Sunday, 5 June 2016

Implementing blocking queue in Java

Blocking Queue

Blocking queue is a queue that has a limit of elements it can hold and once that limit has reached enqueuing thread needs to wait for some thread to dequeue elements and make space. Similarly if queue becomes empty then dequeuing thread  has to wait until some thread enqueues elements in it.

Diagrammatically it is as follows -

Lets see how can we implement it in Java.

Blocking queue implementation in Java

 Code is as follows - 

package com.osfg.models;

import java.util.LinkedList;
import java.util.Queue;

 * @author athakur
 * Model class for blocking queue
public class BlockingQueue<E> {
    private Queue<E> bQueue = new LinkedList<E>();
    private int maxQueueSize; 
    public BlockingQueue(int maxQueueSize) {
        this.maxQueueSize = maxQueueSize; 
    public synchronized void enqueue(E e) throws InterruptedException {
        while(bQueue.size() == maxQueueSize) {
        // notify if any thread is waiting to dequeue as data is now available
    public synchronized E dequeue() throws InterruptedException{
        while(bQueue.size() == 0) {
        E e = bQueue.remove();
        return e;


Notice how we are using wait() and notifyAll() calls. Also observer both enqueue and dequeue methods are synchronized. So at a time only one thread can execute them. You can also find this code in my data structure github repository. Also see the Thread Pool implementation in Java which uses the blocking queue internally.

Producer - Consumer design can be built using blocking queue. Producers enqueue jobs in the queue and wait when the queue is full where as consumers dequeue jobs in the queue and wait when the queue is empty. Famous example of producer consumer design in Thread pool implementation where you can enqueue your tasks in the queue and threads from Threadpool will dequeue and process it. As mentioned before you can see the code for blocking queue and thread pool in my github repository (Links in Related Links section below).

Related Links

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