CyclicBarrier
- 柵欄內部使用了ReentrantLock和Condition兩個類,用於實現到達柵欄處等待與喚醒
/** The lock for guarding barrier entry */
private final ReentrantLock lock = new ReentrantLock();
/** Condition to wait on until tripped */
private final Condition trip = lock.newCondition();
- 構造函數
//parties到達柵欄線程數開始放行
public CyclicBarrier(int parties) {
this(parties, null);
}
//barrierAction開始放行後,action將會被調用一次
public CyclicBarrier(int parties, Runnable barrierAction) {
if (parties <= 0) throw new IllegalArgumentException();
this.parties = parties;
this.count = parties;
this.barrierCommand = barrierAction;
}
- 核心方法await()在柵欄前等待
public int await() throws InterruptedException, BrokenBarrierException {
try {
return dowait(false, 0L);
} catch (TimeoutException toe) {
throw new Error(toe); // cannot happen
}
}
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;
//index == 0 說明都到了柵欄處,可以喚醒處於柵欄的線程
if (index == 0) { // tripped
boolean ranAction = false;
try {
final Runnable command = barrierCommand;
if (command != null)
command.run();
ranAction = true;
//nextGeneration喚醒await的線程
//把count重置成parties使得柵欄可被複用
nextGeneration();
return 0;
} finally {
if (!ranAction)
breakBarrier();
}
}
// loop until tripped, broken, interrupted, or timed out
//線程到了柵欄,則要阻塞在這裏
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 {
//喚醒同步隊列的後續節點線程
lock.unlock();
}
}