CountDownLatch，Cyclic Barrier，Semaphore

参考
CS108 handout30
https://blog.youkuaiyun.com/a347911/article/details/53465445

CountDownLatch

一个thread等待其他的thread。首先创建latch，会开始一个counter，然后用await()设定等待的thread，等待其他thread完成，其他thread需要使用latch.countDown()。

static CountDownLatch latch; 

public static void testThreadingSupport(int numThreads) {
	latch = new CountDownLatch(numThreads); // 创建latch
	for(int i=0; i< numThreads; i++) {
		new TestListThread().start(); // 开始一个thread
}
	try {
		latch.await(); // 这句话使得main thread开始等待
	} catch (InterruptedException e) {
	…
	}
}

public void run() {
	… // do stuff
	latch.countDown();
}

Cyclic Barrier

The CyclicBarrier can be used to halt a number of threads until a given number have reached a particular point in their code.
one set signals with countdown but does not block, the other set blocks until the counter hits zero.
通过它可以实现让一组线程等待至某个状态之后再全部同时执行。叫做回环是因为当所有等待线程都被释放以后，CyclicBarrier可以被重用。我们暂且把这个状态就叫做barrier，当调用await()方法之后，线程就处于barrier了。

static CyclicBarrier barrier;
	public static void main(String[] args) {
		barrier = new CyclicBarrier(NUM_THREADS);// cyclicBarrier object
		for(int i=0; i< NUM_THREADS; i++) {
			new TestListThread().start(); // threads start
	}
	System.out.println("Main Thread Done");
}

public void run() {
	// get work done
	System.out.println(getName() + " is working");
	try {
		barrier.await();
	} catch (InterruptedException e) {
	// handle interrupt
	// The thread that is interrupted receives an InterruptedException.
	} catch (BrokenBarrierException e) {
	// handle broken barrier
	// In addition if one thread receives InterruptedException, every other thread waiting on the same barrier will receive a BrokenBarrierException.
	}
	// do whatever needs to get done after synching up
	System.out.println(getName() + " is done");
	}
}

Semaphore

Semaphore可以控同时访问的线程个数，通过 acquire() 获取一个许可，如果没有就等待，而 release() 释放一个许可。

Semaphores are a natural means of managing access to a limited pool of resources