Java synchronization/locking

Question

I'm relatively new to concurrency in Java (still have yet to read JCIP, but it's on my list!) and I have a question regarding locking behavior. Specifically, does Java lock on the reference of an object, or on the object itself?

Code example (not an sscce because I don't know how to demo this behavior in practice):

static final Object lockA = new Object();

public void method1() {
    synchronized(lockA) {
        // do stuff here
    }
}

public void method2() {
    Object lockB = lockA;
    synchronized(lockB) {
        // do stuff
    }
}

If another thread is executing method1() (and therefore has a lock on lockA), will method2() be allowed to execute?

Thanks!

Jack Edmonds · Accepted Answer · 2011-11-07 16:13:42Z

6

Synchronization is done on the object, so the synchronized block in method2 will need to wait for the synchronized block in method1 to finish.

Each object has an "intrinsic lock" associated with it (see Intrinsic Locks and Synchronization). The synchronized block uses the intrinsic lock associated with the object on which you are synchronizing.

edited Nov 7, 2011 at 16:13

answered Nov 7, 2011 at 16:06

Jack Edmonds

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NPE · Accepted Answer · 2011-11-07 16:05:54Z

3

No, method2() will block since the two methods are synchronizing on the same object (lockB is merely a reference pointing to the same object as lockA).

answered Nov 7, 2011 at 16:05

NPE

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