jstack - Detecting Java Thread Deadlocks

Java Tools Tutorials - Herong's Tutorial Examples

∟jstack - Detecting Java Thread Deadlocks

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:

herong> jps -l -m
16308 jdk.jcmd/sun.tools.jps.Jps -l -m
12888 SimpleDeadLock

herong> jstack 12888

Full thread dump Java HotSpot(TM) 64-Bit Server VM (10.0.1+10 mixed mode):
Threads class SMR info:
_java_thread_list=0x0000000024138f00, length=13, elements={...
}
...

"Thread-0" #12 prio=5 os_prio=0 tid=0x000000002413e800 nid=0x30e0
   waiting for monitor entry  [0x0000000024a4e000]
   java.lang.Thread.State: BLOCKED (on object monitor)
  at SimpleDeadLock$Thread1.run(SimpleDeadLock.java:23)
  - waiting to lock <0x00000000c7a49f40> (a java.lang.Object)
  - locked <0x00000000c7a49f30> (a java.lang.Object)

"Thread-1" #13 prio=5 os_prio=0 tid=0x0000000024141800 nid=0x1cc8
   waiting for monitor entry  [0x0000000023f4f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
  at SimpleDeadLock$Thread2.run(SimpleDeadLock.java:36)
  - waiting to lock <0x00000000c7a49f30> (a java.lang.Object)
  - locked <0x00000000c7a49f40> (a java.lang.Object)

...

"G1 Young RemSet Sampling" os_prio=2 tid=0x0000000022077800
   nid=0x401c runnable
"VM Periodic Task Thread" os_prio=2 tid=0x000000002413a000
   nid=0x1c40 waiting on condition

JNI global references: 2

Found one Java-level deadlock:
=============================
"Thread-0":
  waiting to lock monitor 0x000000002334a280 (object 0x00000000c7a49f40,
     a java.lang.Object),
  which is held by "Thread-1"
"Thread-1":
  waiting to lock monitor 0x000000002334a480 (object 0x00000000c7a49f30,
     a java.lang.Object),
  which is held by "Thread-0"

Java stack information for the threads listed above:
===================================================
"Thread-0":
  at SimpleDeadLock$Thread1.run(SimpleDeadLock.java:23)
  - waiting to lock <0x00000000c7a49f40> (a java.lang.Object)
  - locked <0x00000000c7a49f30> (a java.lang.Object)
"Thread-1":
  at SimpleDeadLock$Thread2.run(SimpleDeadLock.java:36)
  - waiting to lock <0x00000000c7a49f30> (a java.lang.Object)
  - locked <0x00000000c7a49f40> (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 waiting threads for each deadlock.
Stack traces of waiting threads with source code line numbers, if source codes were compile with debug information.