CountDownLatch 源码解析―― countDown()
上一篇文章从源码层面说了一下CountDownLatch 中 await() 的原理。这篇文章说一下countDown() 。
public void countDown() { //CountDownLatch sync.releaseShared(1);} ↓public final boolean releaseShared(int arg) { //AQS if (tryReleaseShared(arg)) { doReleaseShared(); return true; } return false;} ↓protected boolean tryReleaseShared(int releases) { //CountDownLatch.Sync // 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; }}通过构造器 CountDownLatch end = new CountDownLatch(2); state 被设置为2，所以c == 2，nextc = 2-1,
然后通过下面这个CAS操作将state设置为1。
protected final boolean compareAndSetState(int expect, int update) { // See below for intrinsics setup to support this return unsafe.compareAndSwapInt(this, stateOffset, expect, update); }此时nextc还不为0，返回false。一直等到countDown() 方法被调用两次，state == 0，nextc ==0,此时返回true。
进入doReleaseShared()方法。
doReleaseShared(); ↓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. */ for (;;) { Node h = head; if (h != null && h != tail) { int ws = h.waitStatus; if (ws == Node.SIGNAL) { if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) continue; // loop to recheck cases unparkSuccessor(h); } else if (ws == 0 && !compareAndSetWaitStatus(h, 0, Node.PROPAGATE)) continue; // loop on failed CAS } if (h == head) // loop if head changed break; }}回顾一下此时的等待队列模型。
+--------------------------+ prev +------------------+head | waitStatus = Node.SIGNAL | <---- node(tail) | currentThread | +--------------------------+ +------------------+
此时head 不为null，也不为tail，waitStatus == Node.SIGNAL，所以进入 if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) 这个判断。
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) ↓ /** * CAS waitStatus field of a node. */private static final boolean compareAndSetWaitStatus(Node node, int expect, int update) { return unsafe.compareAndSwapInt(node, waitStatusOffset, expect, update);}这个CAS 操作将 state 设置为 0 ，也就是说此时Head 中的 waitStatus 是0.此时队列模型如下所示
+----------------+ prev +------------------+head | waitStatus = 0 | <---- node(tail) | currentThread | +----------------+ +------------------+
该方法返回true。进入unparkSuccessor(h);
unparkSuccessor(h); ↓private void unparkSuccessor(Node node) { /* * If status is negative (i.e., possibly needing signal) try * to clear in anticipation of signalling. It is OK if this * fails or if status is changed by waiting thread. */ int ws = node.waitStatus; if (ws < 0) compareAndSetWaitStatus(node, ws, 0); /* * Thread to unpark is held in successor, which is normally * just the next node. But if cancelled or apparently null, * traverse backwards from tail to find the actual * non-cancelled successor. */ Node s = node.next; if (s == null || s.waitStatus > 0) { s = null; for (Node t = tail; t != null && t != node; t = t.prev) if (t.waitStatus <= 0) s = t; } if (s != null) LockSupport.unpark(s.thread);}s 就是head的后继结点，也就是装有当前线程的结点。s != null ，并且s.waitStatus ==0 ，所以进入 LockSupport.unpark(s.thread);
public static void unpark(Thread thread) { if (thread != null) UNSAFE.unpark(thread); }也就是unlock 被阻塞的线程。裁判被允许吹哨了！
countDown() 的原理就此就非常清晰了。
每执行一次countDown() 方法，state 就是减1，直到state == 0，则开始释放被阻塞在队列中的线程，根据前驱结点中waitStatus的状态，释放后续结点中的线程。
OK,回到上一篇文章的问题，什么时候跳出下面这个循环（await方法中的循环）
for (;;) { final Node p = node.predecessor(); if (p == head) { int r = tryAcquireShared(arg); if (r >= 0) { setHeadAndPropagate(node, r); p.next = null; // help GC failed = false; return; } } if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) throw new InterruptedException();}此时state == 0，所以进入 setHeadAndPropagate 方法。
setHeadAndPropagate(node, r); ↓private void setHeadAndPropagate(Node node, int propagate) { Node h = head; // Record old head for check below 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. */ if (propagate > 0 || h == null || h.waitStatus < 0 || (h = head) == null || h.waitStatus < 0) { Node s = node.next; if (s == null || s.isShared()) doReleaseShared(); }} ↓private void setHead(Node node) { head = node; node.thread = null; node.prev = null;}这个方法将head 的后继结点变为head。该方法过后，又将node的next结点设置为null，模型变成下图
prev +---------+ nextnull <---- node(tail/head) | null | ----> null +---------+
也就是node head tail 什么的都被置为null，等待GC回收了，这个时候return，跳出了for循环，队列被清空。
下面演示一下整个过程
setHeadAndPropagate(node, r); +----------------+ head(tail) | waitStatus=0 | | thread =null | +----------------+ ↓ +----------------+ +----------------+ | waitStatus=0 | prev | waitStatus=0 |head(tail) | thread =null | <---- node | currentThread | +----------------+ +----------------+ ↓ +----------------+ +----------------+ | waitStatus=0 | prev | waitStatus=0 |head | thread =null | <---- node(tail) | currentThread | +----------------+ +----------------+ ↓ +----------------+ +----------------+ | Node.SIGNAL | prev | waitStatus=0 |head | thread =null | <---- node(tail) | currentThread | +----------------+ +----------------+ ↓ +----------------+ +----------------+ | waitStatus=0 | prev | waitStatus=0 |head | thread =null | <---- node(tail) | currentThread | +----------------+ +----------------+ ↓ +----------------+ prev | waitStatus=0 | nextnull <---- node(tail/head) | null | ----> null +----------------+
CountDownLatch 的核心就是一个阻塞线程队列，这是由链表构造而成的队列，里面包含thread 和 waitStatus，其中waitStatus说明了后继结点线程状态。
state 是一个非常重要的标志，构造时，设置为对应的n值，如果n != 0，阻塞队列将一直阻塞，除非中断线程。
每次调用countDown() 方法，就是将state-1，而调用await() 方法就是将调用该方法的线程加入到阻塞队列，直到state==0，才能释放线程。
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