Phaser這個類的使用場景為N個線程分階段並行的問題。有這麽一個任務為“做3道題“，每個學生一個進程，5個學生可以並行做，這個就是常規的並發，但是如果加一個額外的 限制條件，必須等所有人都做完類第一題，才能開始做第二題，必須等所有人都做完了第二題，才能做第三題，這個問題就轉變成了分階段並發的問題，最適合用Phaser來解題，下面給出源代碼，大家可以自己嘗試：

MyPhaser.java

import java.util.concurrent.Phaser;

public class MyPhaser extends Phaser {
    @Override
    protected boolean onAdvance(int phase, int registeredParties) { //在每個階段執行完成後回調的方法

        switch (phase) {
            case 0:
                return studentArrived();
            case 1:
                return finishFirstExercise();
            case 2:
                return finishSecondExercise();
            case 3:
                return finishExam();
            default:
                return true;
        }

    }

    private boolean studentArrived(){
        System.out.println("學生準備好了,學生人數："+getRegisteredParties());
        return false;
    }

    private boolean finishFirstExercise(){
        System.out.println("第一題所有學生做完");
        return false;
    }

    private boolean finishSecondExercise(){
        System.out.println("第二題所有學生做完");
        return false;
    }

    private boolean finishExam(){
        System.out.println("第三題所有學生做完，結束考試");
        return true;
    }

}

StudentTask.java

import java.util.concurrent.Phaser;
import java.util.concurrent.TimeUnit;

public class StudentTask implements Runnable {

    private Phaser phaser;

    public StudentTask(Phaser phaser) {
        this.phaser = phaser;
    }

    @Override
    public void run() {
        System.out.println(Thread.currentThread().getName()+"到達考試");
        phaser.arriveAndAwaitAdvance();

        System.out.println(Thread.currentThread().getName()+"做第1題時間...");
        doExercise1();
        System.out.println(Thread.currentThread().getName()+"做第1題完成...");
        phaser.arriveAndAwaitAdvance();

        System.out.println(Thread.currentThread().getName()+"做第2題時間...");
        doExercise2();
        System.out.println(Thread.currentThread().getName()+"做第2題完成...");
        phaser.arriveAndAwaitAdvance();

        System.out.println(Thread.currentThread().getName()+"做第3題時間...");
        doExercise3();
        System.out.println(Thread.currentThread().getName()+"做第3題完成...");
        phaser.arriveAndAwaitAdvance();
    }

    private void doExercise1() {
        long duration = (long)(Math.random()*10);
        try {
            TimeUnit.SECONDS.sleep(duration);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private void doExercise2() {
        long duration = (long)(Math.random()*10);
        try {
            TimeUnit.SECONDS.sleep(duration);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private void doExercise3() {
        long duration = (long)(Math.random()*10);
        try {
            TimeUnit.SECONDS.sleep(duration);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

Main.java

public class Main {

    public static void main(String[] args) {
        MyPhaser phaser = new MyPhaser();
        StudentTask[] studentTask = new StudentTask[5];
        for (int i = 0; i < studentTask.length; i++) {
            studentTask[i] = new StudentTask(phaser);
            phaser.register();  //註冊一次表示phaser維護的線程個數
        }

        Thread[] threads = new Thread[studentTask.length];
        for (int i = 0; i < studentTask.length; i++) {
            threads[i] = new Thread(studentTask[i], "Student "+i);
            threads[i].start();
        }

        //等待所有線程執行結束
        for (int i = 0; i < studentTask.length; i++) {
            try {
                threads[i].join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        System.out.println("Phaser has finished:"+phaser.isTerminated());

    }

}

Java並發編程之Phaser類