Question

Why is this java class not Thread safe.

class TestClass {  
   private int x;

   int get() {
       return x;
   }

   void set(int x) {
       this.x = x;
   }  
}

I read that keyword synchronized is needed to make it thread safe? After all isn’t the operations done inside atomic?

Answer 1

Although the assignment itself is an atomic operation, due to different hardware and compiler implementations, different threads may see different values of the member x. I.e., a modification by one thread may be invisible to the other thread, because of some kind of caching. This is usually called a thread visibility problem.

You can synchronize your code properly either by synchronizing on a monitor (using the synchronized keyword or the java.util.concurrent locks), or by declaring x to be volatile.