主线程提供更多优先级，因为它等待了一段时间
因此，当它询问最后的Future结果时，它还不可用，因为最后一个执行程序线程没有时间片来传播结果，它仍在finally中。

我还没有找到关于Java调度程序如何工作的详细解释，可能是因为它主要依赖于运行JVM的操作系统，但一般来说它试图在一段时间内平均给CPU提供可运行的线程。这就是为什么主线程可以在另一个离开isDone子句之前到达finally测试的原因。

我建议您更改结果'在latch.await之后收集。如您所知，锁存器已降至零（除非主线程被中断），所有结果应该很快就可用。带有超时的get方法让调度程序有机会将时间片分配给仍在finally子句中等待的最后一个线程：

    for (SubmittedJob job : submittedJobs) {
        try {
            ProcessedResponse response = null;
            try {
                // Try to get answer in short timeout, should be available
                response = job.getFuture().get(10, TimeUnit.MILLISECONDS);
            } catch (TimeoutException te) {
                job.getFuture().cancel(true);
                continue;
            }
            results.add(response);
        } catch (ExecutionException cause) {
            // exceptions occurred during execution, in any
        } catch (InterruptedException e) {
            // take care
        }
    }

备注：您的代码不现实，因为getAndProcessResponse方法的结束时间不到一毫秒。在那里随机睡觉，竞争条件不会经常出现。

Answer 4

我对竞争条件的看法排在第二位。我建议忘记闩锁和使用 java.util.concurrent.ThreadPoolExecutor.awaitTermination(long, TimeUnit)

即使任务完成，future.isDone也返回false

4 个答案: