Many ProducerS and many ConsumerS. Making the last producer alive killing the consumers

Question

I have a standard producer consumer problem. Producer puts data into the stack(buffer) consumers take it.

I would like to have many producers and consumers.

the problem is I would like to make only the last living producer to be able to call b.stop()

for(int i = 0; i < 10; i++){
        try{
    //      sleep((int)(Math.random() * 1));                
        }catch(Exception e){e.printStackTrace();}
        b.put((int) (Math.random()* 10));
        System.out.println("i = " + i);
    }
    b.stop();

so then I call b.stop() which changes running field in Buffer to false and notifiesAll()

End then I get:

i = 9 // number of iteration this is 10th iteration
Consumer 2.: no data to take. I wait.  Memory: 0
Consumer 1.: no data to take. I wait.  Memory: 0
Consumer 3.: no data to take. I wait.  Memory: 0

they should die then, so I made method stop() but it did not work.

Code is running please check it

import java.util.Stack;


public class Buffer {
private static int SIZE = 4;
private int i;//number of elements in buffer
public Stack<Integer> stack;
private volatile boolean running;
    public Buffer() {
        stack = new Stack<>();
        running = true;
        i = 0;
    }
    synchronized public void put(int val){
        while (i >= SIZE) {
            try {
                System.out.println("Buffer full, producer waits");
                wait();
            } catch (InterruptedException exc) {
                exc.printStackTrace();
            }
        }   
        stack.push(val);//txt = s;
        i++;
        System.out.println("Producer inserted " + val + " memory: " + i);
        if(i - 1 == 0)
            notifyAll();
        System.out.println(stack);
    }

    public synchronized Integer get(Consumer c) {
        while (i == 0) {
            try {
                System.out.println(c + ": no data to take. I wait.  Memory: " + i);
                wait();
            } catch (InterruptedException exc) {
                exc.printStackTrace();
            }
        }   
        if(running){
            int data = stack.pop();
            i--;    
            System.out.println(c+  ": I took: " + data +" memory: " +  i);
            System.out.println(stack);
            if(i + 1 == SIZE){//if the buffer was full so the producer is waiting
                notifyAll();
                System.out.println(c +  "I notified producer about it");
        }
        return data;}
        else 
            return null;
    }

    public boolean isEmpty(){
        return i == 0;
    }
    public synchronized void stop(){//I THOUGH THIS WOULD FIX IT~!!!!!!!!!!!!!!
        running = false;
        notifyAll();
    }
    public boolean isRunning(){
        return running;
    }

}

public class Producer extends Thread {
private Buffer b;
    public Producer(Buffer b) {
        this.b = b;
    }

    public void run(){
        for(int i = 0; i < 10; i++){
            try{
        //      sleep((int)(Math.random() * 1));                
            }catch(Exception e){e.printStackTrace();}
            b.put((int) (Math.random()* 10));
            System.out.println("i = " + i);
        }
        b.stop();
    }

}

public class Consumer extends Thread {
    Buffer b;
    int nr;
    static int NR = 0;

    public Consumer(Buffer b) {
        this.b = b;
        nr = ++NR;
    }

    public void run() {
        Integer i = b.get(this);
        while (i != null) {
            System.out.println(nr + " I received : " + i);
            i = b.get(this);
        }
        System.out.println("Consumer " + nr + " is dead");
    }

    public String toString() {
        return "Consumer " + nr + ".";
}

}

public class Main {

    public static void main(String[] args) {

        Buffer b = new Buffer();
        Producer p = new Producer(b);
        Consumer c1 = new Consumer(b);
        Consumer c2 = new Consumer(b);
        Consumer c3 = new Consumer(b);  
        p.start();
        c1.start();c2.start();c3.start();

    }

}

OldCurmudgeon OldCurmudgeon · Accepted Answer · 2013-04-20T23:13:29

What you have to realise is that your threads could be waiting in either of two locations:

In the wait loop with i == 0 - in which case notifyall will kick all of them out. However, if i is still 0 they will go straight back to waiting again.
Waiting for exclusive access to the object (i.e. waiting on a synchronized method) - in which case (if you fix issue 1 above and the lock will be released) they will go straight into a while (i == 0) loop.

I would suggest you change your while ( i == 0 ) loop to while ( running && i == 0 ). This should fix your problem. Since your running flag is (correctly) volatile all should tidily exit.

Many ProducerS and many ConsumerS. Making the last producer alive killing the consumers

4 Answers