Multiple arrive() in java phaser

Question

Assuming all threads work in phase 0 of java phaser then one thread make arrive(); arrive(); arriveAndAwaitAdvance(); will this thread be waiting for phaser to end phase 2 or 0?

Answer 1

It depends on the number of threads (parties) on the Phaser. Simply phaser will wait a number of arrive() before advancing the phase.

Here is what will happen if you set the phaser with 2 parties:

1. First, arrive() won't change the phase since we need 2.
2. Second arrive() will change the phase from 0 to 1.
3. arriveAndAwaitAdvance() operation won't change the phase from 1 to 2 since we need 2 arrive in order to advance.

So your thread will wait for phase 1 to end if the number of party in your Phaser is 2 (new Phaser(2);).

Here is an example code that can help you to understand how phasers work:

ExecutorService executorService = Executors.newFixedThreadPool(2);
Phaser phaser = new Phaser(2);

executorService.execute(() -> {
    System.out.println("First thread phase = " + phaser.getPhase());
    phaser.arrive();
    System.out.println("First thread After arrive1 = " + phaser.getPhase());
    phaser.arrive();
    System.out.println("First thread After arrive2 = " + phaser.getPhase());
    phaser.arrive();
    System.out.println("First thread After arrive3 = " + phaser.getPhase());
    phaser.arrive();
    System.out.println("First thread After arrive4 = " + phaser.getPhase());
    phaser.arriveAndAwaitAdvance();
    System.out.println("First thread After arriveAndAwaitAdvance  = " + phaser.getPhase());
});

executorService.execute(() -> {
    try {
        Thread.sleep(2000);
        System.out.println("Second thread current phase = " + phaser.getPhase());
        phaser.arriveAndAwaitAdvance();
        System.out.println("Second thread After arriveAndAwaitAdvance = " + phaser.getPhase());
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
});
executorService.shutdown();

The output of this code piece is:

First thread phase = 0
First thread After arrive1 = 0
First thread After arrive2 = 1
First thread After arrive3 = 1
First thread After arrive4 = 2
Second thread current phase = 2
Second thread After arriveAndAwaitAdvance = 3
First thread After arriveAndAwaitAdvance  = 3

I hope that will answer your question.