文章目录
java中CyclicBarrier的使用
CyclicBarrier是java 5中引入的线程安全的组件。它有一个barrier的概念，主要用来等待所有的线程都执行完毕，然后再去执行特定的操作。
假如我们有很多个线程，每个线程都计算出了一些数据，然后我们需要等待所有的线程都执行完毕，再把各个线程计算出来的数据加起来，的到最终的结果，那么我们就可以使用CyclicBarrier。
CyclicBarrier的方法
我们先看下CyclicBarrier的构造函数：
public CyclicBarrier(int parties, Runnable barrierAction) { if (parties <= 0) throw new IllegalArgumentException(); this.parties = parties; this.count = parties; this.barrierCommand = barrierAction; } public CyclicBarrier(int parties) { this(parties, null); }
CyclicBarrier有两个构造函数，第一个只接受一个参数，表示需要统一行动的线程个数。
第二个参数叫做barrierAction，表示出发barrier是需要执行的方法。
其中barrierAction是一个Runnable，我们可以在其中定义最后需要执行的工作。
再看下重要await方法：
public int await() throws InterruptedException, BrokenBarrierException { try { return dowait(false, 0L); } catch (TimeoutException toe) { throw new Error(toe); // cannot happen } } public int await(long timeout, TimeUnit unit) throws InterruptedException, BrokenBarrierException, TimeoutException { return dowait(true, unit.toNanos(timeout)); }
await也有两个方法，一个是带时间参数的，一个是不带时间参数的。
await本质上调用了lock.newCondition().await()方法。
因为有多个parties，下面我们考虑两种情况。
- 该线程不是最后一个调用await的线程
在这种情况下，该线程将会进入等待状态，直到下面的情况发送：
- 最后一个线程调用await（）
- 其他线程中断了当前线程
- 其他线程中断了其他正在等待的线程
- 其他线程在等待barrier的时候超时
- 其他线程在该barrier上调用的reset（）方法
如果该线程在调用await（）的时候已经设置了interrupted的状态，或者在等待的时候被interrupted，那么将会抛出InterruptedException异常，并清除中断状态。（这里和Thread的interrupt()方法保持一致）
如果任何线程正在等待状态中，这时候barrier被重置。或者在线程调用await方法或者正在等待中，barrier被broken，那么将会抛出BrokenBarrierException。
如果任何线程在等待的时候被中断，那么所有其他等待的线程将会抛出BrokenBarrierException，barrier将会被置为broken状态。
- 如果该线程是最后一个调用await方法的
在这种情况，如果barrierAction不为空，那么该线程将会在其他线程继续执行前调用这个barrierAction。
如果该操作抛出异常，那么barrier的状态将会被置为broken状态。
再看看这个reset() 方法：
public void reset() { final ReentrantLock lock = this.lock; lock.lock(); try { breakBarrier(); // break the current generation nextGeneration(); // start a new generation } finally { lock.unlock(); } }
该方法将会将barrier置为broken状态，并且开启一个新的generation，来进行下一轮的操作。
CyclicBarrier的使用
我们在子线程中生成一个随机的整数队列，当所有的线程都生成完毕之后，我们再将生成的整数全都加起来。看下怎么实现。
定义生成整数队列的子线程：
public class CyclicBarrierUsage implements Runnable { private CyclicBarrier cyclicBarrier; private List<List<Integer>> partialResults; private Random random = new Random(); public CyclicBarrierUsage(CyclicBarrier cyclicBarrier,List<List<Integer>> partialResults){ this.cyclicBarrier=cyclicBarrier; this.partialResults=partialResults; } @Override public void run() { String thisThreadName = Thread.currentThread().getName(); List<Integer> partialResult = new ArrayList<>(); // Crunch some numbers and store the partial result for (int i = 0; i < 10; i++) { Integer num = random.nextInt(10); System.out.println(thisThreadName + ": Crunching some numbers! Final result - " + num); partialResult.add(num); } partialResults.add(partialResult); try { System.out.println(thisThreadName + " waiting for others to reach barrier."); cyclicBarrier.await(); } catch (InterruptedException e) { // ... } catch (BrokenBarrierException e) { // ... } } }
上面的子线程接收外部传入的cyclicBarrier和保存数据的partialResults，并在运行完毕调用cyclicBarrier.await()来等待其他线程执行完毕。
看下CyclicBarrier的构建：
CyclicBarrier cyclicBarrier=new CyclicBarrier(5,()->{ String thisThreadName = Thread.currentThread().getName(); System.out.println( thisThreadName + ": Computing sum of 5 workers, having 10 results each."); int sum = 0; for (List<Integer> threadResult : partialResults) { System.out.print("Adding "); for (Integer partialResult : threadResult) { System.out.print(partialResult+" "); sum += partialResult; } System.out.println(); } System.out.println(thisThreadName + ": Final result = " + sum); });
在CyclicBarrier中，我们定义了一个BarrierAction来做最后数据的汇总处理。
运行：
for (int i = 0; i < 5; i++) { Thread worker = new Thread(new CyclicBarrierUsage(cyclicBarrier,partialResults)); worker.setName("Thread " + i); worker.start(); }
输出结果如下：
Spawning 5 worker threads to compute 10 partial results each Thread 0: Crunching some numbers! Final result - 5 Thread 0: Crunching some numbers! Final result - 3 Thread 1: Crunching some numbers! Final result - 1 Thread 0: Crunching some numbers! Final result - 7 Thread 1: Crunching some numbers! Final result - 8 Thread 0: Crunching some numbers! Final result - 4 Thread 0: Crunching some numbers! Final result - 6 Thread 0: Crunching some numbers! Final result - 9 Thread 1: Crunching some numbers! Final result - 3 Thread 2: Crunching some numbers! Final result - 1 Thread 0: Crunching some numbers! Final result - 0 Thread 2: Crunching some numbers! Final result - 9 Thread 1: Crunching some numbers! Final result - 3 Thread 2: Crunching some numbers! Final result - 7 Thread 0: Crunching some numbers! Final result - 2 Thread 2: Crunching some numbers! Final result - 6 Thread 1: Crunching some numbers! Final result - 6 Thread 2: Crunching some numbers! Final result - 5 Thread 0: Crunching some numbers! Final result - 0 Thread 2: Crunching some numbers! Final result - 1 Thread 1: Crunching some numbers! Final result - 5 Thread 2: Crunching some numbers! Final result - 1 Thread 0: Crunching some numbers! Final result - 7 Thread 2: Crunching some numbers! Final result - 8 Thread 1: Crunching some numbers! Final result - 2 Thread 2: Crunching some numbers! Final result - 4 Thread 0 waiting for others to reach barrier. Thread 2: Crunching some numbers! Final result - 0 Thread 2 waiting for others to reach barrier. Thread 1: Crunching some numbers! Final result - 7 Thread 1: Crunching some numbers! Final result - 6 Thread 1: Crunching some numbers! Final result - 9 Thread 1 waiting for others to reach barrier. Thread 3: Crunching some numbers! Final result - 9 Thread 3: Crunching some numbers! Final result - 3 Thread 3: Crunching some numbers! Final result - 8 Thread 3: Crunching some numbers! Final result - 8 Thread 3: Crunching some numbers! Final result - 1 Thread 3: Crunching some numbers! Final result - 8 Thread 3: Crunching some numbers! Final result - 0 Thread 3: Crunching some numbers! Final result - 5 Thread 3: Crunching some numbers! Final result - 9 Thread 3: Crunching some numbers! Final result - 1 Thread 3 waiting for others to reach barrier. Thread 4: Crunching some numbers! Final result - 2 Thread 4: Crunching some numbers! Final result - 2 Thread 4: Crunching some numbers! Final result - 5 Thread 4: Crunching some numbers! Final result - 5 Thread 4: Crunching some numbers! Final result - 3 Thread 4: Crunching some numbers! Final result - 7 Thread 4: Crunching some numbers! Final result - 4 Thread 4: Crunching some numbers! Final result - 8 Thread 4: Crunching some numbers! Final result - 4 Thread 4: Crunching some numbers! Final result - 3 Thread 4 waiting for others to reach barrier. Thread 4: Computing sum of 5 workers, having 10 results each. Adding 5 3 7 4 6 9 0 2 0 7 Adding 1 9 7 6 5 1 1 8 4 0 Adding 1 8 3 3 6 5 2 7 6 9 Adding 9 3 8 8 1 8 0 5 9 1 Adding 2 2 5 5 3 7 4 8 4 3 Thread 4: Final result = 230 Process finished with exit code 0
本文的例子可以参考https://github.com/ddean2009/learn-java-concurrency/tree/master/CyclicBarrier
