Java线程转储:“等待锁定”和“停车等待”之间的区别?

2022-08-31 21:02:31

在 Java 线程转储中,您可以看到堆栈跟踪中提到的锁。
似乎有三种信息:

1:

- locked <0x00002aab329f7fa0> (a java.io.BufferedInputStream)

2:

- waiting to lock <0x00002aaaf4ff6fa0> (a org.alfresco.repo.lock.LockServiceImpl)

3:

- parking to wait for  <0x00002aaafbf70bb8> (a java.util.concurrent.SynchronousQueue$TransferStack)
  • 1:线程已获得对象0x00002aab329f7fa0的锁定。
  • 2&3:似乎在说线程正在等待所述对象上的锁定变为可用...
    但是2和3的区别是什么?

答案 1

使用固有锁时,您将在线程转储中得到“等待锁定”,在使用 java.util.concurrent 中的锁时,您将在线程转储中获得“等待等待”。请考虑以下示例:

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class LockTest {

    final Lock lock = new ReentrantLock(true);

    synchronized void intrinsicLock() {
        Thread th = new Thread(new Runnable() {
            public void run() {
                intrinsicLock();
            }
        }, "My thread");
        th.start();
        try {
            th.join();
        } catch (InterruptedException e) {
        }
    }

    void reentrantLock() {
        lock.lock();
        Thread th = new Thread(new Runnable() {
            public void run() {
                reentrantLock();
            }
        }, "My thread");
        th.start();
        try {
            th.join();
        } catch (InterruptedException e) {
        }
        lock.unlock();
    }


    public static void main(String[] args) {
        LockTest lockTest = new LockTest();
        lockTest.intrinsicLock();
        //lockTest.reentrantLock();
    }

}

您将获得以下线程转储:lockTest.intrinsicLock()

"My thread" prio=10 tid=0x00007fffec015800 nid=0x1775 waiting for monitor entry [0x00007ffff15e5000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at LockTest.intrinsicLock(LockTest.java:9)
    - waiting to lock <0x00000007d6a33b10> (a LockTest)
    at LockTest$1.run(LockTest.java:11)
    at java.lang.Thread.run(Thread.java:662)

同时生产:lockTest.reentrantLock()

"My thread" prio=10 tid=0x00007fffec082800 nid=0x17e8 waiting on condition [0x00007ffff14eb000]
   java.lang.Thread.State: WAITING (parking)
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <0x00000007d6a33d30> (a java.util.concurrent.locks.ReentrantLock$FairSync)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:156)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:811)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:842)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1178)
    at java.util.concurrent.locks.ReentrantLock$FairSync.lock(ReentrantLock.java:201)
    at java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:262)
    at LockTest.reentrantLock(LockTest.java:22)
    at LockTest$2.run(LockTest.java:25)
    at java.lang.Thread.run(Thread.java:662)

答案 2

在我看来,java.util.concurrent包几乎使用LockSupport.park()方法来阻塞线程,例如属于抽象队列同步框架的CountDownLatch,ReentrantLock。因此,问题中的第三个场景意味着你的代码最终调用了LockSupport.park()方法,对于您在java.util.concurrent包中使用并发类,第二个场景意味着您使用同步的keywork并显式调用wait()方法。