1. CyclicBarrier的介绍与源码分析
CyclicBarrier 的字面意思是可循环（Cyclic）使用的屏障（Barrier）。它要做的事情是，让一组线程到达一个屏障（也可以叫同步点）时被阻塞，直到最后一个线程到达屏障时，屏障才会开门，所有被屏障拦截的线程才会继续干活。线程进入屏障通过CyclicBarrier的await()方法。
CyclicBarrier默认的构造方法是CyclicBarrier(int parties)，其参数表示屏障拦截的线程数量，每个线程调用await方法告诉CyclicBarrier我已经到达了屏障，然后当前线程被阻塞。
CyclicBarrier还提供一个更高级的构造函数CyclicBarrier(int parties, Runnable barrierAction)，用于在线程到达屏障时，优先执行barrierAction这个Runnable对象，方便处理更复杂的业务场景。
构造函数
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public CyclicBarrier(int parties) { this(parties, null);}public int getParties() { return parties;} |
实现原理：在CyclicBarrier的内部定义了一个Lock对象，每当一个线程调用CyclicBarrier的await方法时，将剩余拦截的线程数减1，然后判断剩余拦截数是否为0，如果不是，进入Lock对象的条件队列等待。如果是，执行barrierAction对象的Runnable方法，然后将锁的条件队列中的所有线程放入锁等待队列中，这些线程会依次的获取锁、释放锁，接着先从await方法返回，再从CyclicBarrier的await方法中返回。
await源码
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public int await() throws InterruptedException, BrokenBarrierException { try { return dowait(false, 0L); } catch (TimeoutException toe) { throw new Error(toe); // cannot happen }} |
dowait源码
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private int dowait(boolean timed, long nanos) throws InterruptedException, BrokenBarrierException, TimeoutException { final ReentrantLock lock = this.lock; lock.lock(); try { final Generation g = generation; if (g.broken) throw new BrokenBarrierException(); if (Thread.interrupted()) { breakBarrier(); throw new InterruptedException(); } int index = --count; if (index == 0) { // tripped boolean ranAction = false; try { final Runnable command = barrierCommand; if (command != null) command.run(); ranAction = true; nextGeneration(); return 0; } finally { if (!ranAction) breakBarrier(); } } // loop until tripped, broken, interrupted, or timed out for (;;) { try { if (!timed) trip.await(); else if (nanos > 0L) nanos = trip.awaitNanos(nanos); } catch (InterruptedException ie) { if (g == generation && ! g.broken) { breakBarrier(); throw ie; } else { // We're about to finish waiting even if we had not // been interrupted, so this interrupt is deemed to // "belong" to subsequent execution. Thread.currentThread().interrupt(); } } if (g.broken) throw new BrokenBarrierException(); if (g != generation) return index; if (timed && nanos <= 0L) { breakBarrier(); throw new TimeoutException(); } } } finally { lock.unlock(); }} |
当最后一个线程到达屏障点，也就是执行dowait方法时，会在return 0 返回之前调用finally块中的breakBarrier方法。
breakBarrier源代码
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private void breakBarrier() { generation.broken = true; count = parties; trip.signalAll();} |
CyclicBarrier主要用于一组线程之间的相互等待，而CountDownLatch一般用于一组线程等待另一组些线程。实际上可以通过CountDownLatch的countDown()和await()来实现CyclicBarrier的功能。即 CountDownLatch中的countDown()+await() = CyclicBarrier中的await()。注意：在一个线程中先调用countDown()，然后调用await()。
其它方法：CycliBarrier对象可以重复使用，重用之前应当调用CyclicBarrier对象的reset方法。
reset源码
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public void reset() { final ReentrantLock lock = this.lock; lock.lock(); try { breakBarrier(); // break the current generation nextGeneration(); // start a new generation } finally { lock.unlock(); }} |
2. 使用示例
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package javalearning;import java.util.Random;import java.util.concurrent.BrokenBarrierException;import java.util.concurrent.CyclicBarrier;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class CyclicBarrierDemo { private CyclicBarrier cb = new CyclicBarrier(4); private Random rnd = new Random(); class TaskDemo implements Runnable{ private String id; TaskDemo(String id){ this.id = id; } @Override public void run(){ try { Thread.sleep(rnd.nextInt(1000)); System.out.println("Thread " + id + " will wait"); cb.await(); System.out.println("-------Thread " + id + " is over"); } catch (InterruptedException e) { } catch (BrokenBarrierException e) { } } } public static void main(String[] args){ CyclicBarrierDemo cbd = new CyclicBarrierDemo(); ExecutorService es = Executors.newCachedThreadPool(); es.submit(cbd.new TaskDemo("a")); es.submit(cbd.new TaskDemo("b")); es.submit(cbd.new TaskDemo("c")); es.submit(cbd.new TaskDemo("d")); es.shutdown(); }} |
在这个示例中，我们创建了四个线程a、b、c、d，这四个线程提交给了线程池。四个线程不同时间到达cb.await()语句，当四个线程都输出“Thread x will wait”以后才会输出“Thread x is over”。
运行结果
Thread d will wait
Thread a will wait
Thread c will wait
Thread b will wait
-------Thread b is over
-------Thread d is over
-------Thread a is over
-------Thread c is over