Locks, monitors, and deadlocks (LOCKS)

An example of the LOCKS component part of a Java™ dump taken during a deadlock.

A lock typically prevents more than one entity from accessing a shared resource. Each object in the Java language has an associated lock, also referred to as a monitor, which a thread obtains by using a synchronized method or block of code. In the case of the JVM, threads compete for various resources in the JVM and locks on Java objects.

When you take a Java dump, the JVM attempts to detect deadlock cycles. The JVM can detect cycles that consist of locks that are obtained through synchronization, locks that extend the java.util.concurrent.locks.AbstractOwnableSynchronizer class, or a mix of both lock types.

The following example is from a deadlock test program where two threads, DeadLockThread 0 and DeadLockThread 1, unsuccessfully attempt to synchronize on a java/lang/String object, and lock an instance of the java.util.concurrent.locks.ReentrantLock class.

The Locks section in the example (highlighted) shows that thread DeadLockThread 1 locked the object instance java/lang/String@0x00007F5E5E18E3D8. The monitor was created as a result of a Java code fragment such as synchronize(aString), and this monitor has DeadLockThread 0 waiting to get a lock on this same object instance (aString). The deadlock section also shows an instance of the java.util.concurrentlocks.ReentrantLock$NonfairSync class, that is locked by DeadLockThread 0, and has Deadlock Thread 1 waiting.

This classic deadlock situation is caused by an error in application design; Javadump is a major tool in the detection of such events.

Blocked thread information is also available in the Threads section of the Java dump, in lines that begin with 3XMTHREADBLOCK, for threads that are blocked, waiting or parked. For more information, see Blocked thread information.

NULL           ------------------------------------------------------------------------
0SECTION       LOCKS subcomponent dump routine
NULL           ===============================
NULL           
1LKPOOLINFO    Monitor pool info:
2LKPOOLTOTAL     Current total number of monitors: 2
NULL           
1LKMONPOOLDUMP Monitor Pool Dump (flat & inflated object-monitors):
2LKMONINUSE      sys_mon_t:0x00007F5E24013F10 infl_mon_t: 0x00007F5E24013F88:
3LKMONOBJECT      java/lang/String@0x00007F5E5E18E3D8: Flat locked by "Deadlock Thread 1" (0x00007F5E84362100), entry count 1
3LKWAITERQ            Waiting to enter:
3LKWAITER                "Deadlock Thread 0" (0x00007F5E8435BD00)
NULL           
1LKREGMONDUMP  JVM System Monitor Dump (registered monitors):
2LKREGMON          Thread global lock (0x00007F5E84004F58): <unowned>
2LKREGMON          &(PPG_mem_mem32_subAllocHeapMem32.monitor) lock (0x00007F5E84005000): <unowned>
2LKREGMON          NLS hash table lock (0x00007F5E840050A8): <unowned>
            < lines removed for brevity >

1LKDEADLOCK    Deadlock detected !!!
NULL           ---------------------
NULL           
2LKDEADLOCKTHR  Thread "Deadlock Thread 0" (0x00007F5E8435BD00)
3LKDEADLOCKWTR    is waiting for:
4LKDEADLOCKMON      sys_mon_t:0x00007F5E24013F10 infl_mon_t: 0x00007F5E24013F88:
4LKDEADLOCKOBJ      java/lang/String@0x00007F5E5E18E3D8
3LKDEADLOCKOWN    which is owned by:
2LKDEADLOCKTHR  Thread "Deadlock Thread 1" (0x00007F5E84362100)
3LKDEADLOCKWTR    which is waiting for:
4LKDEADLOCKOBJ      java/util/concurrent/locks/ReentrantLock$NonfairSync@0x00007F5E7E1464F0
3LKDEADLOCKOWN    which is owned by:
2LKDEADLOCKTHR  Thread "Deadlock Thread 0" (0x00007F5E8435BD00)