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 <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 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)