Question

I wrote this program to check if a thread t1 holding lock on two different objects :
Lock.class and MyThread.class goes into waiting mode on MyThread.class instance using MyThread.class.wait().It does not release lock on Lock.class instance. why so ? I have been thinking that once a thread goes into wait mode or it dies it releases all the acquired locks.

public class Lock {

protected static volatile boolean STOP = true;
public static void main(String[] args) throws InterruptedException {
    MyThread myThread = new MyThread();
    Thread t1 = new Thread(myThread);
    t1.start();
    while(STOP){
    }
    System.out.println("After while loop");
    /*
     * 
     */
    Thread.sleep(1000*60*2);
    /*
     * Main thread should be Blocked.
     */
    System.out.println("now calling Check()-> perhaps i would be blocked. t1 is holding lock on class instance.");
    check();
}

public static synchronized void check(){
    System.out.println("inside Lock.check()");
    String threadName = Thread.currentThread().getName();
    System.out.println("inside Lock.Check() method : CurrrentThreadName : "+ threadName);
}
}


class MyThread implements Runnable{
public MyThread() {
}

@Override
public void run() {
    try {
        System.out.println("inside Mythread's run()");
        classLocking();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

public static synchronized void classLocking() throws InterruptedException{
    System.out.println("inside Mythread.classLocking()");
    String threadName = Thread.currentThread().getName();
    System.out.println("inside MyThread.classLocking() : CurrrentThreadName : "+ threadName);
    /*
     * outer class locking 
     */
    synchronized (Lock.class) {
        System.out.println("I got lock on Lock.class definition");
        Lock.STOP = false;
        /*
         * Outer class lock is not released. Lock on MyThread.class instance is released.
         */
        MyThread.class.wait();
    }
}
}

Answer 1

You are correct that it doesn’t release the other lock. As for why, it’s because it isn’t safe to do so. If it was safe to release the outer lock during the call to the inner function, why would the inner function be called with the other lock held at all?

Having a function release a lock it didn’t acquire behind the programmer’s back would destroy the logic of synchronized functions.