Java并发包中CyclicBarrier的工作原理、使用示例
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
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
public int await() throws InterruptedException, BrokenBarrierException {
try {
return dowait(false, 0L);
} catch (TimeoutException toe) {
throw new Error(toe); // cannot happen
}
}
dowait源码
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
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
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
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
3. 参考内容
[1] http://ifeve.com/concurrency-cyclicbarrier/
