How to indefinitely pause a thread in Java and later resume it?

Question

Maybe this question has been asked many times before, but I never found a satisfying answer.

The problem:

I have to simulate a process s

Answer 1

Yes, there is:

Object.wait( ), Object.notify() and a bunch of other much nicer synchronization primitives in java.util.concurrent.

Answer 2

Who said Java is not low level enough?

Here is my 3 minute solution. I hope it fits your needs.

import java.util.ArrayList;
import java.util.List;

public class ThreadScheduler {

    private List<RoundRobinProcess> threadList
            = new ArrayList<RoundRobinProcess>();

    public ThreadScheduler(){
        for (int i = 0 ; i < 100 ; i++){
            threadList.add(new RoundRobinProcess());
            new Thread(threadList.get(i)).start();
        }
    }


    private class RoundRobinProcess implements Runnable{

        private final Object lock = new Object();
        private volatile boolean suspend = false , stopped = false;

        @Override
        public void run() {
            while(!stopped){
                while (!suspend){
                    // do work
                }
                synchronized (lock){
                    try {
                        lock.wait();
                    } catch (InterruptedException e) {
                        Thread.currentThread().interrupt();
                        return;
                    }
                }
            }
        }

        public void suspend(){
            suspend = true;
        }
        public void stop(){
            suspend = true;stopped = true;
            synchronized (lock){
                lock.notifyAll();
            }
        }

        public void resume(){
            suspend = false;
            synchronized (lock){
                lock.notifyAll();
            }
        }

    }
}

Please note that "do work" should not be blocking.

Answer 3

Short answer: no. You don't get to implement a thread scheduler in Java, as it doesn't operate at a low enough level.

If you really do intend to implement a process scheduler, I would expect you to need to hook into the underlying operating system calls, and as such I doubt this will ever be a good idea (if remotely possible) in Java. At the very least, you wouldn't be able to use java.lang.Thread to represent the running threads so it may as well all be done in a lower-level language like C.