Multiple Consumer Producer In Java

Question

I tried to create 2 threads for consumer and 2 threads for producer. All the 4 threads are racing against one resource. Two of them are trying to consume from resource and two of them are trying to produce.

Below is the code

package com.threading;

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

public class TestConsumerProducer2 {

    protected static int maxSize = 2;

    static class Consumer implements Runnable {
        List<Integer> goods;

        public Consumer(List<Integer> goods) {
            this.goods = goods;
        }

        public void consume() {
            synchronized (goods) {

                if (goods.size() <= 0) {
                    try {
                        goods.wait();
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
                System.out.println(Thread.currentThread().getName() + " >>>> consuming >>>" + goods.remove(0));
                goods.notifyAll();
            }
        }

        @Override
        public void run() {
            for (int i = 0; i < 10; i++) {
                consume();
                try {
                    Thread.currentThread().sleep(100);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }
    }

    static class Producer implements Runnable {
        List<Integer> goods;

        public Producer(List<Integer> goods) {
            this.goods = goods;
        }

        public void produce(int i) {
            synchronized (goods) {

                if (goods.size() >= maxSize) {
                    try {
                        goods.wait();
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
                System.out.println(Thread.currentThread().getName() + ">>> producing >> " + i);
                goods.add(i);
                goods.notifyAll();
            }
        }

        @Override
        public void run() {
            // TODO Auto-generated method stub
            for (int i = 0; i < 10; i++) {
                produce(i);
                try {
                    Thread.currentThread().sleep(100);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }
    }

    public static void main(String[] args) {
        List<Integer> goods = new ArrayList<>();
        Consumer consumer = new Consumer(goods);
        Producer producer = new Producer(goods);
        Thread consumerWorker1 = new Thread(consumer);
        Thread consumerWorker2 = new Thread(consumer);

        Thread prroducerWorker1 = new Thread(producer);
        Thread prroducerWorker2 = new Thread(producer);

        consumerWorker1.start();
        consumerWorker2.start();
        prroducerWorker1.start();
        prroducerWorker2.start();

        try {
            consumerWorker1.join();
            consumerWorker2.join();
            prroducerWorker1.join();
            prroducerWorker2.join();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        System.out.println("Job completed >>>>");
    }
}

Output Of the program

Thread-2>>> producing >> 0

Thread-1 >>>> consuming >>>0

Thread-3>>> producing >> 0

Exception in thread "Thread-0" java.lang.IndexOutOfBoundsException: Index: 0, Size: 0 at java.util.ArrayList.rangeCheck(ArrayList.java:657) at java.util.ArrayList.remove(ArrayList.java:496) at com.threading.TestConsumerProducer2$Consumer.consume(TestConsumerProducer2.java:27) at com.threading.TestConsumerProducer2$Consumer.run(TestConsumerProducer2.java:35) at java.lang.Thread.run(Thread.java:748) Thread-2>>> producing >> 1

Thread-1 >>>> consuming >>>0

Thread-3>>> producing >> 1

Thread-1 >>>> consuming >>>1

Thread-2>>> producing >> 2

Thread-1 >>>> consuming >>>1

Thread-3>>> producing >> 2

Thread-1 >>>> consuming >>>2

Thread-2>>> producing >> 3

Thread-1 >>>> consuming >>>2

Thread-2>>> producing >> 4

Thread-3>>> producing >> 3

Thread-1 >>>> consuming >>>3

Thread-2>>> producing >> 5

Thread-1 >>>> consuming >>>4

Thread-3>>> producing >> 4

Thread-1 >>>> consuming >>>3

Thread-2>>> producing >> 6 Thread-1 >>>> consuming >>>5

Thread-2>>> producing >> 7

Thread-3>>> producing >> 5

Problem Statement: Why are none of the threads executing 10 times? Where is the deadlock situation in the code? Why is there an IndexOutOfBoundsException when Goods object is locked by consumer thread and if size <=0 It should go into waiting state?

Answer 1

Changing occurrences of if to while in your code fixes the problem.

There is relevant advice in the Guarded Blocks tutorial on Oracle's site where it says:

Note: Always invoke wait inside a loop that tests for the condition being waited for. Don't assume that the interrupt was for the particular condition you were waiting for, or that the condition is still true.

(By interrupt they mean return from wait, not necessarily an actual interruption originating from somebody calling Thread.interrupt.)

Key points:

• A thread knows the contents of the goods list only if it has the lock held at the time that it's checking.

• Calling wait relinquishes the lock, allowing other threads to make progress while this one goes dormant.

• Once a thread relinquishes the lock, any checks it made previously about the state of the goods list are not valid any more.

Once a thread returns from wait it has reacquired the lock, but the thread needs to re-evaluate the condition check, otherwise it is acting on stale information. A lot may have happened between the time the thread last checked the condition and the present time. You get the IllegalArgumentException because the current thread is assuming something is there that another thread removed while the current thread was waiting.

package com.threading;

import java.util.ArrayList; 

import java.util.List;

public class TestConsumerProducer2 {

protected static int maxSize = 2;

static class Consumer implements Runnable {
    List<Integer> goods;

    public Consumer(List<Integer> goods) {
        this.goods = goods;
    }

    public void consume() {
        synchronized (goods) {

            while (goods.size() <= 0) {
                try {
                    goods.wait();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
            System.out.println(Thread.currentThread().getName() + " >>>> consuming >>>" + goods.remove(0));
            goods.notifyAll();
        }
    }

    @Override
    public void run() {
        for (int i = 0; i < 10; i++) {
            consume();
            try {
                Thread.currentThread().sleep(100);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }
}

static class Producer implements Runnable {
    List<Integer> goods;

    public Producer(List<Integer> goods) {
        this.goods = goods;
    }

    public void produce(int i) {
        synchronized (goods) {

            while (goods.size() >= maxSize) {
                try {
                    goods.wait();
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
            System.out.println(Thread.currentThread().getName() + ">>> producing >> " + i);
            goods.add(i);
            goods.notifyAll();
        }
    }

    @Override
    public void run() {
        // TODO Auto-generated method stub
        for (int i = 0; i < 10; i++) {
            produce(i);
            try {
                Thread.currentThread().sleep(100);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    }
}

public static void main(String[] args) {
    List<Integer> goods = new ArrayList<>();
    Consumer consumer = new Consumer(goods);
    Producer producer = new Producer(goods);
    Thread consumerWorker1 = new Thread(consumer);
    Thread consumerWorker2 = new Thread(consumer);

    Thread prroducerWorker1 = new Thread(producer);
    Thread prroducerWorker2 = new Thread(producer);

    consumerWorker1.start();
    consumerWorker2.start();
    prroducerWorker1.start();
    prroducerWorker2.start();

    try {
        consumerWorker1.join();
        consumerWorker2.join();
        prroducerWorker1.join();
        prroducerWorker2.join();
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    System.out.println("Job completed >>>>");
}

}

Code now completes successfully, output follows:

C:\>java com.threading.TestConsumerProducer2
Thread-2>>> producing >> 0
Thread-1 >>>> consuming >>>0
Thread-3>>> producing >> 0
Thread-0 >>>> consuming >>>0
Thread-2>>> producing >> 1
Thread-3>>> producing >> 1
Thread-0 >>>> consuming >>>1
Thread-1 >>>> consuming >>>1
Thread-2>>> producing >> 2
Thread-3>>> producing >> 2
Thread-0 >>>> consuming >>>2
Thread-1 >>>> consuming >>>2
Thread-2>>> producing >> 3
Thread-0 >>>> consuming >>>3
Thread-3>>> producing >> 3
Thread-1 >>>> consuming >>>3
Thread-2>>> producing >> 4
Thread-0 >>>> consuming >>>4
Thread-3>>> producing >> 4
Thread-1 >>>> consuming >>>4
Thread-2>>> producing >> 5
Thread-0 >>>> consuming >>>5
Thread-3>>> producing >> 5
Thread-1 >>>> consuming >>>5
Thread-2>>> producing >> 6
Thread-0 >>>> consuming >>>6
Thread-3>>> producing >> 6
Thread-1 >>>> consuming >>>6
Thread-2>>> producing >> 7
Thread-0 >>>> consuming >>>7
Thread-3>>> producing >> 7
Thread-1 >>>> consuming >>>7
Thread-2>>> producing >> 8
Thread-0 >>>> consuming >>>8
Thread-3>>> producing >> 8
Thread-1 >>>> consuming >>>8
Thread-2>>> producing >> 9
Thread-0 >>>> consuming >>>9
Thread-3>>> producing >> 9
Thread-1 >>>> consuming >>>9
Job completed >>>>