Java 7 Phaser 有什么新特点

     <p><strong>1 Overview</strong></p>    <p>Java 7的并发包中推出了Phaser，其功能跟CyclicBarrier和CountDownLatch有些重叠，但是提供了更灵活的用法，例如支持动态调整注册任务的数量等。本文在Phaser自带的示例代码基础上进行一下简单的分析。</p>    <p></p>    <p><strong>2 Glossary</strong></p>    <p><strong>2.1 Registration</strong></p>    <p>Phaser支持通过register()和bulkRegister(int parties)方法来动态调整注册任务的数量，此外也支持通过其构造函数进行指定初始数量。在适当的时机，Phaser支持减少注册任务的数量，例如arriveAndDeregister()。单个Phaser实例允许的注册任务数的上限是65535。</p>    <p><strong><br /> </strong></p>    <p><strong>2.2 Arrival</strong></p>    <p>正如Phaser类的名字所暗示，每个Phaser实例都会维护一个phase number，初始值为0。每当所有注册的任务都到达Phaser时，phase number累加，并在超过Integer.MAX_VALUE后清零。arrive()和arriveAndDeregister()方法用于记录到 达，arriveAndAwaitAdvance()方法用于记录到达，并且等待其它未到达的任务。</p>    <p><strong><br /> </strong></p>    <p><strong>2.3 Termination</strong></p>    <p>Phaser支持终止。Phaser终止之后，调用register()和bulkRegister(int parties)方法没有任何效果，arriveAndAwaitAdvance()方法也会立即返回。触发终止的时机是在protected boolean onAdvance(int phase, int registeredParties)方法返回时，如果该方法返回true，那么Phaser会被终止。默认实现是在注册任务数为0时返回true（即return registeredParties == 0;）。此外，forceTermination()方法用于强制终止，isTerminated()方法用于判断是否已经终止。</p>    <p><strong><br /> </strong></p>    <p><strong>2.4 Tiering</strong></p>    <p>Phaser支持层次结构，即通过构造函数Phaser(Phaser parent)和Phaser(Phaser parent, int parties)构造一个树形结构。这有助于减轻因在单个的Phaser上注册过多的任务而导致的竞争，从而提升吞吐量，代价是增加单个操作的开销。</p>    <p></p>    <p><strong>3 Sample Usage</strong></p>    <p><strong>3.1 Sample 1</strong></p>    <p>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。</p>    <pre class="brush:java; toolbar: true; auto-links: false;">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);         }     } }</pre>    <p></p>    <p>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。</p>    <p>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：</p>    <pre class="brush:java; toolbar: true; auto-links: false;">awaitAdvance(arrive())  // 等效于arriveAndAwaitAdvance() awaitAdvanceInterruptibly(int phase) awaitAdvanceInterruptibly(int phase, long timeout, TimeUnit unit)</pre>    <p></p>    <p><strong>3.2 Sample 2</strong></p>    <p>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：</p>    <pre class="brush:java; toolbar: true; auto-links: false;">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);         }     } }</pre>    <p></p>    <p>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）。</p>    <p></p>    <p><strong>3.3 Sample 3</strong></p>    <p>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。</p>    <pre class="brush:java; toolbar: true; auto-links: false;">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());         }     } }</pre>    <p></p> 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。    <p><strong><br /> </strong></p>    <p><strong>3.4 Sample 4</strong></p>    <p>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：</p>    <pre class="brush:java; toolbar: true; auto-links: false;">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();             }         }     } }</pre>    <p></p> 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：    <pre class="brush:java; toolbar: true; auto-links: false;">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(); }</pre>    <p></p> 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。    <p><strong><br /> </strong></p>    <p><strong>3.5 Sample 5</strong></p>    <p>以下对Phaser进行分层的例子：</p>    <pre class="brush:java; toolbar: true; auto-links: false;">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();             }         }     } }</pre>    <p></p>    <p>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。</p>    <p>原文出处：<a href="/misc/goto?guid=4959500085143036191" rel="nofollow">http://whitesock.iteye.com/blog/1135457</a></p>