Question

Considering the following code snippet:

 Object bar1  = new ... ; 
 Object bar2 = new ... ; 


 Object foo = (either bar1 or bar2) ; 

Now, foo can be either bar1 or bar2 at various times of the programs. I just wanted to check that synchronized(foo) will lock the corresponding bar1 or bar2. This seems like the most likely scenario given that objects aren’t copied in Java. Is this correct?

Answer 1

Object bar1 = new Object();
Object foo = bar1;
synchronized(foo) {
    ...
}

will lock on foo==bar1.

However this is a weird and error-prone construct. For example:

• Thread 1 arrives in your method and foo == bar1
• Thread 1 arrives in the synchronized block and locks on bar1
• Thread 2 arrives in your method and foo == bar2
• Thread 2 arrives in the synchronized block and locks on bar2

Now you have 2 threads running your synchronized block concurrently. I can’t really find a reason why you would want that. And if you do, then the block should probably not be synchronized and you should use a different locking strategy.

See also this related post.