Synchronize on object

Question

What is the problem in this case:

private static final Integer LOCK=0;
synchronized(LOCK){

//work
}

This is advised to be not a recommended monitor to lock on.

Answer 1

This is advised to be not a recommended monitor to lock on.

private static final Integer LOCK=0;

is the same as

private static final Integer LOCK=Integer.valueOf(0);

and Integer.valueOf(0) is cached by specification .

What this means is that anybody else can also get Integer.valueOf(0) and synchronize on it; so you might get unexpected contention (at best) or deadlocks (at worst).

You can either use new Integer(0) , which is not a cached instance; or just use new Object() , since the fact it's an Integer is irrelevant if it is only used as a monitor.