AQS---abstractQueuedSynchronizer java并发的主要实现类，内部实现了获取锁和释放锁的主要流程，自定义的同步器,如果是排他锁，实现tryAcquire、tryRelease；如果是共享锁，实现tryAcquireShared,tryReleaseShared。
排他锁和共享锁的区别是，排他锁在同一时刻只能有一个线程获取锁，而共享锁则可以运行多个线程获取到锁去执行。
1. 内部主要实现原理：
volatile类型的state表示同步锁的状态，FIFO的双向链表用来存放未获取锁而进入等待区的线程。
（1）CountDownLatch 实现
Demo:
CountDownLatch countDownLatch = new CountDownLatch(5); //建立state是5的AQS
for (int i=0;i<5;i++) {
threadPoolExecutor.submit(() -> {
try {
System.out.println("threadName:"+Thread.currentThread().getName());
}catch (Exception ex) {
ex.printStackTrace();
}finally {
countDownLatch.countDown();//每执行完一个线程，countdownlatch-1,到0以后会通知阻塞的主线程继续执行
}
});
}
countDownLatch.await();//主线程阻塞
运行流程：
1. 初始处设置 new CountDownLatch(5) 是建立state是5的AQS
2. Demo里有一个主线程和5个子线程，主线程中启动5个子线程提交到线程池中运行。
3. 主线程继续运行，在countDownLatch.await()，进入
/**
* Causes the current thread to wait until the latch has counted down to
* zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
*
* <p>If the current count is zero then this method returns immediately.
*
* <p>If the current count is greater than zero then the current
* thread becomes disabled for thread scheduling purposes and lies
* dormant until one of two things happen:
* <ul>
* <li>The count reaches zero due to invocations of the
* {@link #countDown} method; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread.
* </ul>
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* @throws InterruptedException if the current thread is interrupted
* while waiting
*/
public void await() throws InterruptedException {//如果计数器（就是刚刚设置的Sync的state）是0，就返回；如果大于0，那么就会一直阻塞，直到计数器变为0唤醒他，或者是他被中断
sync.acquireSharedInterruptibly(1);
}
/**
* Causes the current thread to wait until the latch has counted down to
* zero, unless the thread is {@linkplain Thread#interrupt interrupted},
* or the specified waiting time elapses.
*
* <p>If the current count is zero then this method returns immediately
* with the value {@code true}.
*
* <p>If the current count is greater than zero then the current
* thread becomes disabled for thread scheduling purposes and lies
* dormant until one of three things happen:
* <ul>
* <li>The count reaches zero due to invocations of the
* {@link #countDown} method; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
* <li>The specified waiting time elapses.
* </ul>
*
* <p>If the count reaches zero then the method returns with the
* value {@code true}.
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* <p>If the specified waiting time elapses then the value {@code false}
* is returned. If the time is less than or equal to zero, the method
* will not wait at all.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the {@code timeout} argument
* @return {@code true} if the count reached zero and {@code false}
* if the waiting time elapsed before the count reached zero
* @throws InterruptedException if the current thread is interrupted
* while waiting
*/
public boolean await(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); //和上面方法类似，多了个阻塞的时间，如果在指定的时间内还未获取锁，那么就返回。
}
/**
* Acquires in shared mode, aborting if interrupted. Implemented
* by first checking interrupt status, then invoking at least once
* {@link #tryAcquireShared}, returning on success. Otherwise the
* thread is queued, possibly repeatedly blocking and unblocking,
* invoking {@link #tryAcquireShared} until success or the thread
* is interrupted.
* @param arg the acquire argument.
* This value is conveyed to {@link #tryAcquireShared} but is
* otherwise uninterpreted and can represent anything
* you like.
* @throws InterruptedException if the current thread is interrupted
*/
public final void acquireSharedInterruptibly(int arg)
throws InterruptedException {
if (Thread.interrupted()) //判断是否是被中断，如果是被中断，那么直接抛出中断异常
throw new InterruptedException();
if (tryAcquireShared(arg) < 0) //如果获取共享锁失败，则线程进入等待队列，可能会重复的发生阻塞，解开阻塞。一直到有线程唤醒他或者有中断发生
doAcquireSharedInterruptibly(arg);
}
protected int tryAcquireShared(int acquires) { //CountDownLatch内部实现的Sync，实现了AQS的这个方法，如果state是0表示获取锁成功返回1，否则返回-1。开始时候如果子线程都没有执行的情况，这个值肯定是大于0的。
return (getState() == 0) ? 1 : -1;
}
/**
* Acquires in shared interruptible mode.
* @param arg the acquire argument
*/
private void doAcquireSharedInterruptibly(int arg)
throws InterruptedException {
final Node node = addWaiter(Node.SHARED); //添加到等待队列，见下面方法
boolean failed = true;
try {
for (;;) { //节点被加入到等待队列以后，需要在for循环中不断的尝试去获取锁。
final Node p = node.predecessor();//获取当前节点的前驱节点
if (p == head) {//如果是头节点
int r = tryAcquireShared(arg);//直接尝试是否能获取锁
if (r >= 0) {//如果尝试成功，
setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();
}
} finally {
if (failed)
cancelAcquire(node);
}
}
/**
* Creates and enqueues node for current thread and given mode.
*
* @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
* @return the new node
*/
private Node addWaiter(Node mode) {
Node node = new Node(Thread.currentThread(), mode); //建立{共享}的节点，以为是CountDownLatch，如果是ReentrantLock则建立的是排他模式的节点。
// Try the fast path of enq; backup to full enq on failure //先通过快速路径添加试一下，加到队尾，如果添加成功就返回当前节点；如果失败则进入enq(node)
Node pred = tail;
if (pred != null) {
node.prev = pred;
if (compareAndSetTail(pred, node)) {
pred.next = node;
return node;
}
}
enq(node);
return node;
}
/**
* Inserts node into queue, initializing if necessary. See picture above.
* @param node the node to insert
* @return node's predecessor
*/
private Node enq(final Node node) { //在for循环中不断的去尝试。如果队列是空，则他就是头节点，用compareAndSetHead加入到头部；如果非空，则通过compareAndSetTail加入到尾部。
for (;;) {
Node t = tail;
if (t == null) { // Must initialize
if (compareAndSetHead(new Node()))
tail = head;
} else {
node.prev = t;
if (compareAndSetTail(t, node)) {
t.next = node;
return t;
}
}
}
}
/**
* Sets head of queue, and checks if successor may be waiting
* in shared mode, if so propagating if either propagate > 0 or
* PROPAGATE status was set.
*
* @param node the node
* @param propagate the return value from a tryAcquireShared
*/
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 || //如果当前AQS的信号量>0,或者是头节点为空，或者头节点等待状态<0;
(h = head) == null || h.waitStatus < 0) { //或者在setHead之后的head((h=head)==null)为空或事等待状态<0，则表示当前节点的后继节点可以被唤醒。
Node s = node.next;
if (s == null || s.isShared())
doReleaseShared();
}
}
/**
* Release action for shared mode -- signals successor and ensures
* propagation. (Note: For exclusive mode, release just amounts
* to calling unparkSuccessor of head if it needs signal.)
*/
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 (;;) {//在for循环中，当前节点如果是SIGNAL状态，说明后继节点需要被唤醒。则设置节点状态是0，同时唤醒后继节点
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); //后继节点被释放，然后在获取锁的地方继续执行，回到doAcquireSharedInterruptibly，回在里面的for循环继续看能否获取到锁。然后释放它后面的Shared模式的线程。
}
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE)) //否则如果节点是0状态，则循环设置直到他状态为PROPAGATE模式。保障后续节点可以被释放
continue; // loop on failed CAS
}
if (h == head) // loop if head changed
break;
}
}
运行流程继续：
4. 子线程运行，在countDownLatch.countDown()，进入
/**
* Decrements the count of the latch, releasing all waiting threads if
* the count reaches zero.
*
* <p>If the current count is greater than zero then it is decremented.
* If the new count is zero then all waiting threads are re-enabled for
* thread scheduling purposes.
*
* <p>If the current count equals zero then nothing happens.
*/
public void countDown() {
sync.releaseShared(1);
}
/**
* Releases in shared mode. Implemented by unblocking one or more
* threads if {@link #tryReleaseShared} returns true.
*
* @param arg the release argument. This value is conveyed to
* {@link #tryReleaseShared} but is otherwise uninterpreted
* and can represent anything you like.
* @return the value returned from {@link #tryReleaseShared}
*/
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) { //可以释放锁
doReleaseShared();//调用上面提到的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; //如果减到0，说明锁已经释放，返回false
int nextc = c-1;
if (compareAndSetState(c, nextc))//CAS设置-1
return nextc == 0;//一直减到0返回true，否则返回false。countDownlatch的线程只有最后一个运行完的线程在这里可以把状态减到0可以去释放锁。其他的都不可以。
}
}
参考文章：
https://www.cnblogs.com/waterystone/p/4920797.html
https://tech.meituan.com/2019/12/05/aqs-theory-and-apply.html
https://zhuanlan.zhihu.com/p/268364895
原创文章，作者：，如若转载，请注明出处：https://blog.ytso.com/275837.html