Local Variable in Static Inner Class

Question

Suppose I have the following inner class, now method1() is accessed by 2 threads, say thread 1 and thread 2, here we are declaring a local variable and incrementing it. Now this local variable is of primitive data type and will exist on stack, since each Thread has it's own memory stack it should not be shared between the threads, but int local is also a part of a static inner class, so I wanted to know whether int local will be shared between the threads or not?

As in if 2 threads simultaneously call the method1() how will the memory distribution be like?

private static class SharedClass {
        int a = 0;
 
        public void method1() {
            int local = 0;
            local++;
            a=local;
        }     
   }

Answer 1

Local variables are never shared with other threads in Java. It doesn't matter whether we are talking about ordinary methods, or lambdas. It doesn't matter whether the method is declared in a top level class, an inner class, a nested class, an anonymous class or a local classes.

So in your example, the two threads will have their own copies of the local variable, but they may be updating a shared a variable ... depending on which SharedClass instance they are calling the method on.

Or, to put it another way, you don't need to worry about thread safety for local = 0; and local++, but you do for access to a and the a=local; assignment.

---

There some scenario where local variables appear to be shared. However, it is an illusion. Consider this:

public void test() {
    final int arg = 42;
    new Thread(new Runnable(){
        public void run() {
            System.out.println(arg);
        }
    }).start();
}

It looks like the arg variable is accessed by the child thread. But in fact, what the child thread is actually accessing will be a synthetic variable in the Runnable instance whose value has been initialized to the value of arg; i.e. 42.

(If you compile the above code and use javap to examine the bytecodes, you will see how it works.)

Note that this is only allowed by the Java compiler when arg is final or effectively final. If was not final or effectively final, the trick of using a synthetic variable would not work.