[英]how can more than one thread have “locked” on the same object (as shown in a thread-dump)
我有以下线程转储,它显示两个线程都锁定在同一个对象上。 我对它的真正含义感到困惑
"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)
“锁定”在这里意味着什么?
在此上下文中, locked表示您正在运行的java代码已进入synchronous
块但尚未退出该块。
在您的线程转储显示时,您正在调用wait()
,它在内部解锁与synchronous
块关联的监视器。 但是,由于您在等待时阻塞并且未退出同步块,因此线程转储仍显示已locked
。 因此,尽管底层监视器已解锁,但仍有可能在线程转储中locked
多个线程。
通过简单的测试可以很容易地证明这一点:
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,尽管wait
,显示器仍处于locked
状态。
如果单线程情况不令人信服,您可以运行稍微修改的上述示例,在这种情况下,您将看到在线程转储中的同一监视器上locked
了多个线程:
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();
}
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