Multiple Java threads seemingly locking same monitor?

Question

In a Java threaddump I found the following:

"TP-Processor184" daemon prio=10 tid=0x00007f2a7c056800 nid=0x47e7 waiting for monitor entry [0x00007f2a21278000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at org.apache.jackrabbit.core.state.SharedItemStateManager.getNonVirtualItemState(SharedItemStateManager.java:1725)
    - locked <0x0000000682f99d98> (a org.apache.jackrabbit.core.state.SharedItemStateManager)
    at org.apache.jackrabbit.core.state.SharedItemStateManager.getItemState(SharedItemStateManager.java:257)

"TP-Processor137" daemon prio=10 tid=0x00007f2a7c00f800 nid=0x4131 waiting for monitor entry [0x00007f2a1ace7000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at org.apache.jackrabbit.core.state.SharedItemStateManager.getNonVirtualItemState(SharedItemStateManager.java:1725)
    - locked <0x0000000682f99d98> (a org.apache.jackrabbit.core.state.SharedItemStateManager)
    at org.apache.jackrabbit.core.state.SharedItemStateManager.getItemState(SharedItemStateManager.java:257)

The point here being that both threads have locked monitor <0x0000000682f99d98> (regardless of them now waiting for two different other monitors).

When looking at Thread Dump Analyzer, with that monitor being selected, it really says "Threads locking monitor: 2" at the bottom, and "2 Thread(s) locking". Please see https://lh4.googleusercontent.com/-fCmlnohVqE0/T1D5lcPerZI/AAAAAAAAD2c/vAHcDiGOoMo/s971/locked_by_two_threads_3.png for the screenshot, I'm not allowed to paste images here.

Does this mean threaddumps aren't atomic with respect to monitor lock information? I can't imagine this really being a locking bug of the JVM (1.6.0_26-b03).

A similar question has already been asked in Can several threads hold a lock on the same monitor in Java?, but the answer to me didn't see the real point of multiple threads locking the same monitor, even though they may be waiting for some other.

Update May 13th 2014:

Newer question Multiple threads hold the same lock? has code to reproduce the behaviour, and @rsxg has filed an according bug report https://bugs.openjdk.java.net/browse/JDK-8036823 along the lines of his answer here.

Answer 1

I don't think that your thread dump is saying that your two threads are "waiting for two different other monitors". I think it is saying that they are both waiting on the same monitor but at two different code points. That may be a stack location or an object instance location or something. This is a great document about analyzing the stack dumps.

Can several threads hold a lock on the same monitor in Java?

No. Your stack dump is showing two threads locked on the same monitor at the same code location but in different stack frames -- or whatever that value is which seems OS dependent.

Edit:

I'm not sure why the thread dump seems to be saying that both threads have a line locked since that seems to only be allowed if they are in a wait() method. I noticed that you are linking to version 1.6.5. Is that really the version you are using? In version 2.3.6 (which may be the latest), the 1725 line actually is a wait.

1722        synchronized (this) {
1723            while (currentlyLoading.contains(id)) {
1724                try {
1725                    wait();
1726                } catch (InterruptedException e) {

---

You could also see this sort of stack trace even if it was an exclusive synchronized lock. For example, the following stack dump under Linux is for two threads locked on the same object from the same code line but in two different instances of the Runnable.run() method. Here's my stupid little test program. Notice that the monitor entry numbers are different, even thought it is the same lock and same code line number.

"Thread-1" prio=10 tid=0x00002aab34055c00 nid=0x4874
  waiting for monitor entry [0x0000000041017000..0x0000000041017d90]
java.lang.Thread.State: BLOCKED (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x00002aab072a1318> (a java.lang.Object)
    at com.mprew.be.service.auto.freecause.Foo$OurRunnable.run(Foo.java:38)
    - locked <0x00002aab072a1318> (a java.lang.Object)
    at java.lang.Thread.run(Thread.java:619)

"Thread-0" prio=10 tid=0x00002aab34054c00 nid=0x4873
  waiting for monitor entry [0x0000000040f16000..0x0000000040f16d10]
java.lang.Thread.State: BLOCKED (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <0x00002aab072a1318> (a java.lang.Object)
    at com.mprew.be.service.auto.freecause.Foo$OurRunnable.run(Foo.java:38)
    - locked <0x00002aab072a1318> (a java.lang.Object)
    at java.lang.Thread.run(Thread.java:619)

On my Mac, the format is different but again the number after the "monitor entry" is not the same for the same line number.

"Thread-2" prio=5 tid=7f8b9c00d000 nid=0x109622000
  waiting for monitor entry [109621000]
java.lang.Thread.State: BLOCKED (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <7f3192fb0> (a java.lang.Object)
    at com.mprew.be.service.auto.freecause.Foo$OurRunnable.run(Foo.java:38)
    - locked <7f3192fb0> (a java.lang.Object)

"Thread-1" prio=5 tid=7f8b9f80d800 nid=0x10951f000
  waiting for monitor entry [10951e000]
java.lang.Thread.State: BLOCKED (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <7f3192fb0> (a java.lang.Object)
    at com.mprew.be.service.auto.freecause.Foo$OurRunnable.run(Foo.java:38)
    - locked <7f3192fb0> (a java.lang.Object)

This Oracle document describe that value as the following:

Address range, which gives an estimate of the valid stack region for the thread