Java thread dump: Difference between "waiting to lock" and "parking to wait for"?

Nicolas Raoul picture Nicolas Raoul · Jul 5, 2012 · Viewed 22.9k times · Source

In a Java thread dump, you can see locks mentioned within stack traces.
There seems to be three kinds of information:

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: The thread has obtained a lock on object 0x00002aab329f7fa0.
  • 2&3: Seem to say that the thread is waiting for the lock on said object to become available...
    but what is the difference 2 and 3?

Answer

rgerganov picture rgerganov · Jul 5, 2012

You will get "waiting to lock" in the thread dump when using intrinsic locks and "parking to wait for" when using locks from java.util.concurrent. Consider the following example:

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();
    }

}

With lockTest.intrinsicLock() you will get the following thread dump:

"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)

while lockTest.reentrantLock() produce:

"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)