Racing threads

[Arnold]

I made a test class that will let several threads race against each other.
In the constructor all the threads are made and started, but they need to start racing when the start() method is called. Until then the threads need to wait in their "Start Positions". Synchronization is not necessary in this situation, but I want to make the threads wait for a common resource. Java doesn't accept that.
Does anyone know a better alternative?

// No PHP Code! This is Java! I only use it for syntax highlighting.
// I mean a JAVA forum supports HTML tags and PHP tags, but no JAVA tags?
// Oh well here is the code:

import java.util.Random;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class Race {
	public final int numRacers, units, minTimeout, maxTimeout;

	private boolean started = false;

	private final Racer[] racers;
	private final ExecutorService racePool;
	private final Random r = new Random();
	private final Race instance = this;

	public Race(int numRacers, int units, int minTimeout, int maxTimeout) {
		this.numRacers = numRacers;
		this.units = units;
		this.minTimeout = minTimeout;
		this.maxTimeout = maxTimeout;
		racers = new Racer[numRacers];
		racePool = Executors.newFixedThreadPool(numRacers);
		for (int i = 0; i < numRacers; i++) {
			racers[i] = new Racer(i + 1);
			racePool.execute(racers[i]);
		}
		racePool.shutdown();
	}

	public void start() {
		started = true;
		System.out.println("Race started");
//		notifyAll();
	}
	public void finish(Racer r) {
		System.out.println(r + " finished");
	}

	private class Racer implements Runnable {
		public final int index;
		public Racer(int index) {
			this.index = index;
		}
		public void run() {
			try {
				// WAIT here until the start() method was called
//				while (! started)
//					instance.wait(); // don't know how else to refer to the parent of an inner class...
				// That didn't work because of an IllegalMonitorStateException
				System.out.println(this + " started");
				for (int i = 0; i < units; i++)
					Thread.sleep(r.nextInt(maxTimeout - minTimeout) + minTimeout);
				finish(this);
			} catch (InterruptedException e) {}
		}
		public String toString() {
			return "Racer " + index;
		}
	}

	public static void main(String[] args) throws InterruptedException {
		Race r = new Race(8, 100, 0, 500);
		Thread.sleep(5000); // wait on purpose before starting the race
		r.start();
	}
}

[Norm]

I want to make the threads wait for a common resource

Can you explain a bit?
After creating all the threads, you can call start() for each in turn.

[Singing Boyo]

To call Object.wait, Object.notify(), and Object.notifyAll(), you need to have obtained the objects lock first. There may be other methods that need it too, but I don't think there are, and these three are the only ones that are relevant. (Object.notify() is unused in your case, but it's so similar to notifyAll() that I included it. It just makes one thread continue from a wait() call, rather than all of them.)

//this is valid
synchronized(instance){
     obj.notifyAll();
     obj.wait();
}

//this isn't - you don't hold the objects lock
obj.notifyAll();
obj.wait();

To refer the the instance of the outer class from an inner class, use SuperClassName.this, or in your case, Race.this. Note that this won't work for static inner classes because they are not associated with an instance of the parent. Same thing as the way you can't access 'this' from a static method

Try adding in the synchronization and making the correct reference to the instance of the outer class. It should help. You probably also want to synchronized the finish(Racer) method, or you could get some very odd output :P the run() method of Racer could probably use just an if(!started) condition as well, rather than a while loop. By the time the wait call returns (because notifyAll() is called) started is set to true, so you don't need to have it be checked again, and it was slightly confusing when I first read the code, so it might help readability a bit too.

Hope this helps.

Singing Boyo

[Norm]

To get the lock for the object obj:

synchronized(obj){  // get obj's lock
     obj.notifyAll();
     obj.wait();
}

[Arnold]

Well basicly I want to make multiple threads wait for a singe condition and have all of them resume at the same time when that condition is met.
I was wondering if this is possible without using locks.

BTW thanks for the tip on SuperClassName.this.

I used the while-block to wait, because I read somewhere that wait() does not wait forever.

Yes I know I could make all threads just start() when the method is called in my example, but what if threads have to wait for this kind of condition mid-progress?

Right now I see there is a seperate forum for Threads and Synchronization...

[Norm]

multiple threads wait for a singe condition

Look at the Semaphore class.
You could hold up the other threads by having 0 permits available until the condition happens. Then if you do a release() of the number of threads waiting on an acquire() they would all be unblocked.

[Arnold]

Look at the Semaphore class.
You could hold up the other threads by having 0 permits available until the condition happens. Then if you do a release() of the number of threads waiting on an acquire() they would all be unblocked.

I see! You mean something like this:

public Race(int numRacers, int units, int minTimeout, int maxTimeout) {
  //...
  semaphore = new Semaphore(numRacers, true);
  semaphore.drainPermits();
  //...
}

public void start() {
  started = true;
  System.out.println("Race started");
  s.release(numRacers);
}

private class Racer implements Runnable {
  //...
  public void run() {
    //...
    s.acquire();
    System.out.println(this + " started");
    //...
  }
  //...
}

Thanx

[Norm]

Yes. Try it and report back how it works.

[Arnold]

Yes. Try it and report back how it works.

Actually I already did try before posting and it works fine.
Thanks anyway.