1、循坏屏障可以用一句话来总结：集齐七颗龙珠，方能召唤神龙
/**
* CyclicBarrier
* 的字面意思是可循环（Cyclic）使用屏障（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。
*/
public class CyclicBarrierDemo {
private static final int NUMBER = 7;
public static void main(String[] args) {
CyclicBarrier cyclicBarrier = new CyclicBarrier(NUMBER, () -> {
System.out.println("集齐七颗龙珠，方能召唤神龙");
});
for (int i = 1; i <= NUMBER; i++) {
final int temp = i;
new Thread(() -> {
System.out.println(Thread.currentThread().getName()+"\t 收集到第；"+temp+"\t 颗龙珠");
try {
cyclicBarrier.await();
} catch (InterruptedException|BrokenBarrierException e) {
e.printStackTrace();
}
}, String.valueOf(i)).start();
}
}
}
2、循环屏障源码分析
/**
* desc：循坏屏障的源码分析
*
* @author Leslie
*/
public class CyclicBarrier {
/**
* 每一个屏障代表一个Generation实例
*/
private static class Generation {
boolean broken = false;
}
/**
* 可重用锁用于守护屏障
*/
private final ReentrantLock lock = new ReentrantLock();
/**
* 守护屏障锁的通信用于通知屏障被推到
*/
private final Condition trip = lock.newCondition();
/**
* 屏障的线程组成数量
*/
private final int parties;
/**
* 屏障被推到要执行的任务
*/
private final Runnable barrierCommand;
/**
* 当前屏障的 Generation 实例
*/
private CyclicBarrier.Generation generation = new CyclicBarrier.Generation();
/**
* 屏障任然在等待的组成成员数，当成员数为0的时候，
* 屏障被推到，开始执行屏障任务，同时会被重置成和初始parties值一样
*/
private int count;
/**
* 更新状态，当屏障被推到的时候，同时唤醒所有的等待的线程，
* 只有当持有锁的时候，才能被调用
*/
private void nextGeneration() {
// signal completion of last generation
trip.signalAll();
// set up next generation
count = parties;
generation = new CyclicBarrier.Generation();
}
/**
* 设置屏障被推到
*/
private void breakBarrier() {
generation.broken = true;
count = parties;
trip.signalAll();
}
/**
* 每一个线程执行完一个线程任务之后，就调用此方法，让屏障中的
* count 数量减 1
*/
private int dowait(boolean timed, long nanos)
throws InterruptedException, BrokenBarrierException,
TimeoutException {
final ReentrantLock lock = this.lock;
lock.lock();
try {
final CyclicBarrier.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 {
// 当所有的线程都执行完任务之后，就开始执行屏障推到之后的任务
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)
//如果没有超时，就进行等待
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();
}
}
/**
* 屏障的构造器
* @param parties 屏障的线程组成数量
* @param barrierAction 屏障被推到时要执行的操作
* @throws IllegalArgumentException 线程数量小于1 时抛出的异常
*/
public CyclicBarrier(int parties, Runnable barrierAction) {
if (parties <= 0) throw new IllegalArgumentException();
this.parties = parties;
this.count = parties;
this.barrierCommand = barrierAction;
}
/**
* 构造一个没有任何动作执行的一个屏障
*
* @param parties 屏障线程组成数量
* @throws IllegalArgumentException 参数异常
*/
public CyclicBarrier(int parties) {
this(parties, null);
}
/**
* 获取屏障的线程组成数量
*
* @return 屏障的线程组成数量
*/
public int getParties() {
return parties;
}
/**
* 屏障中的线程成员，执行完线程任务之后就调用此方法
* 这样可以使屏障统计当前线程成员中多少线程执行完了任务
*/
public int await() throws InterruptedException, BrokenBarrierException {
try {
return dowait(false, 0L);
} catch (TimeoutException toe) {
throw new Error(toe); // cannot happen
}
}
/**
* 调用时 屏障线程执行任务数量减少 1
*/
public int await(long timeout, TimeUnit unit)
throws InterruptedException,
BrokenBarrierException,
TimeoutException {
return dowait(true, unit.toNanos(timeout));
}
/**
* 获取屏障的状态
*/
public boolean isBroken() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return generation.broken;
} finally {
lock.unlock();
}
}
/**
* 重置屏障中的状态
*/
public void reset() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
breakBarrier(); // break the current generation
nextGeneration(); // start a new generation
} finally {
lock.unlock();
}
}
/**
* 获取还有多少个线程还没有执行完其相应的任务
*/
public int getNumberWaiting() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return parties - count;
} finally {
lock.unlock();
}
}
}
相应到此，你绝对掌握了CyclicBarrierl了。
