This section provides a tutorial example on how to detect Java thread deadlocks with the thread stack trace dump tool, 'jstack'.
With the deadlock demo program, SimpleDeadLock.java, running in a locked status as described in the previous section,
I am ready to run "jstack" to print the deadlock information and stack traces of 2 locked threads:
C:\herong>\Progra~1\java\jdk1.6.0_02\bin\jps -l -m
1464 SimpleDeadLock
464 sun.tools.jps.Jps -l -m
C:\herong>\Progra~1\java\jdk1.6.0_02\bin\jstack 1464
Full thread dump Java HotSpot(TM) Client VM (1.6.0_02-b06 mixed mod...
"DestroyJavaVM" prio=6 tid=0x00296000 nid=0x198 waiting on conditio...
java.lang.Thread.State: RUNNABLE
"Thread-1" prio=6 tid=0x02a99c00 nid=0xee0 waiting for monitor entr...
java.lang.Thread.State: BLOCKED (on object monitor)
at SimpleDeadLock$Thread2.run(SimpleDeadLock.java:37)
- waiting to lock <0x229bd238> (a java.lang.Object)
- locked <0x229bd240> (a java.lang.Object)
"Thread-0" prio=6 tid=0x02a99000 nid=0xefc waiting for monitor entr...
java.lang.Thread.State: BLOCKED (on object monitor)
at SimpleDeadLock$Thread1.run(SimpleDeadLock.java:24)
- waiting to lock <0x229bd240> (a java.lang.Object)
- locked <0x229bd238> (a java.lang.Object)
"Low Memory Detector" daemon prio=6 tid=0x02a7c800 nid=0xd2c runnab...
java.lang.Thread.State: RUNNABLE
"CompilerThread0" daemon prio=10 tid=0x02a78000 nid=0x500 waiting o...
java.lang.Thread.State: RUNNABLE
"Attach Listener" daemon prio=10 tid=0x02a76c00 nid=0x32c waiting o...
java.lang.Thread.State: RUNNABLE
"Signal Dispatcher" daemon prio=10 tid=0x02a75c00 nid=0x190 runnabl...
java.lang.Thread.State: RUNNABLE
"Finalizer" daemon prio=8 tid=0x02a6e000 nid=0xdb0 in Object.wait()...
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x22990b38> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:116)
- locked <0x22990b38> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:132)
at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:159)
"Reference Handler" daemon prio=10 tid=0x02a6d000 nid=0xa44 in Obje...
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x22990a38> (a java.lang.ref.Reference$Lock)
at java.lang.Object.wait(Object.java:485)
at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:116)
- locked <0x22990a38> (a java.lang.ref.Reference$Lock)
"VM Thread" prio=10 tid=0x02a63c00 nid=0xe88 runnable
"VM Periodic Task Thread" prio=10 tid=0x02a7e000 nid=0xf58 waiting ...
JNI global references: 571
Found one Java-level deadlock:
=============================
"Thread-1":
waiting to lock monitor 0x02a6ee64 (object 0x229bd238,
a java.lang.Object), which is held by "Thread-0"
"Thread-0":
waiting to lock monitor 0x02a6eecc (object 0x229bd240,
a java.lang.Object), which is held by "Thread-1"
Java stack information for the threads listed above:
===================================================
"Thread-1":
at SimpleDeadLock$Thread2.run(SimpleDeadLock.java:37)
- waiting to lock <0x229bd238> (a java.lang.Object)
- locked <0x229bd240> (a java.lang.Object)
"Thread-0":
at SimpleDeadLock$Thread1.run(SimpleDeadLock.java:24)
- waiting to lock <0x229bd240> (a java.lang.Object)
- locked <0x229bd238> (a java.lang.Object)
Found 1 deadlock.
The output of "jstack" is very useful for debugging. It tells me:
How many deadlocks exist in this JVM process.
What are the 2 waiting threads for each deadlock.
Stack traces of waiting threads with source code line numbers, if source codes were compile with debug options.
