concurrency - Synchronizing on an object in java, then changing the value of the synchronized-on variable

Question

I came across a code like this

synchronized(obj) {

   obj = new Object();

}

Something does not feel right about this , I am unable to explain, Is this piece of code OK or there is something really wrong in it, please point it out. Thanks

Answer 1

It's probably not what you want to do. You're synchronizing on an object that you're no longer holding a reference to. Consider another thread running this method: they may enter and try to hit the lock at the moment after the reference to obj has been updated to point to the new object. At that point, they're synchronizing on a different object than the first thread. This is probably not what you're expecting.

Unless you have a good reason not to, you probably want to synchronize on a final Object (for visibility's sake.) In this case, you would probably want to use a separate lock variable. For example:

class Foo
{
    private final Object lock = new Object();
    private Object obj;

    public void method()
    {
        synchronized(lock)
        {
            obj = new Object();
        }
    }
}