在前面已经学习过
Java并发编程-同步辅助类之CountDownLatch
Java并发编程-同步辅助类之CyclicBarrier
这篇文章介绍另一个辅助类Phaser,它是Jdk 1.7才添加的新的功能,它可以实现和CountDownLatch/CyclicBarrier类似的功能,但Phaser的功能更多,更加的灵活,它支持任务在多个点都进行同步,支持动态调整注册任务的数量。当然你也可以使用CountDownLatch,但你必须创建多个CountDownLatch对象,每一阶段都需要一个CountDownLatch对象。
操作方法
Phaser(int parties),构造方法,与CountDownLatch一样,传入同步的线程数,也支持层次构造Phaser(Phaser parent)。
register(),bulkRegister(int Parties),动态添加一个或多个参与者。
arriveAndDeregister()方法,动态撤销线程在phaser的注册,通知phaser对象,该线程已经结束该阶段且不参与后面阶段。
isTerminated(),当phaser没有参与同步的线程时(或者onAdvance返回true),phaser是终止态(如果phaser进入终止态arriveAndAwaitAdvance()和awaitAdvance()都会立即返回,不在等待)isTerminated返回true。
arrive()方法,通知phaser该线程已经完成该阶段,但不等待其他线程。
arriveAndAwaitAdvance()方法,类似await()方法,记录到达线程数,阻塞等待其他线程到达同步点后再继续执行。
awaitAdvance(int phase) /awaitAdvanceInterruptibly(int phase) ,传入阶段数,只有当前阶段等于phase阶段时才阻塞等待。后者如果线程在休眠被中断会抛出InterruptedException异常(phaser的其他方法对中断都不会抛出异常)。
onAdvance(int phase, int registeredParties)方法。参数phase是阶段数,每经过一个阶段该数加1,registeredParties是当前参与的线程数。此方法有2个作用:1、当每一个阶段执行完毕,此方法会被自动调用,因此,重载此方法写入的代码会在每个阶段执行完毕时执行,相当于CyclicBarrier的barrierAction。2、当此方法返回true时,意味着Phaser被终止,因此可以巧妙的设置此方法的返回值来终止所有线程。例如:若此方法返回值为 phase>=3,其含义为当整个线程执行了4个阶段后,程序终止。
forceTermination()方法,强制phaser进入终止态。
使用实例
对在CountDownLatch中的例子使用Phaser代替CountDownLatch进行改进:
package MyThread;
import java.util.concurrent.Phaser;
public class Match {
// 模拟了100米赛跑,10名选手,只等裁判一声令下。当所有人都到达终点时,比赛结束。
public static void main(String[] args) throws InterruptedException {
final Phaser phaser=new Phaser(1) ;
// 十名选手
for (int index = 0; index < 10; index++) {
phaser.register();
new Thread(new player(phaser),"player"+index).start();
}
System.out.println("Game Start");
//注销当前线程,比赛开始
phaser.arriveAndDeregister();
//是否非终止态一直等待
while(!phaser.isTerminated()){
}
System.out.println("Game Over");
}
}
class player implements Runnable{
private final Phaser phaser ;
player(Phaser phaser){
this.phaser=phaser;
}
@Override
public void run() {
// TODO Auto-generated method stub
try {
// 第一阶段——等待创建好所有线程再开始
phaser.arriveAndAwaitAdvance();
// 第二阶段——等待所有选手准备好再开始
Thread.sleep((long) (Math.random() * 10000));
System.out.println(Thread.currentThread().getName() + " ready");
phaser.arriveAndAwaitAdvance();
// 第三阶段——等待所有选手准备好到达,到达后,该线程从phaser中注销,不在进行下面的阶段。
Thread.sleep((long) (Math.random() * 10000));
System.out.println(Thread.currentThread().getName() + " arrived");
phaser.arriveAndDeregister();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
3 Sample Usage
3.1 Sample 1
在有些场景下,我们希望控制多个线程的启动时机:例如在并发相关的单元测试中,有时需要控制线程的启动时机,以期获得最大程度的并发,通常我们会使用CountDownLatch,以下是使用Phaser的版本。
- import java.util.concurrent.Phaser;
- public class PhaserTest1 {
- public static void main(String args[]) {
- //
- final int count = 5;
- final Phaser phaser = new Phaser(count);
- for(int i = 0; i < count; i++) {
- System.out.println("starting thread, id: " + i);
- final Thread thread = new Thread(new Task(i, phaser));
- thread.start();
- }
- }
- public static class Task implements Runnable {
- //
- private final int id;
- private final Phaser phaser;
- public Task(int id, Phaser phaser) {
- this.id = id;
- this.phaser = phaser;
- }
- @Override
- public void run() {
- phaser.arriveAndAwaitAdvance();
- System.out.println("in Task.run(), phase: " + phaser.getPhase() + ", id: " + this.id);
- }
- }
- }
以上例子中,由于线程是在一个循环中start,因此start的时机有一定的间隔。本例中这些线程实际开始工作的时机是在
所有的线程都调用了phaser.arriveAndAwaitAdvance()之后。
此外,如果留心arriveAndAwaitAdvance()方法的签名,会发现它并没有抛出InterruptedException,实际上,即使 当前线
程被中断,arriveAndAwaitAdvance()方法也不会返回,而是继续等待。如果在等待时希望可中断,或者可超时,那么需要使
用以下 方法:
- awaitAdvance(arrive()) // 等效于arriveAndAwaitAdvance()
- awaitAdvanceInterruptibly(int phase)
- awaitAdvanceInterruptibly(int phase, long timeout, TimeUnit unit)
3.2 Sample 2
有些时候我们希望只有在某些外部条件满足时,才真正开始任务的执行,例如:
- import java.io.BufferedReader;
- import java.io.InputStreamReader;
- import java.util.concurrent.Phaser;
- public class PhaserTest2 {
- public static void main(String args[]) throws Exception {
- //
- final Phaser phaser = new Phaser(1);
- for(int i = 0; i < 5; i++) {
- phaser.register();
- System.out.println("starting thread, id: " + i);
- final Thread thread = new Thread(new Task(i, phaser));
- thread.start();
- }
- //
- System.out.println("Press ENTER to continue");
- BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
- reader.readLine();
- phaser.arriveAndDeregister();
- }
- public static class Task implements Runnable {
- //
- private final int id;
- private final Phaser phaser;
- public Task(int id, Phaser phaser) {
- this.id = id;
- this.phaser = phaser;
- }
- @Override
- public void run() {
- phaser.arriveAndAwaitAdvance();
- System.out.println("in Task.run(), phase: " + phaser.getPhase() + ", id: " + this.id);
- }
- }
- }
以上例子中,只有当用户按下回车之后,任务才真正开始执行。需要注意的是,arriveAndDeregister()方法不会被阻塞,并且
返回到达时的phase number(arrive方法也是如此)。
3.3 Sample 3
CyclicBarrier支持barrier action, Phaser同样也支持。不同之处是Phaser的barrier action需要改写onAdvance方法来进行定制。
- import java.util.concurrent.Phaser;
- public class PhaserTest3 {
- public static void main(String args[]) throws Exception {
- //
- final int count = 5;
- final int phaseToTerminate = 3;
- final Phaser phaser = new Phaser(count) {
- @Override
- protected boolean onAdvance(int phase, int registeredParties) {
- System.out.println("====== " + phase + " ======");
- return phase >= phaseToTerminate || registeredParties == 0;
- }
- };
- //
- for(int i = 0; i < count; i++) {
- System.out.println("starting thread, id: " + i);
- final Thread thread = new Thread(new Task(i, phaser));
- thread.start();
- }
- }
- public static class Task implements Runnable {
- //
- private final int id;
- private final Phaser phaser;
- public Task(int id, Phaser phaser) {
- this.id = id;
- this.phaser = phaser;
- }
- @Override
- public void run() {
- do {
- try {
- Thread.sleep(500);
- } catch(InterruptedException e) {
- // NOP
- }
- System.out.println("in Task.run(), phase: " + phaser.getPhase() + ", id: " + this.id);
- phaser.arriveAndAwaitAdvance();
- } while(!phaser.isTerminated());
- }
- }
- }
本例中的barrier action只是简单地打印了一条信息,此外在超过指定的迭代次数后终止了Phaser。
3.4 Sample 4
在Smaple 3的例子中,主线程在其它工作线程结束之前已经终止。如果希望主线程等待这些工作线程结束,除了使用Thread.join()之外,也可以尝试以下的方式:
- import java.util.concurrent.Phaser;
- public class PhaserTest4 {
- public static void main(String args[]) throws Exception {
- //
- final int count = 5;
- final int phaseToTerminate = 3;
- final Phaser phaser = new Phaser(count) {
- @Override
- protected boolean onAdvance(int phase, int registeredParties) {
- System.out.println("====== " + phase + " ======");
- return phase == phaseToTerminate || registeredParties == 0;
- }
- };
- //
- for(int i = 0; i < count; i++) {
- System.out.println("starting thread, id: " + i);
- final Thread thread = new Thread(new Task(i, phaser));
- thread.start();
- }
- //
- phaser.register();
- while (!phaser.isTerminated()) {
- phaser.arriveAndAwaitAdvance();
- }
- System.out.println("done");
- }
- public static class Task implements Runnable {
- //
- private final int id;
- private final Phaser phaser;
- public Task(int id, Phaser phaser) {
- this.id = id;
- this.phaser = phaser;
- }
- @Override
- public void run() {
- while(!phaser.isTerminated()) {
- try {
- Thread.sleep(500);
- } catch(InterruptedException e) {
- // NOP
- }
- System.out.println("in Task.run(), phase: " + phaser.getPhase() + ", id: " + this.id);
- phaser.arriveAndAwaitAdvance();
- }
- }
- }
- }
如果希望主线程在特定的phase结束之后终止,那么可以在主线程中调用下述方法:
- public static void awaitPhase(Phaser phaser, int phase) {
- int p = phaser.register(); // assumes caller not already registered
- while (p < phase) {
- if (phaser.isTerminated()) {
- break; // ... deal with unexpected termination
- } else {
- p = phaser.arriveAndAwaitAdvance();
- }
- }
- phaser.arriveAndDeregister();
- }
需要注意的是,awaitPhase方法中的if (phaser.isTerminated()) 分支里需要能够正确处理Phaser终止的情况。否则由于
在Phaser终止之后, phaser.register()和arriveAndAwaitAdvance()方法均返回负值,那么上述方法可能陷入死循环。
3.5 Sample 5
以下对Phaser进行分层的例子:
- import java.util.concurrent.Phaser;
- public class PhaserTest6 {
- //
- private static final int TASKS_PER_PHASER = 4;
- public static void main(String args[]) throws Exception {
- //
- final int phaseToTerminate = 3;
- final Phaser phaser = new Phaser() {
- @Override
- protected boolean onAdvance(int phase, int registeredParties) {
- System.out.println("====== " + phase + " ======");
- return phase == phaseToTerminate || registeredParties == 0;
- }
- };
- //
- final Task tasks[] = new Task[10];
- build(tasks, 0, tasks.length, phaser);
- for (int i = 0; i < tasks.length; i++) {
- System.out.println("starting thread, id: " + i);
- final Thread thread = new Thread(tasks[i]);
- thread.start();
- }
- }
- public static void build(Task[] tasks, int lo, int hi, Phaser ph) {
- if (hi - lo > TASKS_PER_PHASER) {
- for (int i = lo; i < hi; i += TASKS_PER_PHASER) {
- int j = Math.min(i + TASKS_PER_PHASER, hi);
- build(tasks, i, j, new Phaser(ph));
- }
- } else {
- for (int i = lo; i < hi; ++i)
- tasks[i] = new Task(i, ph);
- }
- }
- public static class Task implements Runnable {
- //
- private final int id;
- private final Phaser phaser;
- public Task(int id, Phaser phaser) {
- this.id = id;
- this.phaser = phaser;
- this.phaser.register();
- }
- @Override
- public void run() {
- while (!phaser.isTerminated()) {
- try {
- Thread.sleep(200);
- } catch (InterruptedException e) {
- // NOP
- }
- System.out.println("in Task.run(), phase: " + phaser.getPhase() + ", id: " + this.id);
- phaser.arriveAndAwaitAdvance();
- }
- }
- }
- }
需要注意的是,TASKS_PER_PHASER的值取决于具体的Task实现。对于Task执行时间很短的场景(也就是竞争相对激烈)
,可以考虑使用较小的TASKS_PER_PHASER值,例如4。反之可以适当增大TASKS_PER_PHASER。