BlockingQueue Interface in Java (original) (raw)

The **BlockingQueue Interface in Java is a part of the **java.util.concurrent package and was introduced in **Java 1.5. It is a specialized **Queue implementation designed to handle thread-safe operations in concurrent environments. BlockingQueue is commonly used in **Producer-Consumer patterns and other multithreaded scenarios.

• **Thread Safe: BlockingQueue implementations are thread-safe, with all methods being **atomic.
• **Blocking Operation: Has blocking behavior if the queue is **full (for producers) or **empty (for consumers).
• **No Null Elements: Attempts to insert a null will result in a NullPointerException.
• **Bounded and Unbounded Queues: Can use both as the required case arrives:
  • **Bounded BlockingQueue: Fixed capacity, blocking producers when full.
  • **Unbounded BlockingQueue: Expands as needed (e.g., backed by a LinkedList), though subject to memory constraints.

Java provides several BlockingQueue implementations such as LinkedBlockingQueue, ArrayBlockingQueue, PriorityBlockingQueue, SynchronousQueue, etc.

**Example:

Java `

// Java Program to BlockingQueue import java.util.concurrent.BlockingQueue; import java.util.concurrent.LinkedBlockingQueue;

public class BlockingQueueExample { public static void main(String[] args) { // Create a BlockingQueue with a capacity of 3 BlockingQueue a = new LinkedBlockingQueue<>(3);

    try {          
        // Add elements
        a.put(1);
        a.put(2);
        a.put(3);

          // Printing the BlockingQueue
        System.out.println("BlockingQueue : " + a);
    }
    catch (InterruptedException e) {
        e.printStackTrace();
    }
}

}

`

Output

BlockingQueue : [1, 2, 3]

A thread trying to enqueue an element in a full queue is blocked until some other thread makes space in the queue, either by dequeuing one or more elements or clearing the queue completely. Similarly, it blocks a thread trying to delete from an empty queue until some other threads insert an item.

**Usage of BlockingQueue

A BlockingQueue accessed by a producer(put) thread and a consumer(take) thread

**The Hierarchy of BlockingQueue

**Declaration

public interface BlockingQueue extends Queue

Here, E is the type of elements stored in the Collection.

Classes that Implement BlockingQueue

We directly cannot provide an instance of BlockingQueue since it is an interface, so to utilize the functionality of the BlockingQueue, we need to make use of the classes implementing it. Also, to use BlockingQueue in your code, use this import statement.

import java.util.concurrent.BlockingQueue;
(or)
import java.util.concurrent.*;

• LinkedBlockingQueue
• ArrayBlockingQueue

The implementing class of BlockingDeque is LinkedBlockingDeque. This class is the implementation of the BlockingDeque and the linked list data structure. The LinkedBlockingDeque can be optionally bounded using a constructor, however, if the capacity is unspecified it is Integer.MAX_VALUE by default. The nodes are added dynamically at the time of insertion obeying the capacity constraints.

**The syntax for creating objects:

BlockingQueue objectName = new LinkedBlockingDeque();
(or)
LinkedBlockingDeque objectName = new LinkedBlockingDeque();

Example: In the code given below we perform some basic operations on a LinkedBlockingDeque, like creating an object, adding elements, deleting elements, and using an iterator to traverse through the LinkedBlockingDeque.

BlockingQueue Types

The BlockingQueue are **two types:

**1. Unbounded Queue: The Capacity of the blocking queue will be set to Integer.MAX_VALUE. In the case of an unbounded blocking queue, the queue will never block because it could grow to a very large size. when you add elements its size grows.

**Syntax:

BlockingQueue blockingQueue = new LinkedBlockingDeque();

**2. Bounded Queue: The second type of queue is the bounded queue. In the case of a bounded queue you can create a queue passing the capacity of the queue in queues constructor:

**Syntax:

// Creates a Blocking Queue with capacity 5
BlockingQueue blockingQueue = new LinkedBlockingDeque(5);

**To implement Bounded Semaphore using BlockingQueue

Java `

// Java program that explains the internal // implementation of BlockingQueue

import java.io.; import java.util.;

class BlockingQueue {

// BlockingQueue using LinkedList structure
// with a constraint on capacity
private List<E> queue = new LinkedList<E>();

// limit variable to define capacity
private int limit = 10;

// constructor of BlockingQueue
public BlockingQueue(int limit) { this.limit = limit; }

// enqueue method that throws Exception
// when you try to insert after the limit
public synchronized void enqueue(E item)
    throws InterruptedException
{
    while (this.queue.size() == this.limit) {
        wait();
    }
    if (this.queue.size() == 0) {
        notifyAll();
    }
    this.queue.add(item);
}

// dequeue methods that throws Exception
// when you try to remove element from an
// empty queue
public synchronized E dequeue()
    throws InterruptedException
{
    while (this.queue.size() == 0) {
        wait();
    }
    if (this.queue.size() == this.limit) {
        notifyAll();
    }

    return this.queue.remove(0);
}

public static void main(String []args)
{
}

}

`

**Example:

Java `

// Java Program to demonstrate usage of BlockingQueue

import java.util.concurrent.; import java.util.;

public class GFG {

public static void main(String[] args)
    throws InterruptedException
{

    // define capacity of ArrayBlockingQueue
    int capacity = 5;

    // create object of ArrayBlockingQueue
    BlockingQueue<String> queue
        = new ArrayBlockingQueue<String>(capacity);

    // Add elements to ArrayBlockingQueue using put
    // method
    queue.put("StarWars");
    queue.put("SuperMan");
    queue.put("Flash");
    queue.put("BatMan");
    queue.put("Avengers");

    // print Queue
    System.out.println("queue contains " + queue);

    // remove some elements
    queue.remove();
    queue.remove();

    // Add elements to ArrayBlockingQueue
    // using put method
    queue.put("CaptainAmerica");
    queue.put("Thor");

    System.out.println("queue contains " + queue);
}

}

`

Output

queue contains [StarWars, SuperMan, Flash, BatMan, Avengers] queue contains [Flash, BatMan, Avengers, CaptainAmerica, Thor]

Basic Operations

**1. Adding Elements

Elements can be added into a LinkedBlockedDeque in different ways depending on the type of structure we are trying to use it as. The most common method is the add() method using which we can add elements at the end of the deque. We can also use the addAll() method (which is a method of the Collection interface) to add an entire collection to LinkedBlockingDeque. If we wish to use the deque as a queue we can use add() and put().

Java `

// Java Program Demonstrate add() // method of BlockingQueue

import java.util.concurrent.LinkedBlockingDeque; import java.util.concurrent.BlockingQueue; import java.util.*;

public class GFG {

public static void main(String[] args)
    throws IllegalStateException
{

    // create object of BlockingQueue
    BlockingQueue<Integer> BQ
        = new LinkedBlockingDeque<Integer>();

    // Add numbers to the BlockingQueue
    BQ.add(7855642);
    BQ.add(35658786);
    BQ.add(5278367);
    BQ.add(74381793);

    // before removing print BlockingQueue
    System.out.println("Blocking Queue: " + BQ);
}

}

`

Output

Blocking Queue: [7855642, 35658786, 5278367, 74381793]

**2. Accessing Elements

The elements of the LinkedBlockingDeque can be accessed using contains(), element(), peek(), poll(). There are variations of these methods too which are given in the table above along with their descriptions.

Java `

// Java Program for Accessing the elements of a // LinkedBlockingDeque

import java.util.concurrent.*;

public class AccessingElements {

public static void main(String[] args)
{

    // Instantiate an object of LinkedBlockingDeque
    // named lbdq
    BlockingQueue<Integer> lbdq
        = new LinkedBlockingDeque<Integer>();

    // Add elements using add()
    lbdq.add(22);
    lbdq.add(125);
    lbdq.add(723);
    lbdq.add(172);
    lbdq.add(100);

    // Print the elements of lbdq on the console
    System.out.println(
        "The LinkedBlockingDeque, lbdq contains:");
    System.out.println(lbdq);

    // To check if the deque contains 22
    if (lbdq.contains(22))
        System.out.println(
            "The LinkedBlockingDeque, lbdq contains 22");
    else
        System.out.println("No such element exists");

    // Using element() to retrieve the head of the deque
    int head = lbdq.element();
    System.out.println("The head of lbdq: " + head);
}

}

`

Output

The LinkedBlockingDeque, lbdq contains: [22, 125, 723, 172, 100] The LinkedBlockingDeque, lbdq contains 22 The head of lbdq: 22

**3. Deleting Elements

Elements can be deleted from a LinkedBlockingDeque using remove(). Other methods such as take() and poll() can also be used in a way to remove the first and the last elements.

Java `

// Java Program for removing elements from a // LinkedBlockingDeque

import java.util.concurrent.*;

public class RemovingElements {

public static void main(String[] args)
{

    // Instantiate an object of LinkedBlockingDeque
    // named lbdq
    BlockingQueue<Integer> lbdq
        = new LinkedBlockingDeque<Integer>();

    // Add elements using add()
    lbdq.add(75);
    lbdq.add(86);
    lbdq.add(13);
    lbdq.add(44);
    lbdq.add(10);

    // Print the elements of lbdq on the console
    System.out.println(
        "The LinkedBlockingDeque, lbdq contains:");
    System.out.println(lbdq);

    // Remove elements using remove();
    lbdq.remove(86);
    lbdq.remove(44);

    // Trying to remove an element
    // that doesn't exist
    // in the LinkedBlockingDeque
    lbdq.remove(1);

    // Print the elements of lbdq on the console
    System.out.println(
        "\nThe LinkedBlockingDeque, lbdq contains:");
    System.out.println(lbdq);
}

}

`

Output

The LinkedBlockingDeque, lbdq contains: [75, 86, 13, 44, 10]

The LinkedBlockingDeque, lbdq contains: [75, 13, 10]

**4. Iterating through the Elements

To iterate through the elements of a LinkedBlockingDeque we can create an iterator and use the methods of the Iterable interface, which is the root of the Collection Framework of Java, to access the elements. The next() method of Iterable returns the element of any collection.

Java `

// Java Program to iterate // through the LinkedBlockingDeque import java.util.Iterator; import java.util.concurrent.*;

public class IteratingThroughElements {

public static void main(String[] args) {
    
    // Instantiate an object of LinkedBlockingDeque named lbdq
    BlockingQueue<Integer> lbdq = new LinkedBlockingDeque<Integer>();
    
    // Add elements using add()
    lbdq.add(166);
    lbdq.add(246);
    lbdq.add(66);
    lbdq.add(292);
    lbdq.add(98);
    
    // Create an iterator to traverse lbdq
    Iterator<Integer> lbdqIter = lbdq.iterator();
    
    // Print the elements of lbdq on to the console
    System.out.println("The LinkedBlockingDeque, lbdq contains:");
    
    for(int i = 0; i<lbdq.size(); i++)
    {
        System.out.print(lbdqIter.next() + " ");
    }        
}

}

`

Output

The LinkedBlockingDeque, lbdq contains: 166 246 66 292 98

Methods of BlockingQueue

Methods declared in interface java.util.Collection

Methods declared in interface java.lang.Iterable

Methods declared in interface java.util.Queue

The Behavior of BlockingQueue Methods

The following are the methods provided by the BlockingQueue for insertion, removal, and examine operations on the queue. Each of the four sets of methods behaves differently if the requested operation is not satisfied immediately.

• **Throws Exception: An exception will be thrown, if the requested operation is not satisfied immediately.
• **Special value: A special value is returned if the operation is not satisfied immediately.
• **Blocks: The method call is blocked if the attempted operation is not satisfied immediately and it waits until it gets executed.
• **Times out: A special value is returned telling whether the operation succeeded or not. If the requested operation is not possible immediately, the method call blocks until it is, but waits no longer than the given timeout.

**Reference: https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/concurrent/BlockingQueue.html