想到了之前一直想看又不能静下心去细看的AQS，于是乎今天上班摸鱼的时候又去看了一遍
自己看的时候做的笔记都放在了下面的代码注释里面，方便以后自己去回顾，也分享给同样有需要的小伙伴们（代码有点多，希望能够耐心看完）
不得不感叹，Doug Lea大佬真的是太强了！
另外，如果有写的不对的地方，希望大佬们能够帮忙指正
/**
* 获取锁及获取不到锁的处理逻辑
*/
// ReentrantLock
public void lock() {
sync.lock();
}
final void lock() {
acquire(1);
}
// tryAcquire为协作类即ReentrantLock实现的（不在此次范围）
// AQS
// 尝试获取锁
public final void acquire(int arg) {
// tryAcquire 尝试获取锁 acquireQueued 入队 addWaiter 新增一个节点
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
// 中断当前线程
selfInterrupt();
}
private Node addWaiter(Node mode) {
// 新建一个节点
Node node = new Node(Thread.currentThread(), mode);
// Try the fast path of enq; backup to full enq on failure
// pred = 尾节点
Node pred = tail;
// 尾节点不为空
if (pred != null) {
// 入队并将当前节点作为尾节点
node.prev = pred;
if (compareAndSetTail(pred, node)) {
pred.next = node;
return node;
}
}
// 入队
enq(node);
return node;
}
// 入队
private Node enq(final Node node) {
for (;;) {
// t = 尾节点
Node t = tail;
if (t == null) { // Must initialize
// 新建一个节点，并将其设置为头节点
if (compareAndSetHead(new Node()))
tail = head;
} else {
node.prev = t;
// 将当前节点作为尾节点
if (compareAndSetTail(t, node)) {
t.next = node;
return t;
}
}
}
}
// 加入队列
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
boolean interrupted = false;
// 循环
for (;;) {
// 获取当前节点的前置节点
final Node p = node.predecessor();
// 假如为头节点，则尝试获取一下锁
if (p == head && tryAcquire(arg)) {
// 将当前节点设置为头节点，并清空当前节点的信息（线程，前置节点置为null）
setHead(node);
p.next = null; // help GC
failed = false;
// 获取锁成功后，就不需要中断了
return interrupted;
}
// 获取锁失败，判断是否需要挂起当前线程，
if (shouldParkAfterFailedAcquire(p, node) &&
// 挂起线程
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
// 前置节点的waitStatus
int ws = pred.waitStatus;
if (ws == Node.SIGNAL)
// 前置节点待唤醒，需要挂起当前节点
/*
* This node has already set status asking a release
* to signal it, so it can safely park.
*/
return true;
if (ws > 0) {
// 前置节点取消，再往前找，一直找到不为取消的位置（删除那些已经取消的节点）
/*
* 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.
*/
// 回去将前置节点的waitStatus修改为SIGNAL
compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
return false;
}
/**
* 获取锁及获取不到锁的处理逻辑
*/
public void unlock() {
sync.release(1);
}
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为协作类即ReentrantLock实现的（不在此次范围）
// AQS
// 释放锁
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
// 有头节点且waitStatus不为0
if (h != null && h.waitStatus != 0)
// 唤醒线程
unparkSuccessor(h);
return true;
}
return false;
}
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;
// ws<0 有未取消，在等待唤醒的线程（将头节点waitStatus设置为0）
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.
*/
// s = head.next
Node s = node.next;
// 假如头节点的下一个节点=s为空或已经取消，将s置为空，并从后往前找
if (s == null || s.waitStatus > 0) {
s = null;
// 从后往前找，直到找到距离头节点最近的待唤醒的节点
for (Node t = tail; t != null && t != node; t = t.prev)
if (t.waitStatus <= 0)
s = t;
}
if (s != null)
// 如果s不为空，则唤醒s对应的线程
LockSupport.unpark(s.thread);
}
/**
* 在队列中放弃等待
*/
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;
// 删除cancel的节点
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.
// compareAndSetTail(node, pred) 将前置节点设置为尾节点
if (node == tail && compareAndSetTail(node, pred)) {
// 删除当前节点
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)
// 如果后继的节点需要被唤醒，则将当前节点的前置节点的next设置为当前节点的后置节点
compareAndSetNext(pred, predNext, next);
} else {
// 唤醒后继节点（后续节点被唤醒后拿不到锁，又会被挂起）
unparkSuccessor(node);
}
node.next = node; // help GC
}
}
/**
* Node的waitStatus枚举
*/
/** 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;
补充一下看代码的思路
AQS.png
学习对应AQS协作类的思路
主要关注三个内容
1.对State的修改
2.对应的队列（FIFO）
3.获取和释放锁的方式（分为公平与非公平两种情况）
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