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

Question

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

1:

- locked <0x00002aab329f7         


        

Answer 1

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)