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Thread Local Variables

08 Jan 2017   Java  #java  #threads 

Normally, when an object is shared by multiple threads, depending on the access modifier each thread can access the same field in object and change in that field can be propagated to all other threads.

That’s the default behaviour, but sometimes we want each thread to have its own local copy of the field. In this way, every change to that field will be visible in that thread only.

Java allows us to achieve this behaviour using ThreadLocal variables.

Usage

ThreadLocal is a generic class which can store any kind of object. When object is first get from ThreadLocal variable using Threadlocal.get method, it calls initialValue method if variable is not initialized already.

See MessageQueue class in the example below.

Example

Say, we have a MessageQueue class which contains a ThreadLocal Queue. We want to share an instance of this class with 2 threads, each having its own copy of Queue and inserts 5 items in the Queue.

MessageQueue

class MessageQueue<T> {

    private final ThreadLocal<Queue<T>> queue;

    MessageQueue(){
        queue = new ThreadLocal<Queue<T>>(){
            @Override
            protected Queue<T> initialValue() {
                return new ArrayDeque<>();
            }
        };
    } // MessageQueue


    Queue<T> getQueue() {
        return queue.get();
    } // getQueue


} // MessageQueue

ThreadA

class ThreadA extends Thread {

    private MessageQueue<String> messageQueue;

    ThreadA(MessageQueue<String> messageQueue) {
        super("ThreadA");
        this.messageQueue = messageQueue;
    }

    @Override
    public void run() {
        super.run();
        Queue<String> queue = messageQueue.getQueue();
        for (int i = 0; i < 5; i++) {
          queue.add(String.format("%s - %s", i, Thread.currentThread().getName()));
        } // end for
        System.out.println(queue.toString());
    }
} // ThreadA

ThreadB

class ThreadB extends Thread {

    private MessageQueue<String> messageQueue;

    ThreadB(MessageQueue<String> messageQueue) {
        super("ThreadB");
        this.messageQueue = messageQueue;
    }

    @Override
    public void run() {
        super.run();
        Queue<String> queue = messageQueue.getQueue();
        for (int i = 0; i < 5; i++) {
            queue.add(String.format("%s - %s", i, Thread.currentThread().getName()));
        } // end for
        System.out.println(queue.toString());
    } // run

} // ThreadB

Main

public class App {

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

        MessageQueue<String> messageQueue = new MessageQueue<>();

        // init 
        ThreadA threadA = new ThreadA(messageQueue);
        ThreadB threadB = new ThreadB(messageQueue);

        // start threads
        threadA.start();
        threadB.start();

        // wait for threads
        threadA.join();
        threadB.join();

    } // main

} // App

Output

[0 - ThreadB, 1 - ThreadB, 2 - ThreadB, 3 - ThreadB, 4 - ThreadB]
[0 - ThreadA, 1 - ThreadA, 2 - ThreadA, 3 - ThreadA, 4 - ThreadA]

Without ThreadLocal, each thread will insert data in same Queue.

Output Without ThreadLocal

[0 - ThreadB, 1 - ThreadA, 1 - ThreadB, 2 - ThreadA, 2 - ThreadB, 3 - ThreadB, 4 - ThreadA]
[0 - ThreadB, 1 - ThreadA, 1 - ThreadB, 2 - ThreadA, 2 - ThreadB, 3 - ThreadB, 4 - ThreadA]

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