Preempting a thread that is executing synchronized method

Question

Give the following code

    class Test{
       double x;
       public void synchronized a()
       { 
          x = 0;
          //do some more stuff
       }
       public void b() 
       { 
          x = -1; 
       } 
    }

Can the thread in a(), in the middle of modifying x be preempted by a thread that calls b() on the same object?

Isn't synchronized method be executed like one single atomic operation?

I believe the other way is possible(thread in b() can be preempted by the thread that calls a() on the same object since b() is not guarded my the Test object lock).

Can some one shed some light on this?

Answer 1

synchronized only stops other threads from acquiring the same monitor. It in no way makes the operation atomic. In particular:

• Side-effects of the method can be observed by other threads which aren't trying to synchronize on the same monitor
• If an exception occurs, there's no sort of roll-back
• Other threads can access and modify the same data used by the synchronized method, if they aren't synchronized on the same monitor

b() isn't synchronized, so it's entirely possible for one thread to be executing a() and another to be executing b() at the same time.