CyclicBarrier意为“循环栅栏”，是一个可循环利用的屏障。CyclicBarrier可以使指定数量线程到达阻塞点后继续后续任务。
演示示例：
首先，新建一个线程类，用于模拟多线程环境。
package com.securitit.serialize.juc;
import java.util.concurrent.CyclicBarrier;
public class CyclicBarrierThread extends Thread {
// CyclicBarrier实例.
private CyclicBarrier cyclicBarrier;
// Constructor.
public CyclicBarrierThread(CyclicBarrier cyclicBarrier) {
this.cyclicBarrier = cyclicBarrier;
}
@Override
public void run() {
try {
Thread.sleep(1000);
System.out.println(Thread.currentThread().getName() + " 到达循环栅栏-A");
cyclicBarrier.await();
System.out.println(Thread.currentThread().getName() + " 通过循环栅栏-A");
Thread.sleep(2000);
System.out.println(Thread.currentThread().getName() + " 到达循环栅栏-B");
cyclicBarrier.await();
System.out.println(Thread.currentThread().getName() + " 通过循环栅栏-B");
} catch (Exception e) {
e.printStackTrace();
}
}
}
在测试类中，启动多线程对循环栅栏CyclicBarrier进行测试。
package com.securitit.serialize.juc;
import java.util.concurrent.CyclicBarrier;
public class CyclicBarrierTester {
// CyclicBarrier实例.
private static CyclicBarrier cyclicBarrier = new CyclicBarrier(3);
public static void main(String[] args) throws Exception {
new CyclicBarrierThread(cyclicBarrier).start();
new CyclicBarrierThread(cyclicBarrier).start();
new CyclicBarrierThread(cyclicBarrier).start();
}
}
输出结果：
Thread-2 到达循环栅栏-A
Thread-1 到达循环栅栏-A
Thread-0 到达循环栅栏-A
Thread-1 通过循环栅栏-A
Thread-0 通过循环栅栏-A
Thread-2 通过循环栅栏-A
Thread-2 到达循环栅栏-B
Thread-0 到达循环栅栏-B
Thread-1 到达循环栅栏-B
Thread-1 通过循环栅栏-B
Thread-0 通过循环栅栏-B
Thread-2 通过循环栅栏-B
从输出结果可以看出，与线程启动顺序无关，所有线程都是到达栅栏位置然后继续运行，到达栅栏线程的多少取决于CyclicBarrier实例化是指定的栅栏数量。
源码分析：
package java.util.concurrent;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
public class CyclicBarrier {
//
private static class Generation {
boolean broken = false;
}
// 守卫栅栏进入的锁.
private final ReentrantLock lock = new ReentrantLock();
// 等待条件.
private final Condition trip = lock.newCondition();
// 总共需要等待的线程数.
private final int parties;
// 所有等待线程都到达时执行该命令.
private final Runnable barrierCommand;
// Generation.
private Generation generation = new Generation();
// 在等待栅栏的数量.
private int count;
// 下一栅栏.
private void nextGeneration() {
// 唤醒所有正在等待的吃的人.
trip.signalAll();
// 重置部分数据.
count = parties;
generation = new Generation();
}
// 打破栅栏.
private void breakBarrier() {
generation.broken = true;
count = parties;
trip.signalAll();
}
// 等待线程.
private int dowait(boolean timed, long nanos)
throws InterruptedException, BrokenBarrierException,
TimeoutException {
final ReentrantLock lock = this.lock;
lock.lock();
try {
final Generation g = generation;
// 屏障已被破坏.抛出异常.
if (g.broken)
throw new BrokenBarrierException();
// 线程响应中断.
if (Thread.interrupted()) {
breakBarrier();
throw new InterruptedException();
}
// 当前线程到达后剩余线程数.
int index = --count;
// 线程已全部到达.
if (index == 0) { // tripped
boolean ranAction = false;
try {
final Runnable command = barrierCommand;
if (command != null)
command.run();
ranAction = true;
// 生成新一代唤醒等待线程，重置count、parties.
nextGeneration();
return 0;
} finally {
// 执行失败，打破栅栏.
if (!ranAction)
breakBarrier();
}
}
//
for (;;) {
// 设置线程阻塞.
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();
}
}
// 栅栏被打破，抛出异常.
if (g.broken)
throw new BrokenBarrierException();
if (g != generation)
return index;
// 超时，打破栅栏，抛出异常.
if (timed && nanos <= 0L) {
breakBarrier();
throw new TimeoutException();
}
}
} finally {
lock.unlock();
}
}
// Construtor.
public CyclicBarrier(int parties, Runnable barrierAction) {
if (parties <= 0) throw new IllegalArgumentException();
this.parties = parties;
this.count = parties;
this.barrierCommand = barrierAction;
}
// Construtor.
public CyclicBarrier(int parties) {
this(parties, null);
}
// 获取需要等待的线程数.
public int getParties() {
return parties;
}
// 阻塞线程.
public int await() throws InterruptedException, BrokenBarrierException {
try {
return dowait(false, 0L);
} catch (TimeoutException toe) {
throw new Error(toe); // cannot happen
}
}
// 带超时时间阻塞线程.
public int await(long timeout, TimeUnit unit)
throws InterruptedException,
BrokenBarrierException,
TimeoutException {
return dowait(true, unit.toNanos(timeout));
}
// 栅栏是否已被破解.
public boolean isBroken() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return generation.broken;
} finally {
lock.unlock();
}
}
// 重置栅栏数据和状态，CyclicBarrier
public void reset() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
// 打破当前栅栏信息.
breakBarrier();
// 开始处理下一栅栏信息。
nextGeneration(); // start a new generation
} finally {
lock.unlock();
}
}
// 已达到等待线程数量.
public int getNumberWaiting() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return parties - count;
} finally {
lock.unlock();
}
}
}
从“演示示例“到”源码分析“，CyclicBarrier的使用以及源码相对比较简单，“源码分析”中是CyclicBarrier全部API。
注：文中源码均来自于JDK1.8版本，不同版本间可能存在差异。
如果有哪里有不明白或不清楚的内容，欢迎留言哦！