Question

I have a doubt on the java synchronization mechanism. Let us take the following piece of code–

  class A {
     int instance=0;
     synchronized void met1() {
       instance=instance +1;
        ....
        instance = instance+2*3;

      }

In the above method met1, we need to make it synchronized in a multi threaded enviroment because it is modifying an instance variable of an object. However in this piece of code —

class A {

     synchronized void met1() {
       local=local +1;
        ....
        local = local+2*3;

      }

The method met1 is modifying a local variable, which I think will be unique for each thread that executes that method. So in this case, when a thread is modifying only a local variable, is it necessary to make the method synchronized ?

Answer 1

In your second example, local is not exactly local, because you are not declaring it inside met1. Probably you meant:

 synchronized void met1() {
   int local = 0;
   // ...
   local=local +1;
    ....
    local = local+2*3;

  }

Then yes, you are correct, since each method call happens on the stack of the calling thread, method-local variables exist only the corresponding thread’s stack and don’t need synchronized access.