Question

I’m testing a Java multi-threading sample code but the thread started in the for loop of qB.start() is blocked because it’s waiting for entry of qB monitor. What is the cause of this blockage?

Thank you.

import java.util.*;

class QA {

public synchronized void open() throws Exception {

    Thread o = new Thread() {

        public void run() {

            QB qB = new QB();

            qB.start();
        }
    };

    o.start();
}

public static void main(String args[]) throws Exception {

    new QA().open();
}

public class QB {

private boolean shutdown;
private Vector<Thread> tList;
private final Object waitingLock = new Object();

public QB() {

    tList = new Vector<Thread>();
}

public synchronized void start() {


    for(int i = 0; i < 1; i++) {

        final int id = i;

        Thread t = new Thread("Thread " + id) {

            public void run() {

                load(id);
            }
        };

        tList.add(i, t);

        t.start();

    }

    tMonitor();
    waitUntilFinished();
}

private void tMonitor() {

    Thread cmt = new Thread("T Monitor Thread") {

        public void run() {

            synchronized(waitingLock) {

                while(tList.size() > 0) {

                    try {

                        sleep(10000);

                    } catch(Exception e) {

                        e.printStackTrace();
                    }
                }

                waitingLock.notifyAll();
            }
        }
    };

    cmt.start();
}

private void waitUntilFinished() {

    synchronized(waitingLock) {

        while(!isShutDown()) {

            try {


                waitingLock.wait();


            } catch(Exception e) {

        e.printStackTrace();
            }
        }
    }

}

private synchronized void load(int id) {

    try {

        System.out.println("blocked here");

// some work done here

removeFromTList(id);


    } catch(Exception e) {

        e.printStackTrace();
    }
}


public synchronized boolean isShutDown() {

    return shutdown;
}
}
}

Answer 1

The first problem I see is that QB#start() is synchronized on the instance of QB.
Inside the thread t that you are trying to spawn, load(id) is also synchronized on the same instance of QB. So when you call t.start() the t thread blocks until QB#start() finishes.

Presumably, at the end of the QB#start() method, QB#waitUntilFinished() is supposed to wait for all the t threads to finish, but they can’t even enter the QB#load method because they’re still waiting for the QB#start() method to release the lock on the QB instance.

So, circular deadlock.