Question

Given that a static variable of a class takes only one memory location, is it shared by all threads of process? Or is one memory location for such a variable created for each of the running threads?

Also, if all threads share the same memory location, how can we ensure mutual exclusion?

Answer 1

A static variable of a class in a process will be shared between every thread contained in that process.

You can verify this by creating a simple class with a public static field, and then start up a couple of Threads and have them increment the variable and see what happens.

If you want to ensure mutual exclusion you can make the variable private, and only allow access to it through methods that are defined using the synchronized keyword.

class Foo {

    private static int aVariable = 0;

    public static synchronized void increment() { aVariable++; }
    public static synchronized int getVariable() { return aVariable; }

}