CountDownLatch介绍
CountDownLatch是一个同步辅助类,在完成一组正在其他线程中执行的操作之前,它允许一个或多个线程一直等待。
uml类图
CountDownLatch的数据结构很简单,它是通过”共享锁”实现的。它包含了sync对象,sync是Sync类型。Sync是实例类,它继承于AQS。
CountDownLatch源码分析
1. CountDownLatch(int count)
public CountDownLatch(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
this.sync = new Sync(count);
}
该函数是创建一个Sync对象,而Sync是继承于AQS类。Sync构造函数如下:
Sync(int count) {
setState(count);
}
setState()在AQS中实现,源码如下:
protected final void setState(long newState) {
state = newState;
}
说明:在AQS中,state是一个private volatile long类型的对象。对于CountDownLatch而言,state表示的”锁计数器“。CountDownLatch中的getCount()最终是调用AQS中的getState(),返回的state对象,即”锁计数器“。
2.await()
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
说明:该函数实际上是调用的AQS的acquireSharedInterruptibly(1);
AQS中的acquireSharedInterruptibly()的源码如下:
public final void acquireSharedInterruptibly(long arg)
throws InterruptedException {
//如果当前线程是中断状态,则抛出异常InterruptedException
if (Thread.interrupted())
throw new InterruptedException();
//调用tryAcquireShared(arg)尝试获取共享锁,尝试成功则返回
//否则就调用doAcquireSharedInterruptibly()
if (tryAcquireShared(arg) < 0)
doAcquireSharedInterruptibly(arg);
}
tryAcquireShared()在CountDownLatch.java中被重写,它的源码如下:
protected int tryAcquireShared(int acquires) {
return (getState() == 0) ? 1 : -1;
}
如果”锁计数器=0”,即锁是可获取状态,则返回1;否则,锁是不可获取状态,则返回-1。
doAcquireSharedInterruptibly()方法
private void doAcquireSharedInterruptibly(long arg)
throws InterruptedException {
// 创建"当前线程"的Node节点,且Node中记录的锁是"共享锁"类型;并将该节点添加到CLH队列末尾。
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) {
// 获取上一个节点。
// 如果上一节点是CLH队列的表头,则"尝试获取共享锁"。
final Node p = node.predecessor();
if (p == head) {
long r = tryAcquireShared(arg);
if (r >= 0) {
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
// (上一节点不是CLH队列的表头) 当前线程一直等待,直到获取到共享锁。
// 如果线程在等待过程中被中断过,则再次中断该线程(还原之前的中断状态)。
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
doAcquireSharedInterruptibly()会使当前线程一直等待,直到当前线程获取到共享锁(或被中断)才返回。跟共享锁ReadLock的doAcquireShare()方法非常类似,这里就不进行赘述了。
3. countDown()
public void countDown() {
sync.releaseShared(1);
}
说明:该函数实际上调用releaseShared(1)释放共享锁。
releaseShared()在AQS中实现,源码如下:
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
说明:releaseShared()的目的是让当前线程释放它所持有的共享锁。
它首先会通过tryReleaseShared()去尝试释放共享锁。尝试成功,则直接返回;尝试失败,则通过doReleaseShared()去释放共享锁。
tryReleaseShared()在CountDownLatch.java中被重写,源码如下:
protected boolean tryReleaseShared(int releases) {
// Decrement count; signal when transition to zero
for (;;) {
// 获取“锁计数器”的状态
int c = getState();
if (c == 0)
return false;
// “锁计数器”-1
int nextc = c-1;
// 通过CAS函数进行赋值。
if (compareAndSetState(c, nextc))
return nextc == 0;
}
}
说明:tryReleaseShared()的作用是释放共享锁,将“锁计数器”的值-1。
总结:CountDownLatch是通过“共享锁”实现的。在创建CountDownLatch中时,会传递一个int类型参数count,该参数是“锁计数器”的初始状态,表示该“共享锁”最多能被count给线程同时获取。当某线程调用该CountDownLatch对象的await()方法时,该线程会等待“共享锁”可用时,才能获取“共享锁”进而继续运行。而“共享锁”可用的条件,就是“锁计数器”的值为0!而“锁计数器”的初始值为count,每当一个线程调用该CountDownLatch对象的countDown()方法时,才将“锁计数器”-1;通过这种方式,必须有count个线程调用countDown()之后,“锁计数器”才为0,而前面提到的等待线程才能继续运行!
CountDownLatch的使用示例
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.CyclicBarrier;
public class CountDownLatchTest1 {
private static int LATCH_SIZE = 5;
private static CountDownLatch doneSignal;
public static void main(String[] args) {
try {
doneSignal = new CountDownLatch(LATCH_SIZE);
// 新建5个任务
for(int i=0; i<LATCH_SIZE; i++)
new InnerThread().start();
System.out.println("main await begin.");
// "主线程"等待线程池中5个任务的完成
doneSignal.await();
System.out.println("main await finished.");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
static class InnerThread extends Thread{
public void run() {
try {
Thread.sleep(1000);
System.out.println(Thread.currentThread().getName() + " sleep 1000ms.");
// 将CountDownLatch的数值减1
doneSignal.countDown();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
运行结果:
main await begin.
Thread-0 sleep 1000ms.
Thread-2 sleep 1000ms.
Thread-1 sleep 1000ms.
Thread-4 sleep 1000ms.
Thread-3 sleep 1000ms.
main await finished.