前言
java.concurrent包提供了几个非常实用的并发工具类，这些工具类提供了一种非常有效的并发流程控制手段。
CountDownLatch
CountDownLatch 允许一个或多个线程等待其他线程完成操作。CountDownLatch的具体实现如下：
public class CountDownLatch {
private static final class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 4982264981922014374L;
Sync(int count) {
setState(count);
}
int getCount() {
return getState();
}
protected int tryAcquireShared(int acquires) {
return (getState() == 0) ? 1 : -1;
}
protected boolean tryReleaseShared(int releases) {
// Decrement count; signal when transition to zero
for (;;) {
int c = getState();
if (c == 0)
return false;
int nextc = c-1;
if (compareAndSetState(c, nextc))
return nextc == 0;
}
}
}
private final Sync sync;
public CountDownLatch(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
this.sync = new Sync(count);
}
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
public boolean await(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
public void countDown() {
sync.releaseShared(1);
}
public long getCount() {
return sync.getCount();
}
public String toString() {
return super.toString() + "[Count = " + sync.getCount() + "]";
}
}从内部构造上看，CountDownLatch内部含有一个继承自AbstractQueuedSynchronizer（AQS，队列同步器）的内部类Sync，关于AQS的内容，博主在《重学多线程（三）—— 锁》 一文中有比较详细的介绍。
CountDownLatch的构造函数接收一个int类型的参数作为计数器，表示等待的线程数量。当调用countDown方法时，计数器就会减1，await方法会阻塞当前线程，直至计数器变为0。当计数器为0时，调用await方法不会阻塞当前线程，CountDownLatch不可能重新初始化或者修改计数器的值，说白了就是不能复用。
CyclicBarrier
CyclicBarrier 允许一组线程到达一个 barrier 时被阻塞，直到最后一个线程到达 barrier 时，barrier才会打开，被拦截的线程继续允许。
public CyclicBarrier(int parties, Runnable barrierAction) {
if (parties <= 0) throw new IllegalArgumentException();
this.parties = parties;
this.count = parties;
this.barrierCommand = barrierAction;
}
public CyclicBarrier(int parties) {
this(parties, null);
}CyclicBarrier 的默认构造方法接受一个 int 类型的参数，表示 barrier 拦截的线程数量。同时，也可以通过 barrierAction 参数指定当所有线程到达barrier后，优先执行的动作， 以方便处理更复杂的业务。
CyclicBarrier 最重要的成员方法便是await方法：
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;
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();
}
}可以看到，当调用await方法时，先对计数器 count 自减操作，判断 count 值是否为0，如果不为零，则调用 Condition的await方法使当前线程进入等待状态，当最后一个线程进入临界区对 count 值自减操作后，此时 count 值为0，便调用Condition的 signalAll 方法唤醒一个等待的线程，而被唤醒的线程在退出临界区后再次调用Condition的 signalAll 方法唤醒一个等待线程，通过线程间的循环唤醒，使得所有线程再次进入运行状态。
CountDownLatch的计数器只能使用一次，而CyclicBarrier 的计数器可以通过调用 reset 方法重置，这是两者比较大的区别。
Semaphore
Semaphore（信号量）是用来控制同时访问特定资源的线程数量，通过协调各个线程，以保证合理的使用公共资源。
abstract static class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 1192457210091910933L;
Sync(int permits) {
setState(permits);
}
final int getPermits() {
return getState();
}
final int nonfairTryAcquireShared(int acquires) {
for (;;) {
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
protected final boolean tryReleaseShared(int releases) {
for (;;) {
int current = getState();
int next = current + releases;
if (next < current) // overflow
throw new Error("Maximum permit count exceeded");
if (compareAndSetState(current, next))
return true;
}
}
final void reducePermits(int reductions) {
for (;;) {
int current = getState();
int next = current - reductions;
if (next > current) // underflow
throw new Error("Permit count underflow");
if (compareAndSetState(current, next))
return;
}
}
final int drainPermits() {
for (;;) {
int current = getState();
if (current == 0 || compareAndSetState(current, 0))
return current;
}
}
}
public Semaphore(int permits) {
sync = new NonfairSync(permits);
}
public Semaphore(int permits, boolean fair) {
sync = fair ? new FairSync(permits) : new NonfairSync(permits);
}跟 CountDownLatch 一样，Semaphore 内部也含有一个继承自AbstractQueuedSynchronizer（AQS，队列同步器）的内部类Sync。Semaphore 的构造方法接受一个 int 类型参数，表示可用的许可证数量，而参数 fair 表示获取许可证的时候是否公平。可以调用Semaphore的 acquire 方法获取一个许可证，调用 release 方法归还许可证。
顺便说一下，Google的Guava框架中有个RateLimiter的工具类也提供了限流的功能，读者有兴趣可以自行了解一下。
总结
本文介绍的三个并发工具类的使用方法，在实际开发过程中如果有类似场景，不妨一试。