Is a deadlock possible in this method? How can I prevent it?

Question

public void function(object a, object b){
    synchronized(a){
        synchronized (b){
           a.performAction(b);
           b.performAction(a);
        }
    }
}

Deadlock with 2 Threads? Thanks for the answers!

Answer 1

Sure,

Suppose we have two objects,

Object one = ...;
Object two = ...;

And suppose thread 1 calls:

function(one, two);

While thread 2 calls:

function(two, one);

In thread 1, a == one and b == two, but in thread 2, a == two and b == one.

So while thread 1 is obtaining a lock on object one, thread 2 can be obtaining the lock on object two. Then when each of the threads tries to take the next step, they will be deadlocked.