CyclicBarrier

//使一组线程互相等待，直到所有线程都到达了，才放行所有阻塞的线程。用了分代思想，只用初始化一次，parties会不变，count到达0时放行。
public class CyclicBarrier{}

//所有的参与者，需要等待的线程数量
private final int parties;
//还需要等待的线程数量
private int count;
//放行的同时，开启的新的线程
private final Runnable barrierCommand;

//放行所有
public CyclicBarrier(int parties, Runnable barrierAction) {
if (parties <= 0) throw new IllegalArgumentException();
this.parties = parties;
this.count = parties;
this.barrierCommand = barrierAction;
}
//进行计数器-1
public int await() throws InterruptedException, BrokenBarrierException {
try {
return dowait(false, 0L);
} catch (TimeoutException toe) {
throw new Error(toe); // cannot happen
}
}
//核心方法，计数器到到0时唤醒所有线程，否者一直被阻塞
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();//唤醒所有被阻塞的线程
throw new InterruptedException();
}

int index = --count;
//如果计数器到达0
if (index == 0) {  // tripped
boolean ranAction = false;
try {
final Runnable command = barrierCommand;
if (command != null)
command.run();
ranAction = true;
nextGeneration();//起用下一代
return 0;
} finally {
if (!ranAction)
breakBarrier();
}
}

// loop until tripped, broken, interrupted, or timed out
//被阻塞，除非超时，中断，被唤醒
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();
}
}