What is Phaser in Java?

A Phaser in Java is a synchronization construct that allows multiple threads to coordinate their execution. It is part of the `java.util.concurrent` package, which provides various concurrency utilities to help manage and control thread behavior. A Phaser works similarly to a countdown latch or barrier, but it is more flexible and can be reused for multiple phases of work.

Phasers are particularly useful when you need to synchronize several threads at certain points in their execution. For example, if we have multiple threads performing tasks that need to wait for each other at specific checkpoints, a Phaser can manage this synchronization seamlessly.

Example:

import java.util.concurrent.Phaser;

public class PhaserExample {
    public static void main(String[] args) {
        Phaser phaser = new Phaser(3); // 3 parties involved
        
        for (int i = 0; i < 3; i++) {
            final int partyId = i;
            new Thread(() -> {
                System.out.println("Thread " + partyId + " is doing some work.");
                phaser.arriveAndAwaitAdvance(); // Wait for all parties to arrive
                System.out.println("Thread " + partyId + " has finished the first phase.");
            }).start();
        }
    }
}