synchronize section does not (always) block!

[Eduard Wirch]

Hi


I've noticed something very strange yesterday. It seems that in app two threads are entering two synchronized blocks locking on the same object at the same time.

The class (MyClass) containing the relevant code looks similar to this:

------------------------------------------------------------------
private static int[]    myLock  = new int[0];

protected static int methodA(final long handle, final byte[] sort) {
    synchronized (myLock) {
        return xsMethodA(handle, sort);
    }
}

protected static int methodB(final long handle) {
    synchronized (myLock) {
        return xsMethodB(handle);
    }
}
------------------------------------------------------------------



I created a thread dump of my application running the above class and was very surprised as I saw this:

------------------------------------------------------------------
"http-8080-136" daemon prio=10 tid=0x00000000447df000 nid=0x70ed waiting for monitor entry [0x00007fd862aea000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at com.MyClass.methodA(MyClass.java:750)
    - locked <0x00007fd8a6b8c790> (a [I)
    at com.SomeOtherClass.otherMethod(SomeOtherClass.java:226)
    ...

"http-8080-111" daemon prio=10 tid=0x00007fd87d1a0000 nid=0x70c8 waiting for monitor entry [0x00007fd86e15f000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at com.MyClass.methodB(MyClass.java:991)
    - locked <0x00007fd8a6b8c790> (a [I)
    at com.SomeOtherClass.yetAnotherMethod(SomeOtherClass.java:3231)
    ...

"http-8080-146" daemon prio=10 tid=0x00007fd786dab000 nid=0x184b waiting for monitor entry [0x00007fd8393b6000]
   java.lang.Thread.State: BLOCKED (on object monitor)
    at com.MyClass.methodC(MyClass.java:750)
    - waiting to lock <0x00007fd8a6b8c790> (a [I)
    at com.SomeOtherClass.yetAnoterMethod2(SomeOtherClass.java:226)
    ...
(43 more threads waiting for the lock)
------------------------------------------------------------------

(I changed the class and method names for the case of simplicity, so don't get confused by the silly names.)


It seems that thread http-8080-136 and http-8080-111 have both acquired the lock on myLock. It is the same object as the object address is the same: 0x00007fd8a6b8c790.

The Java Runtime Specification says this about the synchronized keyword:
------------------------------------------------------------------
A synchronized statement acquires a mutual-exclusion lock (§17.1) on behalf of the executing thread, executes a block, then releases the lock. While the executing thread owns the lock, no other thread may acquire the lock. [The Java Language Specification, 14.19]
------------------------------------------------------------------

So how is this even possible?


Regards,
Eduard