Thread Scheduling - Shared Array

I need two threads to write one a shared array of ints. Both threads need to write on all the elements of that array. Each thread will write either 1 or 7, and the result should be like 171717171 (or 71717171). To do that I have the first Thread1 write at position 0, then wait. Thread2 now writes at position 0 and 1, notifies Thread1, and waits. Thread1 writes at position 1 and 2, notifies Thread2 and waits, etc. With the following code I get correct output, although when run with JPF it finds a deadlock. Its become really frustrating since I can not find whats wrong with it. Any advice would be appreciated.

import java.util.logging.Level;
import java.util.logging.Logger;


public class WriterThreadManager {

    private int[] array = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
    private Thread thread7;
    private Thread thread1;

    public static void main(String[] args) {
        WriterThreadManager mng = new WriterThreadManager();
        mng.exec();

    }

    public WriterThreadManager() {
        thread7 = new Thread(new WriterRunnable(this, 7));
        thread1 = new Thread(new WriterRunnable(this, 1));
    }

    public void overwriteArray(int pos, int num) {
        array[pos] = num;
        printArray();
    }

    private  void printArray() {
        for (int i = 0; i < array.length; i++) {
            System.out.print(array[i]);
        }
        System.out.println("");
    }

    public synchronized void stopThread() {
        try {
            this.wait();
        } catch (InterruptedException ex) {
            Logger.getLogger(WriterThreadManager.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

    public synchronized void wakeUpThread() {
        notifyAll();
    }

    private void exec() {
        thread7.start();
        thread1.start();
    }

    public int length() {
        return array.length;
    }
}



public class WriterRunnable implements Runnable {

    private WriterThreadManager mng;
    private int numberToWrite;
    private static boolean flag = true;

    @Override
    public void run() {
        int counter = 0;
        int j = 0;

        //first thread to get in should write only at 
        //position 0 and then wait.
        synchronized (mng) {
            if (flag) {
                flag = false;
                mng.overwriteArray(0, numberToWrite);
                j = 1;
                waitForOtherThread();
            }
        }
        for (int i = j; i < mng.length(); i++) {
            mng.overwriteArray(i, numberToWrite);
            counter++;
            if (i == mng.length() - 1) {
                mng.wakeUpThread();
                break;
            }
            if (counter == 2) {
                waitForOtherThread();
                counter = 0;
            }
        }
    }

    private void waitForOtherThread() {
        mng.wakeUpThread();
        mng.stopThread();
    }

    public WriterRunnable(WriterThreadManager ar, int num) {
        mng = ar;
        numberToWrite = num;
    }
}

p.s: an example of the execution:

The error snapshot from JPF is the following:

thread java.lang.Thread:{id:1,name:Thread-1,status:WAITING,priority:5,lockCount:1,suspendCount:0}
  waiting on: WriterThreadManager@152
  call stack:
    at java.lang.Object.wait(Object.java)
    at WriterThreadManager.stopThread(WriterThreadManager.java:43)
    at WriterRunnable.waitForOtherThread(WriterRunnable.java:53)
    at WriterRunnable.run(WriterRunnable.java:45)

thread java.lang.Thread:{id:2,name:Thread-2,status:WAITING,priority:5,lockCount:1,suspendCount:0}
  waiting on: WriterThreadManager@152
  call stack:
    at java.lang.Object.wait(Object.java)
    at WriterThreadManager.stopThread(WriterThreadManager.java:43)
    at WriterRunnable.waitForOtherThread(WriterRunnable.java:53)
    at WriterRunnable.run(WriterRunnable.java:45)

Solution

I believe the race is due to this method:

private void waitForOtherThread() {
    mng.wakeUpThread();
    mng.stopThread();
}

While the individual wakeUpThread() and stopThread() methods are synchronized, you have the opportunity for unexpected thread scheduling between these calls.

Consider:

thread7 - notify thread1 to wakup
thread1 - wake up
thread1 - work to completion
thread1 - notify thread7 to wakeup
thread1 - wait to be notified to wakeup
thread7 - wait to be notified to wakeup

In this case you have deadlocked because thread1 sent its notifyAll() before thread7 had a chance to wait() for it.

Running in a different context can mess with your timing and cause these types of behaviors to appear.

To avoid this I suggestion doing this:

private void waitForOtherThread() {
    synchronized(mng) {
        mng.wakeUpThread();
        mng.stopThread();
    }
}

Or better yet, use a semaphore as @KumarVivekMitra suggested. Semaphores combine both the notification system and a counter so that the order of the notify and wait don't matter.