java local variable synchronization

Question

I have problem with following java code, this program always give "KKBB" as the output (so it seems like synchronization works ), So i am unable to understand since i is a local variable why synchronization is working here?

class Test implements Runnable {
    public void run() {
        Integer i=10;
        synchronized(i)
        {
            try {
                System.out.print(Thread.currentThread().getName());
                Thread.sleep(1200);
                System.out.print(Thread.currentThread().getName());
            } catch (InterruptedException e) {
            }
        }
    }

    public static void main(String[] args) {
        new Thread(new Test(), "K").start();
        new Thread(new Test(), "B").start();
    }
}

I heard that since local variables have different copies for each methods, so synchronization won't work, please help me to understand, thanks

Re, "since i is a local variable..." The synchronized(i) statement does not operate on the variable, it operates on the object to which the variable refers. As @chrylis pointed out, Integer.valueOf(10) always refers to the same cached Integer object. — Solomon Slow
– Solomon Slow, Commented Aug 15, 2014 at 13:50

chrylis -cautiouslyoptimistic- · Accepted Answer · 2014-08-15 01:35:42Z

8

The wrapper classes have special behavior for small values. If you use Integer.valueOf() (or Short, Char, or Byte) for a value between -128 and 127, you'll get a shared cached instance.

The autoboxing treats

Integer i = 10;

as

Integer i = Integer.valueOf(10);

so the different i variables are actually referring to the same instance of Integer and thus share a monitor.

answered Aug 15, 2014 at 1:35

chrylis -cautiouslyoptimistic-

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Right -- you don't synchronize on the variable, you synchronize on the object.

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