本来想好好写一下，csdn编辑器 写到一半就崩溃了 ，草稿箱也没有了，随便写写给自己看就行了。。。
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;
volatile int waitStatus;
volatile Node prev;
volatile Node next;
volatile Thread thread;
Node nextWaiter;
AQS中存在首尾Node元素
/**
* Head of the wait queue, lazily initialized. Except for
* initialization, it is modified only via method setHead. Note:
* If head exists, its waitStatus is guaranteed not to be
* CANCELLED.
*/
private transient volatile Node head;
/**
* Tail of the wait queue, lazily initialized. Modified only via
* method enq to add new wait node.
*/
private transient volatile Node tail;
/**
* The synchronization state.
*/
private volatile int state;protected boolean tryAcquire(int arg) : 独占式获取同步状态，试着获取，成功返回true，反之为false
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
官方提供的小例子
package juc;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
/**
* Created by chengxiao on 2017/3/28.
*/
public class Mutex implements java.io.Serializable {
//静态内部类，继承AQS
private static class Sync extends AbstractQueuedSynchronizer {
//是否处于占用状态
protected boolean isHeldExclusively() {
return getState() == 1;
}
//当状态为0的时候获取锁，CAS操作成功，则state状态为1，
public boolean tryAcquire(int acquires) {
if (compareAndSetState(0, 1)) {
setExclusiveOwnerThread(Thread.currentThread());
return true;
}
return false;
}
//释放锁，将同步状态置为0
protected boolean tryRelease(int releases) {
if (getState() == 0) throw new IllegalMonitorStateException();
setExclusiveOwnerThread(null);
setState(0);
return true;
}
}
//同步对象完成一系列复杂的操作，我们仅需指向它即可
private final Sync sync = new Sync();
//加锁操作，代理到acquire（模板方法）上就行，acquire会调用我们重写的tryAcquire方法
public void lock() {
sync.acquire(1);
}
public boolean tryLock() {
return sync.tryAcquire(1);
}
//释放锁，代理到release（模板方法）上就行，release会调用我们重写的tryRelease方法。
public void unlock() {
sync.release(1);
}
public boolean isLocked() {
return sync.isHeldExclusively();
}
}打断方法
/**
* Convenience method to interrupt current thread.
*/
static void selfInterrupt() {
Thread.currentThread().interrupt();
}接下来，我们看看添加队列成功的，首先来看addWaiter()方法
/**
* 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);
// 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;
}首先创建一个Node对象，并判断AQS中尾节点是否为空，
如果不为空，那么指定创建新节点的前置节点为末节点，并对节点最后的值进行cas快速插入。
如果为空，那么进入到enq方法
/**
* CAS tail field. Used only by enq.
*/
private final boolean compareAndSetTail(Node expect, Node update) {
return unsafe.compareAndSwapObject(this, tailOffset, expect, update);
}我们继续来看enq方法
/**
* 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) {
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;
}
}
}
}可以看到，是一个经典的cas自旋，每次都拿到tail节点，如果为空，就设置头节点的值，就cas设置其为头节点，不为空，那么使劲儿中为末节点。
回到addWaiter方法，此时，我们已经添加了node节点到队列尾部，并返回了该节点
并将节点传递到acquireQueued()方法中
/*
* Various flavors of acquire, varying in exclusive/shared and
* control modes. Each is mostly the same, but annoyingly
* different. Only a little bit of factoring is possible due to
* interactions of exception mechanics (including ensuring that we
* cancel if tryAcquire throws exception) and other control, at
* least not without hurting performance too much.
*/
/**
* Acquires in exclusive uninterruptible mode for thread already in
* queue. Used by condition wait methods as well as acquire.
*
* @param node the node
* @param arg the acquire argument
* @return {@code true} if interrupted while waiting
*/
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return interrupted;
}
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}该方法的目的就是询问该线程是否可以park，容易看出也是一个自旋方法，那么我们来看一下这个方法：
/**
* Returns previous node, or throws NullPointerException if null.
* Use when predecessor cannot be null. The null check could
* be elided, but is present to help the VM.
*
* @return the predecessor of this node
*/
final Node predecessor() throws NullPointerException {
Node p = prev;
if (p == null)
throw new NullPointerException();
else
return p;
}首先看一下Node的predecessor()方法，返回值就是p的前节点，如果跑出异常那么被捕获，就会进行到cancelAcquire方法
// Utilities for various versions of acquire
/**
* Cancels an ongoing attempt to acquire.
*
* @param node the node
*/
private void cancelAcquire(Node node) {
// Ignore if node doesn't exist
if (node == null)
return;
node.thread = null;
// Skip cancelled predecessors
Node pred = node.prev;
while (pred.waitStatus > 0)
node.prev = pred = pred.prev;
// predNext is the apparent node to unsplice. CASes below will
// fail if not, in which case, we lost race vs another cancel
// or signal, so no further action is necessary.
Node predNext = pred.next;
// Can use unconditional write instead of CAS here.
// After this atomic step, other Nodes can skip past us.
// Before, we are free of interference from other threads.
node.waitStatus = Node.CANCELLED;
// If we are the tail, remove ourselves.
if (node == tail && compareAndSetTail(node, pred)) {
compareAndSetNext(pred, predNext, null);
} else {
// If successor needs signal, try to set pred's next-link
// so it will get one. Otherwise wake it up to propagate.
int ws;
if (pred != head &&
((ws = pred.waitStatus) == Node.SIGNAL ||
(ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
pred.thread != null) {
Node next = node.next;
if (next != null && next.waitStatus <= 0)
compareAndSetNext(pred, predNext, next);
} else {
unparkSuccessor(node);
}
node.next = node; // help GC
}
}如果不为空，那么返回P，如果p为是头结点，并且当前线程获取同步成功，那么就将node设置为头结点，并返回false，表明当前可以执行该线程。
如果p不是头结点，那么进入到shouldParkAfterFailedAcquire()是否应该park
/**
* Checks and updates status for a node that failed to acquire.
* Returns true if thread should block. This is the main signal
* control in all acquire loops. Requires that pred == node.prev.
*
* @param pred node's predecessor holding status
* @param node the node
* @return {@code true} if thread should block
*/
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;
}查询前节点的等待状态，如果是signal那么返回true，如果大于0，那么只有当线程状态为取消状态的时候，才是大于0的，就进行while循环，使得node成为一个状态不为取消节点的后一个节点。
如果前节点状态不大于0(这里可以认为不大于0且不等于单独占有),那么就为该线程的node节点设置等待状态为Node.SIGNAL。并返回false，表示不用暂停，可以共享锁。
/**
* Convenience method to park and then check if interrupted
*
* @return {@code true} if interrupted
*/
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}返回true，进入到打断中，当前线程暂停，进入park等待中。
参考:https://www.cnblogs.com/chengxiao/archive/2017/07/24/7141160.html#a2-1