Tutorial

Java BlockingQueue Example

Published on August 3, 2022
author

Pankaj

Java BlockingQueue Example

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Today we will look into Java BlockingQueue. java.util.concurrent.BlockingQueue is a java Queue that support operations that wait for the queue to become non-empty when retrieving and removing an element, and wait for space to become available in the queue when adding an element.

Java BlockingQueue

Java BlockingQueue Java BlockingQueue doesn’t accept null values and throw NullPointerException if you try to store null value in the queue. Java BlockingQueue implementations are thread-safe. All queuing methods are atomic in nature and use internal locks or other forms of concurrency control. Java BlockingQueue interface is part of java collections framework and it’s primarily used for implementing producer consumer problem. We don’t need to worry about waiting for the space to be available for producer or object to be available for consumer in BlockingQueue because it’s handled by implementation classes of BlockingQueue. Java provides several BlockingQueue implementations such as ArrayBlockingQueue, LinkedBlockingQueue, PriorityBlockingQueue, SynchronousQueue etc. While implementing producer consumer problem in BlockingQueue, we will use ArrayBlockingQueue implementation. Following are some important methods you should know.

  • put(E e): This method is used to insert elements to the queue. If the queue is full, it waits for the space to be available.
  • E take(): This method retrieves and remove the element from the head of the queue. If queue is empty it waits for the element to be available.

Let’s implement producer consumer problem using java BlockingQueue now.

Java BlockingQueue Example - Message

Just a normal java object that will be produced by Producer and added to the queue. You can also call it as payload or queue message.

package com.journaldev.concurrency;

public class Message {
    private String msg;
    
    public Message(String str){
        this.msg=str;
    }

    public String getMsg() {
        return msg;
    }

}

Java BlockingQueue Example - Producer

Producer class that will create messages and put it in the queue.

package com.journaldev.concurrency;

import java.util.concurrent.BlockingQueue;

public class Producer implements Runnable {

    private BlockingQueue<Message> queue;
    
    public Producer(BlockingQueue<Message> q){
        this.queue=q;
    }
    @Override
    public void run() {
        //produce messages
        for(int i=0; i<100; i++){
            Message msg = new Message(""+i);
            try {
                Thread.sleep(i);
                queue.put(msg);
                System.out.println("Produced "+msg.getMsg());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        //adding exit message
        Message msg = new Message("exit");
        try {
            queue.put(msg);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

Java BlockingQueue Example - Consumer

Consumer class that will process on the messages from the queue and terminates when exit message is received.

package com.journaldev.concurrency;

import java.util.concurrent.BlockingQueue;

public class Consumer implements Runnable{

private BlockingQueue<Message> queue;
    
    public Consumer(BlockingQueue<Message> q){
        this.queue=q;
    }

    @Override
    public void run() {
        try{
            Message msg;
            //consuming messages until exit message is received
            while((msg = queue.take()).getMsg() !="exit"){
            Thread.sleep(10);
            System.out.println("Consumed "+msg.getMsg());
            }
        }catch(InterruptedException e) {
            e.printStackTrace();
        }
    }
}

Java BlockingQueue Example - Service

Finally we have to create BlockingQueue service for producer and consumer. This producer consumer service will create the BlockingQueue with fixed size and share with both producers and consumers. This service will start producer and consumer threads and exit.

package com.journaldev.concurrency;

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;

public class ProducerConsumerService {

    public static void main(String[] args) {
        //Creating BlockingQueue of size 10
        BlockingQueue<Message> queue = new ArrayBlockingQueue<>(10);
        Producer producer = new Producer(queue);
        Consumer consumer = new Consumer(queue);
        //starting producer to produce messages in queue
        new Thread(producer).start();
        //starting consumer to consume messages from queue
        new Thread(consumer).start();
        System.out.println("Producer and Consumer has been started");
    }

}

Output of the above java BlockingQueue example program is shown below.

Producer and Consumer has been started
Produced 0
Produced 1
Produced 2
Produced 3
Produced 4
Consumed 0
Produced 5
Consumed 1
Produced 6
Produced 7
Consumed 2
Produced 8
...

Java Thread sleep is used in producer and consumer to produce and consume messages with some delay.

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JournalDev
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January 24, 2019

Hi Pankaj, How in case of multiple threads trying to write in same blocking queue (consider huge count of threads from business perspective view. practically ). Because synchronization would be needed for queue also i think. Please guide in such scenarios. welcome for suggestions

- venkatesh choudhary

    JournalDev
    DigitalOcean Employee
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    September 13, 2018

    Using Concurrency Linked List import java.util.Collections; import java.util.Iterator; import java.util.LinkedList; import java.util.List; import java.util.Scanner; public class Threadexample { public static void main(String[] args) throws InterruptedException { // Object of a class that has both produce() // and consume() methods Scanner in = new Scanner(System.in); int cakes = in.nextInt(); System.out.println(“Number of cakes::”+cakes); final Cake cake = new Cake(); // Create producer thread Thread t1 = new Thread(new Runnable() { @Override public void run() { try { cake.produce(cakes); } catch (InterruptedException e) { e.printStackTrace(); } } }); // Create consumer thread Thread t2 = new Thread(new Runnable() { @Override public void run() { try { cake.consume(cakes); } catch (InterruptedException e) { e.printStackTrace(); } } }); // Start both threads t1.start(); t2.start(); // t1 finishes before t2 t1.join(); t2.join(); } // This class has a list, producer (adds items to list // and consumber (removes items). public static class Cake { // Create a list shared by producer and consumer List list1 = Collections.synchronizedList(new LinkedList()); List list2 = Collections.synchronizedList(new LinkedList()); List list3 = Collections.synchronizedList(new LinkedList()); int capacity = 3; // Function called by producer thread public void produce(int cakes) throws InterruptedException { int value = 0; while (cakes!=0) { synchronized (this) { // producer thread waits while list // is full while (list1.size() == capacity && list2.size() == capacity && list3.size() == capacity) wait(); System.out.println(“Producer produced-” + value); // to insert the jobs in the list if (list1.size() < 3) { System.out.println(“List1 size:” + list1.size()); list1.add(value++); } else if (list2.size() < 3) { System.out.println(“List1 is full:::”); System.out.println(“List2 size:” + list2.size()); list2.add(value++); } else if (list3.size() < 3) { System.out.println(“List2 is full:::”); System.out.println(“List3 size:” + list3.size()); list3.add(value++); } // notifies the consumer thread that // now it can start consuming notify(); // makes the working of program easier // to understand Thread.sleep(1000); cakes–; System.out.println(“Producer cakes:”+cakes); } } } // Function called by consumer thread public void consume(int cakes) throws InterruptedException { while (cakes!=0) { synchronized (this) { // consumer thread waits while list // is empty while (list1.size() == 0 && list2.size() == 0 && list3.size() == 0) wait(); // to retrieve the cake in the list System.out.println(“Consumer cakes:”+cakes); synchronized (list1) { Iterator it = list1.iterator(); while (it.hasNext()) { System.out.println(“Consumer consumed-” + it.next()); it.remove(); cakes–; } } synchronized (list2) { Iterator it = list2.iterator(); while (it.hasNext()) { System.out.println(“Consumer consumed2-” + it.next()); it.remove(); cakes–; } } synchronized (list3) { Iterator it = list3.iterator(); while (it.hasNext()) { System.out.println(“Consumer consumed3-” + it.next()); it.remove(); cakes–; } } // Wake up producer thread notify(); // and sleep Thread.sleep(1000); //cakes–; //System.out.println(“Consumer cakes:”+cakes); } } } } }

    - Vinothkumar

      JournalDev
      DigitalOcean Employee
      DigitalOcean Employee badge
      July 2, 2018

      Hi, could you change the operator for comparing string from reference to value equality in consumer example ?

      - Simplicity

        JournalDev
        DigitalOcean Employee
        DigitalOcean Employee badge
        November 3, 2017

        Thanks Pankaj for the excellent tutorial!

        - Karthik

          JournalDev
          DigitalOcean Employee
          DigitalOcean Employee badge
          October 27, 2017

          Hello author, Thank you for tutorial. I’m a C# developer, and trying to build java application for multi platform purposes, i want to ask you a question. how if i only create a producer, to create new object and execute it? i think consumer was unnecessary because we have main thread, and we create 2 other for producer and consumer, sorry if the way i thinking is wrong because i’m just learning blockqueue and create asynchronous from scratch. regards

          - Vico Ervanda

            JournalDev
            DigitalOcean Employee
            DigitalOcean Employee badge
            June 10, 2017

            Why Consumer is not able to consume everything that Producer has produced. Why it exits without finishing??

            - Pinesh

              JournalDev
              DigitalOcean Employee
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              June 8, 2016

              HI, It’s really helpful for the beginner. Thanks, Lalit

              - Lalit Patil

                JournalDev
                DigitalOcean Employee
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                May 14, 2015

                Hi Pankaj, Could you kindly put a few more examples where BlockingQueue has been implemented for a larger service e.g. a real-time message queue where it’s being run for almost infinite duration. BTW - This example helps me a lot though! Kindest Regards,

                - Mohammed Manna

                  JournalDev
                  DigitalOcean Employee
                  DigitalOcean Employee badge
                  April 22, 2015

                  i want to create a concurrent asynchronous queue so that the messages or data can be inserted into it using threadpool and another pool retrieves data from the queue and stores it in database (asynchronously) how can i do this if anyone knows about it then please share the code or explain me how to implement it…

                  - Ahmed

                    JournalDev
                    DigitalOcean Employee
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                    July 21, 2014

                    Hello Sir, I am running the same code given by you in above example but its showing given below output: Produced Messagedemo.Message@247cb66a consumednull Not as above given by you.And after adding toString() method in the Consumer class at belwo line it is throwing null pointer exception. System.out.println(“consumed” +msg.getMsg().toString()); Exception in thread “Thread-1” java.lang.NullPointerException at demo.Consumer.run(Consumer.java:24) at java.lang.Thread.run(Unknown Source) Will you please correct me ? Do I need to syncronized them?

                    - Shweta

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