为什么ScheduledExecutorService.shutdown()使用100%的CPU？

Question

我有以下简单的代码:

package main;

import java.util.concurrent.*;

public class Main {

    public static void main(String[] args) throws InterruptedException {
        new Main();
    }

    public Main() throws InterruptedException {
        ScheduledExecutorService executor = Executors.newScheduledThreadPool(1);
        executor.schedule(new MyRunnable(), 10, TimeUnit.SECONDS);
        System.out.println("Shutting down...");
        executor.shutdown();
        System.out.println("Awaiting termination...");
        executor.awaitTermination(Long.MAX_VALUE, TimeUnit.MINUTES);
        System.out.println("Main finished!");
    }

    private class MyRunnable implements Runnable {
        public void run() {
            System.out.println("Finished running!");
        }
    }

}

实际上,虽然我的真实代码比这更复杂,但我可以在这些方面隔离问题.代码基本上等待10秒运行runnable,然后通知主程序的结束.

但是,我注意到了10秒钟,我的核心之一用于100%.

如果我评论这一行:

executor.awaitTermination(Long.MAX_VALUE, TimeUnit.MINUTES);

cpu核心也以100%使用,主程序也在Runnable之前完成.

如果我评论这一行:

executor.shutdown();

cpu已正确使用但程序无法完成.

如果我评论前两行,则cpu正确使用但主程序无法完成.

1. 我的代码有问题吗？
2. 正在executor.shutdown(); 做某种忙碌的等待而不是仅仅禁用提交新任务？
3. 或者我应该责怪JVM？

附加细节:

$ java -version
java version "1.6.0_26"
Java(TM) SE Runtime Environment (build 1.6.0_26-b03)
Java HotSpot(TM) Server VM (build 20.1-b02, mixed mode)

$ uname -a
Linux XPSG 2.6.32-5-686-bigmem #1 SMP Sun May 6 04:39:05 UTC 2012 i686 GNU/Linux

PS:拜托,请不要让我使用CountDownLatch也不是newSingleThreadScheduledExecutor.这跟我问的问题无关.谢谢.

编辑:

这是java转储:

Full thread dump Java HotSpot(TM) Server VM (20.1-b02 mixed mode):

"pool-1-thread-1" prio=10 tid=0x08780c00 nid=0x32ee runnable [0x6fdcc000]
   java.lang.Thread.State: RUNNABLE
    at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:943)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:907)
    at java.lang.Thread.run(Thread.java:662)

"Low Memory Detector" daemon prio=10 tid=0x0874dc00 nid=0x32ec runnable [0x00000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread1" daemon prio=10 tid=0x0874c000 nid=0x32eb waiting on condition [0x00000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread0" daemon prio=10 tid=0x0874a000 nid=0x32ea waiting on condition [0x00000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" daemon prio=10 tid=0x08748800 nid=0x32e9 waiting on condition [0x00000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" daemon prio=10 tid=0x0873a000 nid=0x32e8 in Object.wait() [0x70360000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x9e8f1150> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:118)
    - locked <0x9e8f1150> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:134)
    at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:159)

"Reference Handler" daemon prio=10 tid=0x08735400 nid=0x32e7 in Object.wait() [0x703b1000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x9e8f1050> (a java.lang.ref.Reference$Lock)
    at java.lang.Object.wait(Object.java:485)
    at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:116)
    - locked <0x9e8f1050> (a java.lang.ref.Reference$Lock)

"main" prio=10 tid=0x086b5c00 nid=0x32e3 waiting on condition [0xb6927000]
   java.lang.Thread.State: TIMED_WAITING (parking)
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <0x9e958998> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
    at java.util.concurrent.locks.LockSupport.parkNanos(LockSupport.java:198)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(AbstractQueuedSynchronizer.java:2025)
    at java.util.concurrent.ThreadPoolExecutor.awaitTermination(ThreadPoolExecutor.java:1253)
    at main.Main.<init>(Main.java:19)
    at main.Main.main(Main.java:10)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at org.eclipse.jdt.internal.jarinjarloader.JarRsrcLoader.main(JarRsrcLoader.java:58)

"VM Thread" prio=10 tid=0x08731800 nid=0x32e6 runnable 

"GC task thread#0 (ParallelGC)" prio=10 tid=0x086bd000 nid=0x32e4 runnable 

"GC task thread#1 (ParallelGC)" prio=10 tid=0x086be400 nid=0x32e5 runnable 

"VM Periodic Task Thread" prio=10 tid=0x0874fc00 nid=0x32ed waiting on condition 

JNI global references: 931

Heap
 PSYoungGen      total 18752K, used 645K [0x9e8f0000, 0x9fdd0000, 0xb3790000)
  eden space 16128K, 4% used [0x9e8f0000,0x9e991510,0x9f8b0000)
  from space 2624K, 0% used [0x9fb40000,0x9fb40000,0x9fdd0000)
  to   space 2624K, 0% used [0x9f8b0000,0x9f8b0000,0x9fb40000)
 PSOldGen        total 42880K, used 0K [0x74b90000, 0x77570000, 0x9e8f0000)
  object space 42880K, 0% used [0x74b90000,0x74b90000,0x77570000)
 PSPermGen       total 16384K, used 2216K [0x70b90000, 0x71b90000, 0x74b90000)
  object space 16384K, 13% used [0x70b90000,0x70dba198,0x71b90000)

Answer 1

事实上，它正忙于等待。ThreadPoolExecutor 似乎没有退避逻辑来等待所有任务完成（请注意，这仅在您完成时才会发生shutdown()，否则它将正确挂起线程）。

它不断检查任务是否准备好执行，如果没有，它将重试，直到调度任务的时间过去。

关闭计划线程池需要进行权衡（这种权衡是由实现强加的）。它会忙于旋转，直到任务准备好调度或shutdownNow不会执行任何排队的任务。但是，您可以获取返回的 Runnable 列表并自己执行它们。