Question

What is the difference between a Collections.synchronizedMap() and a wrapper around a HashMap with all the methods synchronized. I dont see any difference becuase Collections.synchronizedMap() internally maintains the same lock for all methods.

Basically, what is the difference between the following code snippets

Class C {    
    Object o;

    public void foo() {
       synchronized(o) {
           // thread safe code here
       }
    }
}

and

Class C {
    Object o;

    public synchronized void foo() {

    }
}

Answer 1

There is only one difference:

Collections.synchronizedMap is able to use a different monitor than itself.

Using synchronized methods is the same as using sychnchonized(this)-blocks, which means, the wrapper would be the monitor and could be locked from the outside of the wrapper.

If you doesn’t want an outside application to lock your monitor, you need to hide it.

On the other side, if you want to call multiple methods in a thread safe fashion, it is the easiest way to lock the whole collection (but it’s not very scaleable, indeed).

Ps: For reuse, it’s better to delegate the method calls to a backup-Map than to override the class, because you can switch to another Map implementation later, without changing your wrapper.