理解java并发工具Phaser

java为我们提供了很多并发工具&＃xff0c;比如Semaphore、CountDownLatch、CyclicBarrier还有我们这里要讲到的phaser。

当某些任务是分成多个步骤来执行时&＃xff0c;并且同一个步骤之间可以并发&＃xff0c;不同步骤之间不能并发的时候&＃xff0c; 此机制是非常有用的。Phaser类提供的机制是在每个步骤的结尾同步线程&＃xff0c;所以除非全部线程完成第一个步骤&＃xff0c;否则线程不能开始进行第二步。

使用phaser你首先需要指示参与同步的线程的个数&＃xff0c;我们可以使用 phaser.register()或者new Phaser(5)&＃xff0c;下面的示意图给出了形象的解释&＃xff1a;

我们假设每个方框代表需要同步的线程&＃xff0c;这里直接使用new Phaser(4)说明有4个线程需要同步&＃xff1a;

如果在同一个phaser对象上使用phaser.register()&＃xff0c;每使用一次就增加一个需要同步的线程。也就是增加一个方框的数量。

如果现在有线程调用了&＃xff1a;phaser.arriveAndAwaitAdvance()那就说明有一个线程到达了同步点&＃xff0c;它将阻塞在调用这个函数的位置&＃xff0c;直到所有需要同步的线程都到达&＃xff0c;将对所有线程统一放行&＃xff0c;进行第二个阶段任务的执行。

在第二阶段的时候&＃xff0c;又会有这四个方框被创建出来&＃xff0c;等待线程的到达&＃xff0c;然后又统一放行&＃xff0c;最后直到方框数量为0&＃xff0c;方框数量是如何降到0的&＃xff0c;就需要了解如果减少方框的数量了。

我们在执行期间&＃xff0c;可以去增加或者减少这些方框的数量。增加的话依旧依旧可以使用phaser.register()如果需要减少方框的数量可以调用phaser.arriveAndDeregister()&＃xff0c;也就是通知Phaser对象&＃xff0c;当前线程已经结束这个阶段&＃xff0c;并且将不再参与接下来的阶段操作。这就相当于说明这个阶段到达了方框&＃xff0c;并且下一个阶段减少一个方框。一般一个线程的所有阶段任务都结束了&＃xff0c;需要调用phaser.arriveAndDeregister();或者一个线程在执行的时候我们希望停止这个线程的执行了&＃xff0c;那么可以调用phaser.arriveAndDeregister();通知phaser后面阶段不用同步这个线程了。

Phaser还提供了一个方法protected boolean onAdvance(int phase, int registeredParties)&＃xff0c;这个方法每当统一放行的时候会自动执行一次&＃xff0c;它接收2个参数&＃xff1a;当前阶段数&＃xff08;从0开始&＃xff09;和注册的参与者数&＃xff1b;

在这里有一个例子&＃xff0c;它实现一个模拟测验&＃xff0c;所有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。作为一个练习&＃xff0c;你可以将例子做个扩展&＃xff0c;比如如果解题时间&＃xff08;代码中表现为线程睡眠时间&＃xff09;大于8s&＃xff0c;就说明学生做不出来这一题&＃xff0c;则退出测验。

下面是例子的输出&＃xff0c;一个线程代表一个学生&＃xff1a;

Thread-4: Is going to do the first exercise. Mon Jul 22 13:40:50 CST 2019
Thread-4: Has done the first exercise. Mon Jul 22 13:40:50 CST 2019
Thread-3: Is going to do the first exercise. Mon Jul 22 13:40:50 CST 2019
Thread-0: Is going to do the first exercise. Mon Jul 22 13:40:50 CST 2019
Thread-2: Has done the first exercise. Mon Jul 22 13:40:52 CST 2019
Thread-0: Has done the first exercise. Mon Jul 22 13:40:55 CST 2019
Thread-1: Has done the first exercise. Mon Jul 22 13:40:56 CST 2019
Thread-3: Has done the first exercise. Mon Jul 22 13:40:57 CST 2019
Phaser: All the students has finished the first exercise.
Phaser: It&＃39;s turn for the second one.
Thread-1: Is going to do the second exercise. Mon Jul 22 13:40:57 CST 2019
Thread-0: Is going to do the second exercise. Mon Jul 22 13:40:57 CST 2019
Thread-2: Is going to do the second exercise. Mon Jul 22 13:40:57 CST 2019
Thread-2: Has done the second exercise. Mon Jul 22 13:40:57 CST 2019
Thread-4: Is going to do the second exercise. Mon Jul 22 13:40:57 CST 2019
Thread-3: Is going to do the second exercise. Mon Jul 22 13:40:57 CST 2019
Thread-0: Has done the second exercise. Mon Jul 22 13:40:59 CST 2019
Thread-4: Has done the second exercise. Mon Jul 22 13:40:59 CST 2019
Thread-3: Has done the second exercise. Mon Jul 22 13:41:03 CST 2019
Thread-1: Has done the second exercise. Mon Jul 22 13:41:04 CST 2019
Phaser: All the students has finished the second exercise.
Phaser: It&＃39;s turn for the third one.
Thread-3: Is going to do the third exercise. Mon Jul 22 13:41:04 CST 2019
Thread-3: Has finished the exam. Mon Jul 22 13:41:04 CST 2019
Thread-4: Is going to do the third exercise. Mon Jul 22 13:41:04 CST 2019
Thread-0: Is going to do the third exercise. Mon Jul 22 13:41:04 CST 2019
Thread-2: Is going to do the third exercise. Mon Jul 22 13:41:04 CST 2019
Thread-1: Is going to do the third exercise. Mon Jul 22 13:41:04 CST 2019
Thread-0: Has finished the exam. Mon Jul 22 13:41:06 CST 2019
Thread-4: Has finished the exam. Mon Jul 22 13:41:09 CST 2019
Thread-2: Has finished the exam. Mon Jul 22 13:41:09 CST 2019
Thread-1: Has finished the exam. Mon Jul 22 13:41:09 CST 2019
Phaser: All the students has finished the exam.
Phaser: Thank you for your time.
Main: The phaser has finished: true.


下面是添加了模拟学生不会做的情况的输出&＃xff1a;

Thread-1: Has arrived to do the exam. Mon Jul 22 14:00:09 CST 2019
Thread-4: Has arrived to do the exam. Mon Jul 22 14:00:09 CST 2019
Thread-3: Has arrived to do the exam. Mon Jul 22 14:00:09 CST 2019
Thread-0: Has arrived to do the exam. Mon Jul 22 14:00:09 CST 2019
Thread-2: Has arrived to do the exam. Mon Jul 22 14:00:09 CST 2019
Phaser: The exam are going to start. The students are ready.
Phaser: We have 5 students.
Thread-2: Is going to do the first exercise. Mon Jul 22 14:00:09 CST 2019
Thread-2: Exit the exam
Thread-4: Is going to do the first exercise. Mon Jul 22 14:00:09 CST 2019
Thread-4: Exit the exam
Thread-0: Is going to do the first exercise. Mon Jul 22 14:00:09 CST 2019
Thread-0: Has done the first exercise. Mon Jul 22 14:00:09 CST 2019
Thread-3: Is going to do the first exercise. Mon Jul 22 14:00:09 CST 2019
Thread-1: Is going to do the first exercise. Mon Jul 22 14:00:09 CST 2019
Thread-1: Exit the exam
Thread-3: Has done the first exercise. Mon Jul 22 14:00:14 CST 2019
Phaser: All the students has finished the first exercise.
Phaser: It&＃39;s turn for the second one.
Thread-0: Is going to do the second exercise. Mon Jul 22 14:00:14 CST 2019
Thread-0: Exit the exam
Thread-3: Is going to do the second exercise. Mon Jul 22 14:00:14 CST 2019
Thread-3: Has done the second exercise. Mon Jul 22 14:00:19 CST 2019
Phaser: All the students has finished the second exercise.
Phaser: It&＃39;s turn for the third one.
Thread-3: Is going to do the third exercise. Mon Jul 22 14:00:19 CST 2019
Thread-3: Exit the exam
Phaser: All the students has finished the exam.
Phaser: Thank you for your time.
Main: The phaser has finished: true.