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)
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)