使用執行緒池時，呼叫Future#get，程式掛掉

我創建了一個執行緒池，并提交了兩個任務。為什么我的應用程式在列印后沒有任何例外掛起task one ,result: null？？？

   private final static ThreadPoolExecutor executorService = new
        ThreadPoolExecutor(1, 1, 1L, TimeUnit.MINUTES,
        new SynchronousQueue<Runnable>(), new ThreadPoolExecutor.DiscardPolicy());

    public static void main(String[] args) throws Exception {
        Future taskOne = executorService.submit(() -> {
            try {
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        Future taskTwo = executorService.submit(() -> System.out.println("task two is working"));;
        System.out.println("task one ,result: "   taskOne.get());
        System.out.println("task two, result: "   taskTwo.get());
        executorService.shutdown();
    }

uj5u.com熱心網友回復：

提交第二個任務時，觸發拒絕策略，因為執行緒池使用SynchronousQueue和maximumPoolSize為1，而第一個任務還沒有完成。您正在使用DiscardPolicy，這意味著執行緒池不執行任何操作并回傳FutureTask狀態始終為 的NEW。

    public static class DiscardPolicy implements RejectedExecutionHandler {
        public DiscardPolicy() { }
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
        }
    }

所以當你打電話時taskTwo#get()，你將永遠被阻止。（當 FutureTask 處于小于 的狀態時，它將始終被阻止COMPLETING，請參閱FutureTask#get）。

您可以使用AbortPolicy（默認策略），這樣當您執行時executorService.submit(() - > submit; System.out.println("task two is working"))，您會立即獲得一個RejectedExecutionException.

或者使用Future#get(timeout)，在這種情況下，TimeoutException如果在指定時間內沒有得到結果，則會得到 。

uj5u.com熱心網友回復：

new ThreadPoolExecutor.DiscardPolicy()當新任務提交失敗時，它會默默地丟棄新任務。這里 taskTwo 想要被執行，它永遠沒有機會執行。

DiscardPolicy()方法void rejectedExecution(Runnable r, ThreadPoolExecutor executor)從RejectedExecutionHandler介面內部呼叫。

我已經展示CustomRejectedExecutionHandler了更好地理解taskTwo執行緒條件。AstaskTwo被悄悄丟棄，因此該taskTwo.get()方法將永遠無法回傳資料。

這就是為什么超時需要設定為 1 秒(taskTwo.get(1000, TimeUnit.MILLISECONDS))。

package example;

import java.util.concurrent.*;

public class ThreadPoolEx {

    public static void main(String[] args) {
        CustomRejectedExecutionHandler rejectionHandler = new CustomRejectedExecutionHandler();
        ThreadPoolExecutor executorService =
                new ThreadPoolExecutor(1, 1, 1L,
                        TimeUnit.MINUTES,
                        new SynchronousQueue<Runnable>(),
                        rejectionHandler
                );


        Future taskOne = executorService.submit(() -> {
            try {
                System.out.println("taskOne is going to sleep");
                Thread.sleep(2000);
                System.out.println("taskOne is wake up");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });

        Future taskTwo = executorService.submit(() -> System.out.println("task two is working"));
        try {
            System.out.println("task one ,result: "   taskOne.get());
            System.out.println("isTerminating "  executorService.isTerminating());
            System.out.println("getActiveCount "  executorService.getActiveCount());
            System.out.println("is cancelled "   taskTwo.isCancelled());
            System.out.println("is isDone "   taskTwo.isDone());
            System.out.println("task two, result: "   taskTwo.get(1000, TimeUnit.MILLISECONDS));
        } catch (Exception e) {
        }
        executorService.shutdown();
    }
}

class CustomRejectedExecutionHandler implements RejectedExecutionHandler {

    @Override
    public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
        System.out.println(r.toString()   " is rejected");
    }
}

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