CH24-CountDownLatch
概览
其底层是由AQS提供支持,所以其数据结构可以参考AQS的数据结构,而AQS的数据结构核心就是两个虚拟队列: 同步队列sync queue 和条件队列condition queue,不同的条件会有不同的条件队列。
CountDownLatch典型的用法是将一个程序分为n个互相独立的可解决任务,并创建值为n的CountDownLatch。当每一个任务完成时,都会在这个锁存器上调用countDown,等待问题被解决的任务调用这个锁存器的await,将他们自己拦住,直至锁存器计数结束。
源码分析
层级结构
CountDownLatch没有显示继承哪个父类或者实现哪个父接口, 它底层是AQS是通过内部类Sync来实现的。
内部类
CountDownLatch类存在一个内部类Sync,继承自AbstractQueuedSynchronizer,其源代码如下。
private static final class Sync extends AbstractQueuedSynchronizer {
// 版本号
private static final long serialVersionUID = 4982264981922014374L;
// 构造器
Sync(int count) {
setState(count);
}
// 返回当前计数
int getCount() {
return getState();
}
// 试图在共享模式下获取对象状态
protected int tryAcquireShared(int acquires) {
return (getState() == 0) ? 1 : -1;
}
// 试图设置状态来反映共享模式下的一个释放
protected boolean tryReleaseShared(int releases) {
// Decrement count; signal when transition to zero
// 无限循环
for (;;) {
// 获取状态
int c = getState();
if (c == 0) // 没有被线程占有
return false;
// 下一个状态
int nextc = c-1;
if (compareAndSetState(c, nextc)) // 比较并且设置成功
return nextc == 0;
}
}
}
对CountDownLatch方法的调用会转发到对Sync或AQS的方法的调用,所以,AQS对CountDownLatch提供支持。
类属性
可以看到CountDownLatch类的内部只有一个Sync类型的属性:
public class CountDownLatch {
// 同步队列
private final Sync sync;
}
构造函数
public CountDownLatch(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
// 初始化状态数
this.sync = new Sync(count);
}
该构造函数可以构造一个用给定计数初始化的CountDownLatch,并且构造函数内完成了sync的初始化,并设置了状态数。
核心函数:await
此函数将会使当前线程在锁存器倒计数至零之前一直等待,除非线程被中断。其源码如下
public void await() throws InterruptedException {
// 转发到sync对象上
sync.acquireSharedInterruptibly(1);
}
对CountDownLatch对象的await的调用会转发为对Sync的acquireSharedInterruptibly(从AQS继承的方法)方法的调用。
- acquireSharedInterruptibly源码如下:
public final void acquireSharedInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (tryAcquireShared(arg) < 0)
doAcquireSharedInterruptibly(arg);
}
acquireSharedInterruptibly又调用了CountDownLatch的内部类Sync的tryAcquireShared和AQS的doAcquireSharedInterruptibly函数。
- tryAcquireShared函数的源码如下:
protected int tryAcquireShared(int acquires) {
return (getState() == 0) ? 1 : -1;
}
该函数只是简单的判断AQS的state是否为0,为0则返回1,不为0则返回-1。
- doAcquireSharedInterruptibly函数的源码如下:
private void doAcquireSharedInterruptibly(int arg) throws InterruptedException {
// 添加节点至等待队列
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
for (;;) { // 无限循环
// 获取node的前驱节点
final Node p = node.predecessor();
if (p == head) { // 前驱节点为头结点
// 试图在共享模式下获取对象状态
int r = tryAcquireShared(arg);
if (r >= 0) { // 获取成功
// 设置头结点并进行繁殖
setHeadAndPropagate(node, r);
// 设置节点next域
p.next = null; // help GC
failed = false;
return;
}
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt()) // 在获取失败后是否需要禁止线程并且进行中断检查
// 抛出异常
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
在AQS的doAcquireSharedInterruptibly中可能会再次调用CountDownLatch的内部类Sync的tryAcquireShared方法和AQS的setHeadAndPropagate方法。
- setHeadAndPropagate方法源码如下
private void setHeadAndPropagate(Node node, int propagate) {
// 获取头结点
Node h = head; // Record old head for check below
// 设置头结点
setHead(node);
/*
* Try to signal next queued node if:
* Propagation was indicated by caller,
* or was recorded (as h.waitStatus either before
* or after setHead) by a previous operation
* (note: this uses sign-check of waitStatus because
* PROPAGATE status may transition to SIGNAL.)
* and
* The next node is waiting in shared mode,
* or we don't know, because it appears null
*
* The conservatism in both of these checks may cause
* unnecessary wake-ups, but only when there are multiple
* racing acquires/releases, so most need signals now or soon
* anyway.
*/
// 进行判断
if (propagate > 0 || h == null || h.waitStatus < 0 ||
(h = head) == null || h.waitStatus < 0) {
// 获取节点的后继
Node s = node.next;
if (s == null || s.isShared()) // 后继为空或者为共享模式
// 以共享模式进行释放
doReleaseShared();
}
}
该方法设置头结点并且释放头结点后面的满足条件的结点,该方法中可能会调用到AQS的doReleaseShared方法,其源码如下。
private void doReleaseShared() {
/*
* Ensure that a release propagates, even if there are other
* in-progress acquires/releases. This proceeds in the usual
* way of trying to unparkSuccessor of head if it needs
* signal. But if it does not, status is set to PROPAGATE to
* ensure that upon release, propagation continues.
* Additionally, we must loop in case a new node is added
* while we are doing this. Also, unlike other uses of
* unparkSuccessor, we need to know if CAS to reset status
* fails, if so rechecking.
*/
// 无限循环
for (;;) {
// 保存头结点
Node h = head;
if (h != null && h != tail) { // 头结点不为空并且头结点不为尾结点
// 获取头结点的等待状态
int ws = h.waitStatus;
if (ws == Node.SIGNAL) { // 状态为SIGNAL
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) // 不成功就继续
continue; // loop to recheck cases
// 释放后继结点
unparkSuccessor(h);
}
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE)) // 状态为0并且不成功,继续
continue; // loop on failed CAS
}
if (h == head) // 若头结点改变,继续循环
break;
}
}
该方法在共享模式下释放,具体的流程再之后会通过一个示例给出。
所以,对CountDownLatch的await调用大致会有如下的调用链。
上图给出了可能会调用到的主要方法,并非一定会调用到,之后,会通过一个示例给出详细的分析。
核心函数:countDown
此函数将递减锁存器的计数,如果计数到达零,则释放所有等待的线程
public void countDown() {
sync.releaseShared(1);
}
对countDown的调用转换为对Sync对象的releaseShared(从AQS继承而来)方法的调用。
- releaseShared源码如下
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
此函数会以共享模式释放对象,并且在函数中会调用到CountDownLatch的tryReleaseShared函数,并且可能会调用AQS的doReleaseShared函数。
- tryReleaseShared源码如下
protected boolean tryReleaseShared(int releases) {
// Decrement count; signal when transition to zero
// 无限循环
for (;;) {
// 获取状态
int c = getState();
if (c == 0) // 没有被线程占有
return false;
// 下一个状态
int nextc = c-1;
if (compareAndSetState(c, nextc)) // 比较并且设置成功
return nextc == 0;
}
}
此函数会试图设置状态来反映共享模式下的一个释放。具体的流程在下面的示例中会进行分析。
- AQS的doReleaseShared的源码如下
private void doReleaseShared() {
/*
* Ensure that a release propagates, even if there are other
* in-progress acquires/releases. This proceeds in the usual
* way of trying to unparkSuccessor of head if it needs
* signal. But if it does not, status is set to PROPAGATE to
* ensure that upon release, propagation continues.
* Additionally, we must loop in case a new node is added
* while we are doing this. Also, unlike other uses of
* unparkSuccessor, we need to know if CAS to reset status
* fails, if so rechecking.
*/
// 无限循环
for (;;) {
// 保存头结点
Node h = head;
if (h != null && h != tail) { // 头结点不为空并且头结点不为尾结点
// 获取头结点的等待状态
int ws = h.waitStatus;
if (ws == Node.SIGNAL) { // 状态为SIGNAL
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) // 不成功就继续
continue; // loop to recheck cases
// 释放后继结点
unparkSuccessor(h);
}
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE)) // 状态为0并且不成功,继续
continue; // loop on failed CAS
}
if (h == head) // 若头结点改变,继续循环
break;
}
}
此函数在共享模式下释放资源。
所以,对CountDownLatch的countDown调用大致会有如下的调用链。
上图给出了可能会调用到的主要方法,并非一定会调用到,之后,会通过一个示例给出详细的分析。
应用示例
import java.util.concurrent.CountDownLatch;
class MyThread extends Thread {
private CountDownLatch countDownLatch;
public MyThread(String name, CountDownLatch countDownLatch) {
super(name);
this.countDownLatch = countDownLatch;
}
public void run() {
System.out.println(Thread.currentThread().getName() + " doing something");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + " finish");
countDownLatch.countDown();
}
}
public class CountDownLatchDemo {
public static void main(String[] args) {
CountDownLatch countDownLatch = new CountDownLatch(2);
MyThread t1 = new MyThread("t1", countDownLatch);
MyThread t2 = new MyThread("t2", countDownLatch);
t1.start();
t2.start();
System.out.println("Waiting for t1 thread and t2 thread to finish");
try {
countDownLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + " continue");
}
}
Waiting for t1 thread and t2 thread to finish
t1 doing something
t2 doing something
t1 finish
t2 finish
main continue
本程序首先计数器初始化为2。根据结果,可能会存在如下的一种时序图。
首先main线程会调用await操作,此时main线程会被阻塞,等待被唤醒,之后t1线程执行了countDown操作,最后,t2线程执行了countDown操作,此时main线程就被唤醒了,可以继续运行。下面,进行详细分析。
- main线程执行countDownLatch.await操作,主要调用的函数如下。
在最后,main线程就被park了,即禁止运行了。此时Sync queue(同步队列)中有两个节点,AQS的state为2,包含main线程的结点的nextWaiter指向SHARED结点。
- t1线程执行countDownLatch.countDown操作,主要调用的函数如下。
此时,Sync queue队列里的结点个数未发生变化,但是此时,AQS的state已经变为1了。
- t2线程执行countDownLatch.countDown操作,主要调用的函数如下。
经过调用后,AQS的state为0,并且此时,main线程会被unpark,可以继续运行。当main线程获取cpu资源后,继续运行。
- main线程获取cpu资源,继续运行,由于main线程是在parkAndCheckInterrupt函数中被禁止的,所以此时,继续在parkAndCheckInterrupt函数运行.
main线程恢复,继续在parkAndCheckInterrupt函数中运行,之后又会回到最终达到的状态为AQS的state为0,并且head与tail指向同一个结点,该节点的额nextWaiter域还是指向SHARED结点。
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