Reputation: 105

Java program doesn't terminate

I am trying to create a deadlock using join in threads (JAVA). According to my understanding the below program should terminate. Can somebody please explain why the below program doesn't terminate?

public class DeadLockUsingJoins2 {

    public static void main(String[] args) {

        AThread a = new AThread(null, "A");
        a.start();
        AThread b = new AThread(a, "B");
        b.start();
        a.t = b;
    }
}

class AThread extends Thread{

    public Thread t;

    public AThread(Thread thread, String name){
        super(name);
        this.t = thread;
    }

    @Override
    public void run(){
        try {
            if(t != null)
                t.join();
            else
            {
                Thread.sleep(5000);
            }
          //  if(t != null)
          //      t.join();
        } catch (InterruptedException e) {           
            e.printStackTrace();
            System.out.println("Somebody interruped thread - " + this.getName());
        }
        System.out.println("Winding Up thread - " + this.getName());
    }

}

Upvotes: 3

Answers (5)

Evgeniy Dorofeev

Reputation: 136162

add a pause before a.t = b;

    b.start();
    Thread.sleep(100);
    a.t = b;

it will work. But to be reliable it needs syncronization, eg like this

public class DeadLockUsingJoins2 {
    public static boolean go;

    public synchronized static void main(String[] args)
            throws InterruptedException {
        AThread a = new AThread(null, "A");
        AThread b = new AThread(a, "B");
        a.start();
        while (!go) {
            DeadLockUsingJoins2.class.wait();
        }
        a.t = b;
        b.start();
        DeadLockUsingJoins2.class.notifyAll();
    }
}

class AThread extends Thread {
    public Thread t;

    public AThread(Thread thread, String name) {
        super(name);
        this.t = thread;
    }

    @Override
    public void run() {
        try {
            if (t != null)
                t.join();
            else {
                synchronized (DeadLockUsingJoins2.class) {
                    DeadLockUsingJoins2.go = true;
                    DeadLockUsingJoins2.class.notifyAll();
                }
                Thread.sleep(5000);
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Winding Up thread - " + this.getName());
    }

Upvotes: 2

Marco Forberg

Reputation: 2644

The only reason i see for not terminating is that your main thread reaches the line

a.t = b;

before thread "a" reaches the lines

if(t != null)
    t.join();

if so, the threads wait for each other to terminate.

To make sure you get a deadlock - your class names suggests this - you should reorder your main method:

public static void main(String[] args) {
    AThread a = new AThread(null, "A");
    AThread b = new AThread(a, "B");
    a.t = b;
    a.start();
    b.start();
}

Upvotes: 3

david a.

Reputation: 5291

It will deadlock if main thread is executed up to a.t = b; before run() on any of the created threads is actually called. The threads then deadlock on join() calls.

EDIT: To avoid this, you need to use some sort of synchronization between the main thread and the threads you're creating. For example, signal the start of run() using Object.notify() and wait for that using Object.wait() in main thread.

Upvotes: 2