5

I have the following thread-dump, which shows two threads both locking on the same object. And I'm confused as to what it really means

    "pool-1-thread-2" prio=10 tid=0x00007fd6dc106000 nid=0x5d15 in Object.wait() [0x00007fd6d2067000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x00000007c3547770> (a java.lang.Object)
    at java.lang.Object.wait(Object.java:503)
    at test.TestDead$Foo.second(TestDead.java:22)
    at test.TestDead$Foo.first(TestDead.java:14)
    - locked <0x00000007c3547770> (a java.lang.Object)
    at test.TestDead$2.run(TestDead.java:45)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
    at java.util.concurrent.FutureTask.run(FutureTask.java:262)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    at java.lang.Thread.run(Thread.java:724)

   Locked ownable synchronizers:
    - <0x00000007c35519e8> (a java.util.concurrent.ThreadPoolExecutor$Worker)

"pool-1-thread-1" prio=10 tid=0x00007fd6dc104800 nid=0x5d14 in Object.wait() [0x00007fd6d2168000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x00000007c3547770> (a java.lang.Object)
    at java.lang.Object.wait(Object.java:503)
    at test.TestDead$Foo.second(TestDead.java:22)
    at test.TestDead$Foo.first(TestDead.java:14)
    - locked <0x00000007c3547770> (a java.lang.Object)
    at test.TestDead$1.run(TestDead.java:37)
    at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
    at java.util.concurrent.FutureTask.run(FutureTask.java:262)
    atjava.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
    atjava.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
    at java.lang.Thread.run(Thread.java:724)

   Locked ownable synchronizers:
    - <0x00000007c3551730> (a java.util.concurrent.ThreadPoolExecutor$Worker)

What does "locked" really mean here?

4

1 回答 1

5

在这种情况下，locked意味着您正在运行的 java 代码已进入一个synchronous块但尚未退出该块。

正如您的线程转储所示，您正在调用wait()which 在内部synchronous解锁与块关联的监视器。但是，由于您在等待时阻塞并且尚未退出同步块，因此线程转储仍然显示locked. locked因此，尽管底层监视器已解锁，但线程转储中仍可能显示多个线程。

这可以通过一个简单的测试很容易地证明：

public class TestMonitor {

    synchronized public void lockAndWait() {
        try {
            wait();
        } catch ( InterruptedException ex ) {
            // Stifle
        }
    }

    public static void main( String args[] ) {
        TestMonitor tm = new TestMonitor();
        tm.lockAndWait();
    }
}

运行时输出以下线程转储：

"main" prio=10 tid=0x00007f86c4008000 nid=0x5d35 in Object.wait() [0x00007f86cbae2000] java.lang.Thread.State: WAITING (on object monitor) at java.lang.Object.wait(Native Method) - waiting on <0x0000000759055df8> (a TestMonitor) at java.lang.Object.wait(Object.java:503) at TestMonitor.lockAndWait(TestMonitor.java:5) - locked <0x0000000759055df8> (a TestMonitor) at TestMonitor.main(TestMonitor.java:13

请注意，locked尽管处于wait.

更新

如果单线程情况不令人信服，您可以运行上面稍作修改的示例，在这种情况下，您将locked在线程转储中的同一监视器上看到多个线程：

public static void main( String args[] ) {
    final TestMonitor tm = new TestMonitor();

    Thread thread1 = new Thread( new Runnable() { public void run() { tm.lockAndWait(); } } );
    Thread thread2 = new Thread( new Runnable() { public void run() { tm.lockAndWait(); } } );
    thread1.start();
    thread2.start();
}
于 2015-05-07T22:11:08.720 回答