AQS
内部类Node
等待队列是CLH有锁队列的变体。
waitStatus的几种状态：
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;
以下面的测试程序为例，简单介绍一下同步队列的变化：
@Test
public void test() {
CountDownLatch countDownLatch = new CountDownLatch(1);
ReentrantLock lock = new ReentrantLock();
try {
for (int i = 0; i < 5; i++) {
new Thread(new Runnable() {
@Override
public void run() {
lock.lock();
}
}, "线程 " + i
).start();
}
countDownLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
// lock.unlock();
}
我们发现，ReentrantLock的lock方法如下：
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}
由于是独占的获取，因此只有一个线程会通过CAS成功获取state，因此其它四个线程都会进入acquire(1)方法。acquire(int arg)是AQS的模板方法，方法内容如下：
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
以非公平锁为例，tryAcquire实际调用nonfairTryAcquire.该方法可以看出，首先还是通过CAS来获取state，如果是owner是之前的那个线程的话，允许重入，acquire加acquires。
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0) // overflow
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
继续回到刚才的acquire方法，会发现tryAcquire方法返回false，调用addWaiter方法：
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;
}
假设最开始是线程0获取了state，后面来的依次是线程1、线程2、线程3、线程4.
线程1进入addWaiter方法，tail为空，进入enq方法，这里会初始化AQS中的head和tail，例子里的话head是一个new Node对象，tail的Node对象是new Node(“线程1”, mode)对象。
private Node enq(final Node node) {
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;
}
}
}
}
继续，执行完addWaiter方法之后会进入acquireQueued方法：
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
// 找到node的前辈节点
final Node p = node.predecessor();
// 如果线程0不释放，则该不会进入
// 如果线程0释放state，并且p是head，也就是同步队列中的第一个任务，这个时候获取state成功，将node设置为AQS的head，返回false，结束acquire方法。
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return interrupted;
}
// 第一个判断，判断node的前辈节点是否为-1或者大于0，否则设置状态为-1，再下一次循环时，返回true进入第二个判断
// 第二个判断，将node对应的线程park，即设置为wait状态
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
其余的线程2/3/4依次在同步队列上，类似于：
+-----------+ +-----------+ + -----------+ + -----------+ +-----------+
| head | | 线程1 | | 线程2 | | 线程3 | | 线程4|
+-----------+ +-----------+ + -----------+ + -----------+ +-----------+
以下面测试程序为例，再看unlock方法(顺便提一下，idea调试多线程需要将断点处的all改为thread， 程序中的countdownlatch是为了不让test线程结束，导致无法调试)调试时看到一个线程进入release方法，其余四个线程处于wait状态，说明程序正确了。
@Test
public void test() {
CountDownLatch countDownLatch = new CountDownLatch(1);
ReentrantLock lock = new ReentrantLock();
try {
for (int i = 0; i < 5; i++) {
new Thread(new Runnable() {
@Override
public void run() {
lock.lock();
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
if ( lock.isHeldByCurrentThread()) {
lock.unlock();
}
}
}, "线程 " + i
).start();
}
countDownLatch.await();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
}
}
讲完了lock()方法，再看unlock()方法，调用release(int arg)方法
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
tryRelease(arg)不再赘述，不过是释放获得的许可，将state设置为0（一般情况下，有些是重入，需要多调用几次unlock才行),置空独占线程。
进入if内部，调用unparkSuccessor方法
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);
}
正常情况下，唤醒同步队列中的第一个任务线程
acquireShared
上面讲的是独占获取，接下来看一下共享获取
这里以ReentrantReadWriteLock为例
简单介绍一下内部类,包含一个同步器Sync，以及公平及非公平类FairSync与NonfairSync，ReadLock和WriteLock
因为读锁非独占，因此lock方法对应的是sync.tryAcquireShared(1),写锁则相反。
其他
AQS使用了模板方法设计模式。