aqs作为java锁、countDownLatch、信号量的实现核心，功能非常强大，理解起来也分费解
数据结构
state的意义
- 1 表示当前线程已经被取消
- -1 表示后继节点线程需要从阻塞变成就绪状态
- -2 表示当前线程在等待一个资源
- -3 表示acquireShare应该被无条件传播
- 0 表示不是任何一种
static final class Node {
/** Marker to indicate a node is waiting in shared mode */
static final Node SHARED = new Node();
/** Marker to indicate a node is waiting in exclusive mode */
static final Node EXCLUSIVE = null;
/** waitStatus value to indicate thread has cancelled */
static final int CANCELLED = 1;
/** waitStatus value to indicate successor's thread needs unparking */
static final int SIGNAL = -1;
/** waitStatus value to indicate thread is waiting on condition */
static final int CONDITION = -2;
/**
* waitStatus value to indicate the next acquireShared should
* unconditionally propagate
*/
static final int PROPAGATE = -3;
countDownLatch的实现,内部定义了一个Sync同步器，并且覆写了tryAcquireShared、tryReleaseShared方法。获取共享锁时判断state==0就返回成功，否则失败。排它锁需要加1，所以state就不为0了。
public class CountDownLatch {
/**
* Synchronization control For CountDownLatch.
* Uses AQS state to represent count.
*/
private static final class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 4982264981922014374L;
Sync(int count) {
setState(count);
}
int getCount() {
return getState();
}
protected int tryAcquireShared(int acquires) {
return (getState() == 0) ? 1 : -1;
}
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))
return nextc == 0;
}
}
}
private final Sync sync;
看看countDown是怎么实现的，初始化countDownLatch的时候已经设置了同步器的state=线程数量，每个线程去执行countDown的时候，会调用同步器tryReleaseShared，减1，并且判断nextc是否等于0，显然，只有当所有线程都调用了一遍才会是0。
//初始化
public CountDownLatch(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
this.sync = new Sync(count);
}
//构造方法
Sync(int count) {
setState(count);
}
//减
public void countDown() {
sync.releaseShared(1);
}
//AQS的实现
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
如果释放成功,则可以释放共享锁
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;
}
}
唤醒后继节点，找不到就从tail开始从后往前遍历
/**
* Wakes up node's successor, if one exists.
*
* @param node the node
*/
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);
}
在看看主线程现在在哪里
public boolean await(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
AQS，tryAcquireShared要等到所有线程都countDown才会返回true，doAcquireSharedNanos
public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
return tryAcquireShared(arg) >= 0 ||
doAcquireSharedNanos(arg, nanosTimeout);
}
那么再看
/**
* Acquires in shared timed mode.
*
* @param arg the acquire argument
* @param nanosTimeout max wait time
* @return {@code true} if acquired
*/
private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
throws InterruptedException {
if (nanosTimeout <= 0L)
return false;
final long deadline = System.nanoTime() + nanosTimeout;
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
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 true;
}
}
nanosTimeout = deadline - System.nanoTime();
if (nanosTimeout <= 0L)
return false;
if (shouldParkAfterFailedAcquire(p, node) &&
nanosTimeout > spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if (Thread.interrupted())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
在队尾加上这个节点，主线程
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;
}
}
enq(node);
return node;
}