Java concurrency: Countdown latch vs Cyclic barrier

Question

I was reading through the java.util.concurrent API, and found that

• CountDownLatch: A synchronization aid that allows one or more threads to wait

Answer 1

When I was studying about Latches and cyclicbarriers I came up with this metaphors. cyclicbarriers: Imagine a company has a meeting room. In order to start the meeting, a certain number of meeting attendees have to come to meeting (to make it official). the following is the code of a normal meeting attendee (an employee)

class MeetingAtendee implements Runnable {

CyclicBarrier myMeetingQuorumBarrier;

public MeetingAtendee(CyclicBarrier myMileStoneBarrier) {
    this.myMeetingQuorumBarrier = myMileStoneBarrier;
}

@Override
public void run() {
    try {
        System.out.println(Thread.currentThread().getName() + " i joined the meeting ...");
        myMeetingQuorumBarrier.await();
        System.out.println(Thread.currentThread().getName()+" finally meeting stared ...");
    } catch (InterruptedException e) {
        e.printStackTrace();
    } catch (BrokenBarrierException e) {
        System.out.println("Meeting canceled! every body dance <by chic band!>");
    }
 }
}

employee joins the meeting, waits for others to come to start meeting. also he gets exited if the meeting gets canceled :) then we have THE BOSS how doses not like to wait for others to show up and if he looses his patient, he cancels meeting.

class MeetingAtendeeTheBoss implements Runnable {

CyclicBarrier myMeetingQuorumBarrier;

public MeetingAtendeeTheBoss(CyclicBarrier myMileStoneBarrier) {
    this.myMeetingQuorumBarrier = myMileStoneBarrier;
}

@Override
public void run() {
    try {
        System.out.println(Thread.currentThread().getName() + "I am THE BOSS - i joined the meeting ...");
        //boss dose not like to wait too much!! he/she waits for 2 seconds and we END the meeting
        myMeetingQuorumBarrier.await(1,TimeUnit.SECONDS);
        System.out.println(Thread.currentThread().getName()+" finally meeting stared ...");
    } catch (InterruptedException e) {
        e.printStackTrace();
    } catch (BrokenBarrierException e) {
        System.out.println("what WHO canceled The meeting");
    } catch (TimeoutException e) {
        System.out.println("These employees waste my time!!");
    }
 }
}

On a normal day, employee come to meeting wait for other to show up and if some attendees don`t come they have to wait indefinitely! in some special meeting the boss comes and he does not like to wait.(5 persons need to start meeting but only boss comes and also an enthusiastic employee) so he cancels the meeting (angrily)

CyclicBarrier meetingAtendeeQuorum = new CyclicBarrier(5);
Thread atendeeThread = new Thread(new MeetingAtendee(meetingAtendeeQuorum));
Thread atendeeThreadBoss = new Thread(new MeetingAtendeeTheBoss(meetingAtendeeQuorum));
    atendeeThread.start();
    atendeeThreadBoss.start();

Output:

//Thread-1I am THE BOSS - i joined the meeting ...
// Thread-0 i joined the meeting ...
// These employees waste my time!!
// Meeting canceled! every body dance <by chic band!>

There is another scenario in which another outsider thread (an earth quake) cancels the meeting (call reset method). in this case all the waiting threads get woken up by an exception.

class NaturalDisasters implements Runnable {

CyclicBarrier someStupidMeetingAtendeeQuorum;

public NaturalDisasters(CyclicBarrier someStupidMeetingAtendeeQuorum) {
    this.someStupidMeetingAtendeeQuorum = someStupidMeetingAtendeeQuorum;
}

void earthQuakeHappening(){
    System.out.println("earth quaking.....");
    someStupidMeetingAtendeeQuorum.reset();
}

@Override
public void run() {
    earthQuakeHappening();
 }
}

running code will result in funny output:

// Thread-1I am THE BOSS - i joined the meeting ...
// Thread-0 i joined the meeting ...
// earth quaking.....
// what WHO canceled The meeting
// Meeting canceled! every body dance <by chic band!>

You can also add a secretary to meeting room, if a meeting is held she will document every thing but she is not part of the meeting:

class MeetingSecretary implements Runnable {

@Override
public void run() {
        System.out.println("preparing meeting documents");
        System.out.println("taking notes ...");
 }
}

Latches: if the angry boss wants to hold an exhibition for company customers, every thing needs to be ready (resources). we provide a to-do list every worker (Thread) dose his job and we check the to-do list (some workers do painting, others prepare sound system ...). when all the items in to-do list are complete (resources are provided) we can open the doors to customers.

public class Visitor implements Runnable{

CountDownLatch exhibitonDoorlatch = null;

public Visitor (CountDownLatch latch) {
    exhibitonDoorlatch  = latch;
}

public void run() {
    try {
        exhibitonDoorlatch .await();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }

    System.out.println("customer visiting exebition");
 }
}

And the workers how are preparing the exhibition:

class Worker implements Runnable {

CountDownLatch myTodoItem = null;

public Worker(CountDownLatch latch) {
    this.myTodoItem = latch;
}

public void run() {
        System.out.println("doing my part of job ...");
        System.out.println("My work is done! remove it from todo list");
        myTodoItem.countDown();
 }
}

    CountDownLatch preperationTodoList = new CountDownLatch(3);

    // exhibition preparation workers  
    Worker      electricalWorker      = new Worker(preperationTodoList);
    Worker      paintingWorker      = new Worker(preperationTodoList);

    // Exhibition Visitors 
    ExhibitionVisitor exhibitionVisitorA = new ExhibitionVisitor(preperationTodoList);
    ExhibitionVisitor exhibitionVisitorB = new ExhibitionVisitor(preperationTodoList);
    ExhibitionVisitor exhibitionVisitorC = new ExhibitionVisitor(preperationTodoList);

    new Thread(electricalWorker).start();
    new Thread(paintingWorker).start();

    new Thread(exhibitionVisitorA).start();
    new Thread(exhibitionVisitorB).start();
    new Thread(exhibitionVisitorC).start();

Answer 2

In CountDownLatch, main threads waits for other threads to complete their execution. In CyclicBarrier, worker threads wait for each other to complete their execution.

You can not reuse same CountDownLatch instance once count reaches to zero and latch is open, on the other hand CyclicBarrier can be reused by resetting Barrier, Once barrier is broken.