CyclicBarrier 是基于同步到达某个点的信号量触发机制。
字面意思:回环栅栏,即可知道它是一个可以循环使用 ( Cyclic )的屏障式 ( Barrier) 多线程协作方式。
结合生活中的例子:
采用这种方式进行刚才的安检服务 , 就是 3 个人同时进去,只有 3 个人都完成安检,才会放下一批进来。这是一种非常低效的安检方式。
但在某种场景 下就是非常正确的方式,假设在机场排队打车时,现场工作人员统一指挥,每次放 3 辆车进来 ,坐满后开走 , 再放下一批车和人进来。
CyclicBarrier与CountDownLatch的区别
1)应用场景不同:都可以实现一个线程等待其他线程执行完后的结果归并处理,但是CountDownLatch的计数器只能使用一次,而CyclicBarrier的计数器可以使用reset() 方法重置。CyclicBarrier可以处理更复杂的场景,比如如果计算发生错误,可 以重置计数器,并让线程们重新执行一次
2)实现方式不同,CyclicBarrier是通过ReentrantLock的”独占锁”和Conditon来实现一组线程的阻塞 唤醒的,而CountDownLatch则是通过AQS的“共享锁”实现
源码分析:
//成员变量
/** The lock for guarding barrier entry */
private final ReentrantLock lock = new ReentrantLock();
/** Condition to wait on until tripped */
private final Condition trip = lock.newCondition();
/** The number of parties */
//副本,用于循环
private final int parties;
/* The command to run when tripped */
private final Runnable barrierCommand;
/** The current generation */
private Generation generation = new Generation();
/**
* Number of parties still waiting. Counts down from parties to 0
* on each generation. It is reset to parties on each new
* generation or when broken.
*/
private int count;
//构造方法
public CyclicBarrier(int parties, Runnable barrierAction) {
if (parties <= 0) throw new IllegalArgumentException();
this.parties = parties;
this.count = parties;
this.barrierCommand = barrierAction;
}
为什么用先调用ReentrantLock.lock ? 因为是遵照MESA管程模型来的
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;
if (index == 0) { // tripped
boolean ranAction = false;
try {
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)
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();
}
}
前半段:进入等待队列,并堵塞线程
过渡阶段:其他调用signal/signalAll的线程将条件队列转到同步队列中,并唤醒同步队列中head的后续节点去获取锁
后半段(独占锁的逻辑):获取锁,释放锁,并唤醒同步队列中head的后续节点所在的线程
ReentrantLock + condition的实现逻辑:
回顾到AQS定义两种队列
同步等待队列: 主要用于维护获取锁失败时入队的线程
条件等待队列: 调用await()的时候会释放锁,然后线程会加入到条件队列,调用 signal()唤醒的时候会把条件队列中的线程节点移动到同步队列中,等待再次获得锁