用法我就不說了,百度一大堆,我在這裏帶大家看看源碼瞭解其原理:
CyclicBarrier barrier = new CyclicBarrier(3);
public CyclicBarrier(int parties) {
this(parties, null);
}
public CyclicBarrier(int parties, Runnable barrierAction) {
if (parties <= 0) throw new IllegalArgumentException();
this.parties = parties;
this.count = parties;
this.barrierCommand = barrierAction;
}
這裏設置了一個變量 count 和 parties 一開始都是初始值。
接着瞭解其核心方法await()
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; 聲明瞭一個鎖,鎖裏面其實有個同步隊列。
lock.lock();
try {
final Generation g = generation;
if (g.broken) //判斷線程是否被破壞,其實就是下面breakBarrier()方法會設置爲被中斷
throw new BrokenBarrierException();
if (Thread.interrupted()) {
breakBarrier();//設置線程中斷
throw new InterruptedException();
}
int index = --count;重頭戲在這裏,判斷之前聲明的count是否降到0,所有的線程都走到這裏的話就會減到0
if (index == 0) { // tripped
boolean ranAction = false; 設置一個標識,
try {
final Runnable command = barrierCommand;
if (command != null)
command.run();自己先跑
ranAction = true;
nextGeneration();喚醒其他線程跑,並且設置count 爲原來的值,就是又可以循環使用了
return 0;return 之前執行finally lock.unlock();方法釋放鎖。
}
} finally {
if (!ranAction)
breakBarrier();
}
}
// loop until tripped, broken, interrupted, or timed out
for (;;) {死循環自旋
try {
if (!timed)
trip.await();超時就進入同步隊列等待
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();
}
}
大概這些邏輯,我們理解理清楚了,大概就是聲明瞭一個數字 count ,然後每次調用await()就會減1 並且判斷是否到0了,如果到了就自己先跑並且喚醒其他線程。並且重新設置count的值。從而實現可循環。