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Exchanger

20 Feb 2017   Java  #java  #threads 

Exchanger is Java’s high level API for sharing data between two threads. It allows one thread to wait while another thread generates or processes data. Once they have prepared data, data objects can be shared between the threads.

Working

There are two important methods in Exchanger class.

V exchange(V x) throws InterruptedException

V exchange(V x, long timeout, TimeUnit unit) throws InterruptedException, TimeoutException

V is the type of data to be exchanged. When exhcange() is invoked on a thread, it stops the current thread and waits for other thread to reach the exchange point.

exchange() method waits until it is not invoked on same object by two separate threads or thread is interrupted.

When exchage() is called by both threads, objects are swaped and thread execution is resumed. If exchange() is already called by a thread and another thread invoked exchange(), other thread is passed the object immidiately and execution is proceeded in both threads.

Exchange with timeout is same as exchange() with one major difference - it waits for specified interval of time. TimeoutException is thrown if no other thread arrives at exchange point in a given time.

Example

We are going to create two threads which will exchange object. I will print object’s hashcode along with thread name to get better understanding of what’s going on.

ThreadA - sends an empty string and prints the string returned from ThreadB.

class ThreadA extends Thread {

    private Exchanger<String> ex;
    private String str;

    ThreadA(Exchanger<String> ex) {
        super("ThreadA");
        this.ex = ex;
        str = "";
    }

    @Override
    public void run() {
        System.out.println(String.format("[%s] - sending object[%s]", getName(), str.hashCode()));
        try {
        	Thread.sleep(1000);
            str = ex.exchange(str);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(String.format("[%s] - receiving object[%s]", getName(), str.hashCode()));
    }
} // ThreadA

ThreadB - sends a string (ThreadA) and prints the string returned from ThreadA.

class ThreadB extends Thread {

    private Exchanger<String> ex;
    private String str;

    ThreadB(Exchanger<String> ex) {
        super("ThreadB");
        this.ex = ex;
        str = "";
    }

    @Override
    public void run() {
        try {
            str = getName();
            System.out.println(String.format("[%s] - sending object[%s]", getName(), str.hashCode()));
            Thread.sleep(500);
            str = ex.exchange(str);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(String.format("[%s] - receiving object[%s]", getName(), str.hashCode()));
    } // run
} // ThreadB

App.java

public class App {

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

        Exchanger<String> exchanger = new Exchanger<>();

        Thread t1 = new ThreadA(exchanger);
        Thread t2 = new ThreadB(exchanger);

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

    } // main

} // App

Output

[ThreadB] - sending object[326716248]
[ThreadA] - sending object[0]
[ThreadB] - receiving object[0]
[ThreadA] - receiving object[326716248]

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