控制并发阶段性任务的改变

控制并发阶段性任务的改变

Phaser 类提供每次phaser改变阶段都会执行的方法。它是 onAdvance() 方法。它接收2个参数：当前阶段数和注册的参与者数；它返回 Boolean 值，如果phaser继续它的执行，则为 false；否则为真，即phaser结束运行并进入 termination 状态。

如果注册参与者为0，此方法的默认的实现值为真，要不然就是false。如果你扩展Phaser类并覆盖此方法，那么你可以修改它的行为。通常，当你要从一个phase到另一个，来执行一些行动时，你会对这么做感兴趣的。

在这个指南，你将学习如何控制phaser的 phase的改变，通过实现自定义版本的 Phaser类并覆盖 onAdvance() 方法来执行一些每个phase 都会改变的行动。你将要实现一个模拟测验，有些学生要完成他们的练习。全部的学生都必须完成同一个练习才能继续下一个练习。

按照这些步骤来实现下面的例子

package com.packtpub.java7.concurrency.chapter3.recipe6.task;import java.util.Date;import java.util.concurrent.Phaser;import java.util.concurrent.TimeUnit;/** * Implements a subclass of the Phaser class. Overrides the onAdvance method to * control the change of phase * */public class MyPhaser extends Phaser {    /**     * This method is called when the last register thread calls one of the     * advance methods in the actual phase     *      * @param phase     *            Actual phase     * @param registeredParties     *            Number of registered threads     * @return false to advance the phase, true to finish     */    @Override    protected boolean onAdvance(int phase, int registeredParties) {        switch (phase) {        case 0:            return studentsArrived();        case 1:            return finishFirstExercise();        case 2:            return finishSecondExercise();        case 3:            return finishExam();        default:            return true;        }    }    /**     * This method is called in the change from phase 0 to phase 1     *      * @return false to continue with the execution     */    private boolean studentsArrived() {        System.out                .printf("Phaser: The exam are going to start. The students are ready.\n");        System.out.printf("Phaser: We have %d students.\n",                getRegisteredParties());        return false;    }    /**     * This method is called in the change from phase 1 to phase 2     *      * @return false to continue with the execution     */    private boolean finishFirstExercise() {        System.out                .printf("Phaser: All the students has finished the first exercise.\n");        System.out.printf("Phaser: It's turn for the second one.\n");        return false;    }    /**     * This method is called in the change form phase 2 to phase 3     *      * @return false to continue with the execution     */    private boolean finishSecondExercise() {        System.out                .printf("Phaser: All the students has finished the second exercise.\n");        System.out.printf("Phaser: It's turn for the third one.\n");        return false;    }    /**     * This method is called in the change from phase 3 to phase 4     *      * @return true. There are no more phases     */    private boolean finishExam() {        System.out.printf("Phaser: All the students has finished the exam.\n");        System.out.printf("Phaser: Thank you for your time.\n");        return true;    }    public static void main(String[] args) {        // Creates the Phaser        MyPhaser phaser = new MyPhaser();        // Creates 5 students and register them in the phaser        Student students[] = new Student[5];        for (int i = 0; i < students.length; i++) {            students[i] = new Student(phaser);            phaser.register();        }        // Create 5 threads for the students and start them        Thread threads[] = new Thread[students.length];        for (int i = 0; i < students.length; i++) {            threads[i] = new Thread(students[i], "Student " + i);            threads[i].start();        }        // Wait for the finalization of the threads        for (int i = 0; i < threads.length; i++) {            try {                threads[i].join();            } catch (InterruptedException e) {                e.printStackTrace();            }        }        // Check that the Phaser is in the Terminated state        System.out.printf("Main: The phaser has finished: %s.\n",                phaser.isTerminated());    }}class Student implements Runnable {    /**     * Phaser to control the execution     */    private Phaser phaser;    /**     * Constructor of the class. Initialize its objects     *      * @param phaser     *            Phaser to control the execution     */    public Student(Phaser phaser) {        this.phaser = phaser;    }    /**     * Main method of the student. It arrives to the exam and does three     * exercises. After each exercise, it calls the phaser to wait that all     * the students finishes the same exercise     */    public void run() {        System.out.printf("%s: Has arrived to do the exam. %s\n", Thread                .currentThread().getName(), new Date());        phaser.arriveAndAwaitAdvance();        System.out.printf("%s: Is going to do the first exercise. %s\n",                Thread.currentThread().getName(), new Date());        doExercise1();        System.out.printf("%s: Has done the first exercise. %s\n", Thread                .currentThread().getName(), new Date());        phaser.arriveAndAwaitAdvance();        System.out.printf("%s: Is going to do the second exercise. %s\n",                Thread.currentThread().getName(), new Date());        doExercise2();        System.out.printf("%s: Has done the second exercise. %s\n", Thread                .currentThread().getName(), new Date());        phaser.arriveAndAwaitAdvance();        System.out.printf("%s: Is going to do the third exercise. %s\n",                Thread.currentThread().getName(), new Date());        doExercise3();        System.out.printf("%s: Has finished the exam. %s\n", Thread                .currentThread().getName(), new Date());        phaser.arriveAndAwaitAdvance();    }    /**     * Does an exercise is to wait a random time     */    private void doExercise1() {        try {            Long duration = (long) (Math.random() * 10);            TimeUnit.SECONDS.sleep(duration);        } catch (InterruptedException e) {            e.printStackTrace();        }    }    /**     * Does an exercise is wait a random time     */    private void doExercise2() {        try {            Long duration = (long) (Math.random() * 10);            TimeUnit.SECONDS.sleep(duration);        } catch (InterruptedException e) {            e.printStackTrace();        }    }    /**     * Does an exercise is wait a random time     */    private void doExercise3() {        try {            Long duration = (long) (Math.random() * 10);            TimeUnit.SECONDS.sleep(duration);        } catch (InterruptedException e) {            e.printStackTrace();        }    }}

结果

Student 0: Has arrived to do the exam. Sun Jul 17 23:58:44 CST 2016
Student 4: Has arrived to do the exam. Sun Jul 17 23:58:44 CST 2016
Student 2: Has arrived to do the exam. Sun Jul 17 23:58:44 CST 2016
Student 1: Has arrived to do the exam. Sun Jul 17 23:58:44 CST 2016
Student 3: Has arrived to do the exam. Sun Jul 17 23:58:44 CST 2016
Phaser: The exam are going to start. The students are ready.
Phaser: We have 5 students.
Student 2: Is going to do the first exercise. Sun Jul 17 23:58:44 CST 2016
Student 3: Is going to do the first exercise. Sun Jul 17 23:58:44 CST 2016
Student 4: Is going to do the first exercise. Sun Jul 17 23:58:44 CST 2016
Student 1: Is going to do the first exercise. Sun Jul 17 23:58:44 CST 2016
Student 0: Is going to do the first exercise. Sun Jul 17 23:58:44 CST 2016
Student 3: Has done the first exercise. Sun Jul 17 23:58:44 CST 2016
Student 2: Has done the first exercise. Sun Jul 17 23:58:47 CST 2016
Student 0: Has done the first exercise. Sun Jul 17 23:58:50 CST 2016
Student 4: Has done the first exercise. Sun Jul 17 23:58:51 CST 2016
Student 1: Has done the first exercise. Sun Jul 17 23:58:52 CST 2016
Phaser: All the students has finished the first exercise.
Phaser: It’s turn for the second one.
Student 2: Is going to do the second exercise. Sun Jul 17 23:58:52 CST 2016
Student 1: Is going to do the second exercise. Sun Jul 17 23:58:52 CST 2016
Student 3: Is going to do the second exercise. Sun Jul 17 23:58:52 CST 2016
Student 4: Is going to do the second exercise. Sun Jul 17 23:58:52 CST 2016
Student 0: Is going to do the second exercise. Sun Jul 17 23:58:52 CST 2016
Student 1: Has done the second exercise. Sun Jul 17 23:58:56 CST 2016
Student 3: Has done the second exercise. Sun Jul 17 23:58:59 CST 2016
Student 2: Has done the second exercise. Sun Jul 17 23:58:59 CST 2016
Student 0: Has done the second exercise. Sun Jul 17 23:58:59 CST 2016
Student 4: Has done the second exercise. Sun Jul 17 23:59:00 CST 2016
Phaser: All the students has finished the second exercise.
Phaser: It’s turn for the third one.
Student 3: Is going to do the third exercise. Sun Jul 17 23:59:00 CST 2016
Student 4: Is going to do the third exercise. Sun Jul 17 23:59:00 CST 2016
Student 1: Is going to do the third exercise. Sun Jul 17 23:59:00 CST 2016
Student 2: Is going to do the third exercise. Sun Jul 17 23:59:00 CST 2016
Student 0: Is going to do the third exercise. Sun Jul 17 23:59:00 CST 2016
Student 0: Has finished the exam. Sun Jul 17 23:59:04 CST 2016
Student 4: Has finished the exam. Sun Jul 17 23:59:06 CST 2016
Student 1: Has finished the exam. Sun Jul 17 23:59:07 CST 2016
Student 2: Has finished the exam. Sun Jul 17 23:59:07 CST 2016
Student 3: Has finished the exam. Sun Jul 17 23:59:08 CST 2016
Phaser: All the students has finished the exam.
Phaser: Thank you for your time.
Main: The phaser has finished: true.

它是怎么工作的…

这个练习模拟了有3个测验的真实测试。全部的学生必须都完成同一个测试才能开始下一个测试。为了实现这个必须使用同步，我们使用了Phaser类，但是你实现了你自己的phaser通过扩展原来的类，并覆盖onAdvance() 方法.

在阶段改变之前和在唤醒 arriveAndAwaitAdvance() 方法中休眠的全部线程们之前，此方法被 phaser 调用。这个方法接收当前阶段数作为参数，0是第一个phase ，还有注册的参与者数。最有用的参数是actual phase。如果你要基于不同的当前阶段执行不同的操作，那么你必须使用选择性结构（if/else 或 switch）来选择你想执行的操作。例子里，我们使用了 switch 结构来为每个phase的改变选择不同的方法。

onAdvance() 方法返回 Boolean 值表明 phaser 终结与否。如果返回 false 值，表示它还没有终结，那么线程将继续执行其他phases。如果phaser 返回真值，那么phaser将叫醒全部待定的线程们，并且转移phaser到terminated 状态，所以之后的任何对phaser的方法的调用都会被立刻返回，还有isTerminated() 方法将返回真值。

在核心类，当你创建 MyPhaser 对象，在phaser中你不用表示参与者的数量。你为每个 Student 对象调用了 register() 方法创建了phaser的参与者的注册。这个调用不会在Student 对象或者执行它的线程与phaser之间这个建立任何关系。 说真的，phaser的参与者数就是个数字而已。phaser与参与者之间没有任何关系。

转自：http://ifeve.com/thread-synchronization-utilities-7/