Thread safety in Java class

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.

Answer 2

With multiple processors, some values may be cached by the processor and may not reflect the changes made by other threads/processors for the same objects. Actually, JVM may be implemented to work this way even with a single processor.

Synchronized methods are explicitly required by language specification to present a memory barrier and require reread of all instance variables from the memory.

Because your code is not synchronized, one thread may set the value, but the other thread will return the value still cached by that thread.

Please read 'Memory and Locks' chapter of Java Language Specification.