Threaded Queue Example - ConcurrentLinkedQueue

We were giving interviews to some prospective developers for our team at work. One of the questions/examples we asked for was to create a multi-threaded application which has a producer thread and a consumer thread which passes messages between them.

After a couple of really bad attempts by prospects, I decided to give it a try. I have not done a lot of multi-thread applications (standalone) since most of my work has to do with applications in containers. I wondered if I could do it. Well my results are in the included application. It is just one way of doing it, but I thought I would post the cleaned-up version.

The NetBeans project can be downloaded from here: ThreadedQueueExample.zip

ThreadedQueueExample.java

---

/*
 * Copyright 2011 Blue Lotus Software, LLC.
 * Copyright 2011 John Yeary
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
/*
 * $Id: ThreadedQueueExample.java 347 2011-05-23 12:22:29Z jyeary $
 */
package com.bluelotussoftware.thread.example;

import java.util.Queue;
import java.util.concurrent.ConcurrentLinkedQueue;

/**
 *
 * @author John Yeary
 * @version 1.0
 */
public class ThreadedQueueExample {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {

        Queue<Message> queue = new ConcurrentLinkedQueue<Message>();

        Producer p = new Producer(queue);
        Consumer c = new Consumer(queue);

        Thread t1 = new Thread(p);
        Thread t2 = new Thread(c);


        t1.start();
        t2.start();
    }
}

Producer.java

---

/*
 * Copyright 2011 Blue Lotus Software, LLC.
 * Copyright 2011 John Yeary
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
/*
 * $Id: Producer.java 347 2011-05-23 12:22:29Z jyeary $
 */
package com.bluelotussoftware.thread.example;

import java.util.Queue;
import java.util.Random;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 *
 * @author John Yeary
 * @version 1.0
 */
public class Producer implements Runnable {

    private static int ctr;
    private final Queue<Message> messageQueue;
    private final Random r;

    public Producer(Queue<Message> messageQueue) {
        this.messageQueue = messageQueue;
        r = new Random();
    }

    @Override
    public void run() {
        while (true) {
            produce();
            int wait = r.nextInt(5000);
            try {
                Thread.sleep(wait);
            } catch (InterruptedException ex) {
                Logger.getLogger(Producer.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
    }

    private void produce() {
        Message m = new Message(++ctr, "Example message.");
        messageQueue.offer(m);
        synchronized (messageQueue) {
            messageQueue.notifyAll();
        }
        System.out.println("Producer: " + m);
    }
}

Consumer.java

---

/*
 * Copyright 2011 Blue Lotus Software, LLC.
 * Copyright 2011 John Yeary
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
/*
 * $Id: Consumer.java 349 2011-05-23 15:25:32Z jyeary $
 */
package com.bluelotussoftware.thread.example;

import java.util.Queue;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 *
 * @author John Yeary
 * @version 1.0
 */
public class Consumer implements Runnable {

    private final Queue<Message> queue;

    public Consumer(Queue<Message> queue) {
        this.queue = queue;
    }

    @Override
    public void run() {
        while (true) {
            consume();
            try {
                synchronized (queue) {
                    queue.wait();
                }
            } catch (InterruptedException ex) {
                Logger.getLogger(Consumer.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
    }

    private void consume() {
        while (!queue.isEmpty()) {
            Message m = queue.poll();
            if (m != null) {
                System.out.println("Consumer: " + m.toString());
            }
        }
    }
}

Message.java

---

/*
 * Copyright 2011 Blue Lotus Software, LLC.
 * Copyright 2011 John Yeary
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
/*
 * $Id: Message.java 347 2011-05-23 12:22:29Z jyeary $
 */
package com.bluelotussoftware.thread.example;

/**
 *
 * @author John Yeary
 * @version 1.0
 */
public class Message {

    private int id;
    private String message;

    public Message(int id, String message) {
        this.id = id;
        this.message = message;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }

    @Override
    public String toString() {
        return "Message{ id:" + id + ",message: " + message + "}";
    }
}