深入分析同步工具类之CountDownLatch
概览：
CountDownLatch又称闭锁，其作用是让一个或者多个线程挂起，直到其他的线程执行完后恢复挂起的线程，使其继续执行。内部维护着一个静态内部类Sync，该类继承AbstractQueuedSynchronizer（这个类之前分析过了，参见 深入分析同步工具类之AbstractQueuedSynchronizer），Sync实例维护着state属性，调用await()方法，使当前线程挂起，当一个线程执行完后，调用countDown()方法，state-1，直到state变为0，被挂起的线程恢复执行。
常用的方法：
public CountDownLatch(int count) //构造函数初始化state值，可以理解为需要优先执行的线程数量
public void await() //调用后，所在的线程挂起
public void countDown() //优先执行的线程执行完调用，state-1，当state=0，执行阻塞队列中的线程
使用实例：
主线程和线程池中的一个线程会被挂起，等线程池中的另外5个线程执行完才会被执行
CountDownLatch latch=new CountDownLatch(5); ExecutorService service= Executors.newFixedThreadPool(6); for (int i=0;i<5;i++){ service.submit(new Runnable() { @Override public void run() { try { Thread.sleep(5000); System.out.println(Thread.currentThread().getName()); } catch (InterruptedException e) { e.printStackTrace(); } latch.countDown(); } }); } service.submit(new Runnable() { @Override public void run() { try { latch.await(); System.out.println(Thread.currentThread().getName()); } catch (InterruptedException e) { e.printStackTrace(); } } }); latch.await(); System.out.println("主线程");
运行结果：
代码分析：
1.await()
//线程等待 public void await() throws InterruptedException { //AQS的实现 sync.acquireSharedInterruptibly(1); } public final void acquireSharedInterruptibly(int arg) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); //尝试获取状态，<0优先执行的线程未执行完，>0已执行完 //Sync子类自己实现 if (tryAcquireShared(arg) < 0) doAcquireSharedInterruptibly(arg); }
//获取AQS的state值，我们调用countDown会改变这个值 protected int tryAcquireShared(int acquires) { return (getState() == 0) ? 1 : -1; }
private void doAcquireSharedInterruptibly(int arg) throws InterruptedException { //向队列中添加一个共享节点 final Node node = addWaiter(Node.SHARED); boolean failed = true; try { for (;;) { //该节点的前驱节点 final Node p = node.predecessor(); //前驱是头节点 if (p == head) {
//获取状态值 int r = tryAcquireShared(arg); if (r >= 0) { //设置节点为头节点，退出循环 setHeadAndPropagate(node, r); p.next = null; // help GC failed = false; return; } } //否则当前线程挂起 if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) throw new InterruptedException(); } } finally { if (failed) cancelAcquire(node); } }
addWaiter
private Node addWaiter(Node mode) { Node node = new Node(Thread.currentThread(), mode); // Try the fast path of enq; backup to full enq on failure Node pred = tail; if (pred != null) { node.prev = pred; if (compareAndSetTail(pred, node)) { pred.next = node; return node; } } //第一个节点添加队列时，头节点和尾节点都为空，需要初始化 enq(node); return node; } private Node enq(final Node node) { for (;;) { Node t = tail; if (t == null) { // Must initialize //CAS设置头节点 if (compareAndSetHead(new Node())) //头节点赋值给tail，此时head和tail指向同一个对象，如果对任何一个对象中的属性做修改，那么2个引用的属性也会跟着变(后面挂起线程的时候会修改waitStatus属性) tail = head; } else { node.prev = t; //设置尾节点为当前结点，将2个节点串起来，即：node.prev = t;t.next = node; if (compareAndSetTail(t, node)) { t.next = node; return t;//退出循环 } } } } //注意：这里新生成的head节点并没有后继节点 head.next==null,并且head==node.prev(该node是第一次插入的节点) 这个特性在countDown的时候会使用到
假设线程A和线程B先后调用await()方法，并且tryAcquireShared(int acquires)<0,那么此时线程A、B分别被挂起，线程A和B在挂起时先后调用shouldParkAfterFailedAcquire方法，这样各自前驱节点的waitStatus就会被设置为-1，代表该线程需要执行；同时因为线程A的前驱和head引用同一个对象，
所以head==Node并且其waitStatus都为-1
此时队列的节点如下图：
调用挂起线程的方法：
//挂起线程前先将该节点的前驱节点的waitStatus设为-1，即表示其后继节点代表的线程需要执行，这样上图Node B的前驱Node的waitStatus==-1， 因为Node在初始化的时候和head同引用一个对象，所以head 的waitStatus也为-1 private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) { int ws = pred.waitStatus; if (ws == Node.SIGNAL) /* * This node has already set status asking a release * to signal it, so it can safely park. */ return true; if (ws > 0) { /* * Predecessor was cancelled. Skip over predecessors and * indicate retry. */ do { node.prev = pred = pred.prev; } while (pred.waitStatus > 0); pred.next = node; } else { /* * waitStatus must be 0 or PROPAGATE. Indicate that we * need a signal, but don't park yet. Caller will need to * retry to make sure it cannot acquire before parking. */ compareAndSetWaitStatus(pred, ws, Node.SIGNAL); } return false; } //挂起前程 private final boolean parkAndCheckInterrupt() { LockSupport.park(this); return Thread.interrupted(); }
此时，若优先执行的线程执行完毕，调用countDown方法，更新state的值，也就是state==0的时候，就会恢复头节点的下一个节点所代表的线程
public void countDown() { sync.releaseShared(1); } public final boolean releaseShared(int arg) { if (tryReleaseShared(arg)) { doReleaseShared(); return true; } return false; } 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)) //最后一个线程执行完，返回true return nextc == 0; } } private void doReleaseShared() { for (;;) { Node h = head; if (h != null && h != tail) { int ws = h.waitStatus; if (ws == Node.SIGNAL) {//true if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) continue; // loop to recheck cases //恢复后继节点代表的线程 unparkSuccessor(h); } else if (ws == 0 && !compareAndSetWaitStatus(h, 0, Node.PROPAGATE)) continue; // loop on failed CAS } if (h == head) // loop if head changed break; } } private void unparkSuccessor(Node node) { int ws = node.waitStatus; if (ws < 0) compareAndSetWaitStatus(node, ws, 0); //此时s==null，从尾节点开始找到最前面的节点(Node A),将其恢复 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); }
这个时候线程A开始运行，若未发生线程中断，则继续执行循环内的代码，p==head成立，此时获取状态成功，恢复后继节点代表的线程，并退出循环：
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; } } ... } private void setHeadAndPropagate(Node node, int propagate) { Node h = head; // Record old head for check below //设置当前结点为头节点 setHead(node); if (propagate > 0 || h == null || h.waitStatus < 0 || (h = head) == null || h.waitStatus < 0) { Node s = node.next; if (s == null || s.isShared()) //恢复后继节点的线程 doReleaseShared(); } }
最后，线程B运行 获取状态，退出循环，程序结束
