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
当第一个节点插入队列的时候，可以用下图来描绘生成的队列：
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())
doReleaseShared();
}
}
private void doReleaseShared() {
for (;;) {
Node h = head;
if (h != null && h != tail) {
int ws = h.waitStatus;
if (ws == Node.SIGNAL) {//表明后继节点需要执行
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;
}
}
//恢复后继节点代表的线程
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);
}
挂起线程：
//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 A的waitStatus==-1，因为Node A 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();
}
2、countDown()
public void countDown() {
//AQS中的方法
sync.releaseShared(1);
}
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {//获取状态
doReleaseShared();//恢复线程，见上面分析
return true;
}
return false;
}
时间： 2024-10-10 21:51:29