Question

Is it that monitor in Java does not restrict access to instance variables and only to the methods which are declared synchronized or code in synchronized statements?

I have created two threads, thread y invokes sync() method, which is declared synchronized while thread r invokes unsync() method which is not declared synchronized. Both invoke methods on shared object s.

Thread r is able to modify the instance variable of object s while the monitor or lock of that object is still being held by the thread y.

Is it that the monitor in Java does not restrict access to instance variables, and only to the methods which are declared synchronized or code in synchronized statements?

public class Stuff {

    private int a = 10;

    public synchronized void sync() {
        long t1 = System.currentTimeMillis();
        System.out.println("Okay, I am in sync() method. "
                        + "I will be waiting for 10 seconds. Current Time = "
                        + System.currentTimeMillis());
        while (System.currentTimeMillis() - t1 < 10000);
        System.out.println("Okay, I have waited for 10 seconds. Current time is "
                        + System.currentTimeMillis()
                        + ". Now I will exit from sync() method, a = " + this.a);
    }

    public void unsync() {
        System.out.println("Alright, I am in unsync() method. The current time is "
                        + System.currentTimeMillis());
        this.a = this.a + 1;
        System.out.println(". The time of exit from unsync() method is "
                        + System.currentTimeMillis());

    }
}

class T1 extends Thread {

    Stuff s;

    public T1(Stuff s) {
        this.s = s;
    }

    public void run() {
        s.sync();
    }
}

class T2 extends Thread {

    Stuff s;

    public T2(Stuff s) {
        this.s = s;
    }

    public void run() {
        s.unsync();
    }
}

class Main {

    public static void main(String args[]) throws Exception {
        Stuff s = new Stuff();
        T1 y = new T1(s);
        T2 r = new T2(s);
        y.start();
        Thread.sleep(2000);
        r.start();
    }
}

The output of the program is below:

 
Okay, I am in sync() method. I will be waiting for 10 seconds. Current Time = 1358801766310  
Alright, I am in unsync() method. The current time is 1358801768343. The time of exit from unsync() method is 1358801768343  
Okay, I have waited for 10 seconds. Current time is 1358801776310. Now I will exit from sync() method, a = 11

Answer 1

Yes. Holding the monitor of an object prevents another thread from executing another block of code or synchronized on the same object. If a method is not synchronized, any thread can call it at any time, whether another thread holds a monitor or not.

Every access to a shared stated, even read-only accessed, must be synchronized if there’s a chance that at least one thread modifies this shared state.