1. CountDownLatch
1.1 说明
一种同步辅助工具，允许一个或多个线程等待其他线程执行的一组操作完成。
给定一个计数值。当每个线程完成后，调用{@link countDown}方法给计数值减一。在当前计数达到零之前，调用{@link#await await}方法的线程将一直阻塞，直到计数值为0后将释放所有等待线程。
1.2 源码
public class CountDownLatch {
/**
* Synchronization control For CountDownLatch.
* Uses AQS state to represent count.
*/
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;
/**
* 初始化
*/
public CountDownLatch(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
this.sync = new Sync(count);
}
/**
* 导致当前线程等待，直到count为0
*/
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
/**
* Causes the current thread to wait until the latch has counted down to
* zero, unless the thread is {@linkplain Thread#interrupt interrupted},
* or the specified waiting time elapses.
*
*/
public boolean await(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
/**
* Decrements the count of the latch, releasing all waiting threads if
* the count reaches zero.
*/
public void countDown() {
sync.releaseShared(1);
}
/**
* Returns the current count.
*
* <p>This method is typically used for debugging and testing purposes.
*
* @return the current count
*/
public long getCount() {
return sync.getCount();
}
}
1.3 示例
public class CountDownLatchDemo {
public static void main(String[] args) {
CountDownLatch countDownLatch = new CountDownLatch(15);
for (int i = 0; i < 15; i++) {
new Thread(() -> {
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName());
countDownLatch.countDown();
}, ""+ i).start();
}
try {
countDownLatch.await();
System.out.println("main thread");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
结果：
2. CyclicBarrier
2.1 说明
一种同步辅助工具，允许一组线程全部等待对方到达一个公共屏障点。CyclicBarrier在涉及固定大小的线程方的程序中非常有用，这些线程偶尔必须相互等待。这个屏障被称为循环的，因为它可以在等待的线程被释放后重新使用。
CyclicBarrier支持可选的可运行命令，该命令在参与方中的最后一个线程到达之后，但在释放任何线程之前，在每个屏障点运行一次。此屏障操作有助于在任何一方继续之前更新共享状态。
2.2 源码
public class CyclicBarrier {
private static class Generation {
boolean broken = false;
}
/** The lock for guarding barrier entry */
private final ReentrantLock lock = new ReentrantLock();
/** Condition to wait on until tripped */
private final Condition trip = lock.newCondition();
//每次拦截的线程数，在构造时进行赋值
private final int parties;
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; //内部计数器与parties相等
/**
* Updates state on barrier trip and wakes up everyone.
* Called only while holding lock.
*/
private void nextGeneration() {
// signal completion of last generation
trip.signalAll();
// set up next generation
count = parties;
generation = new Generation();
}
/**
* Sets current barrier generation as broken and wakes up everyone.
* Called only while holding lock.
*/
private void breakBarrier() {
generation.broken = true;
count = parties;
trip.signalAll();
}
/**
* Main barrier code, covering the various policies.
*/
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(); //若线程中断，唤醒所有线程
throw new InterruptedException();
}
int index = --count; //计数减一
if (index == 0) { // tripped
boolean ranAction = false;
try {
final Runnable command = barrierCommand;
if (command != null)
command.run(); //执行指定任务
ranAction = true;
nextGeneration(); //唤醒所有线程，转到下一代
return 0;
} finally {
if (!ranAction)
breakBarrier();
}
}
// 如果计数器不为0则执行此循环
for (;;) {
try {
//根据传入的参数来决定是定时等待还是非定时等待
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();
}
}
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);
}
public int getParties() {
return parties;
}
public int await() throws InterruptedException, BrokenBarrierException {
try {
return dowait(false, 0L);
} catch (TimeoutException toe) {
throw new Error(toe); // cannot happen
}
}
public int await(long timeout, TimeUnit unit)
throws InterruptedException,
BrokenBarrierException,
TimeoutException {
return dowait(true, unit.toNanos(timeout));
}
public boolean isBroken() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return generation.broken;
} finally {
lock.unlock();
}
}
public void reset() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
breakBarrier(); // break the current generation
nextGeneration(); // start a new generation
} finally {
lock.unlock();
}
}
/**
* Returns the number of parties currently waiting at the barrier.
* This method is primarily useful for debugging and assertions.
*
* @return the number of parties currently blocked in {@link #await}
*/
public int getNumberWaiting() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return parties - count;
} finally {
lock.unlock();
}
}
}
应用场景：多个线程计算最后汇总
CyclicBarrier 和CountDownLatch 区别
1.CountDownLatch 只能使用一次，CyclicBarrier 的计数器可以使用reset()方法重置。所以CyclicBarrier 可以处理更复杂的逻辑。
例如，计算发生错误可以重置计数器，并让线程重新执行一次。
2.CyclicBarrier 还提供其他方法，如getNumberWaiting方法可以获得被阻塞的线程数等
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