java并發程式設計核心JUC，JUC的核心是AQS，而AQS的核心是Unsafe使用的CAS(compare and swap)。
AQS全稱：AbstractQuenedSynchronizer抽象的隊列式同步器。
AQS定義了一套多線程通路共享資源的同步器架構，許多同步類實作都依賴于它。如：ReentrantLock/Semaphore/CountDownLatch...。
大白話，AQS就是基于CLH隊列，用volatile修飾共享變量state，線程通過CAS去改變狀态符，成功則擷取鎖成功，失敗則進入等待隊列，等待被喚醒。
深入了解，多線程通路共享資源涉及步驟（ReentrantLock為例）：
搶鎖
// 嘗試擷取鎖 成功傳回true,失敗傳回false
// 失敗則進行入隊操作
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
// 公平鎖與非公平鎖實作不同 公平鎖
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
// 判斷是否已鎖
if (c == 0) {
// 是否有隊列有隊列直接傳回false 搶鎖失敗
// cas搶鎖，搶鎖成功設定目前線程
if (!hasQueuedPredecessors() &&
compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
// 重入鎖
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0)
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
// 非公平鎖直接強鎖
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
// 判斷是否已鎖
if (c == 0) {
if (compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
// 重入鎖
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;
}
入隊
// 入隊前先建立節點
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
// 建立節點
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;
// 尾節點存在 證明隊列存在 直接放入隊列尾部
if (pred != null) {
node.prev = pred;
if (compareAndSetTail(pred, node)) {
pred.next = node;
return node;
}
}
//
enq(node);
return node;
}
// 隊列不存在情況 建立兩個節點，頭結點作為 目前持有鎖的節點
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;
}
}
}
}
// 入隊操作：總結就是我是下一個頭結點就跳出循環 否則一直循環等待我是下一個頭結點
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true; // 入隊失敗标記
try {
boolean interrupted = false;// 中斷标記
// 自旋
for (;;) {
// 目前節點的前節點是頭結點證明直接下一個就能輪到他 進行擷取鎖，點嘗試擷取鎖，擷取成功把自己設定為頭節點，之前的頭結點需要GC回收
final Node p = node.predecessor();
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return interrupted;
}
// 不是下一個要處理的節點，就通過park()進入等待狀态。直到被喚醒
// 如果自己可以休息了，就通過park()進入waiting狀态，直到被unpark()。如果不可中斷的情況下被中斷了，那麼會從park()中醒過來，發現拿不到資源，進而繼續進入park()等待。
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
// 設定自己等待waiting 如果
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
int ws = pred.waitStatus; // 等待标記
if (ws == Node.SIGNAL) // 前節點 正處于等待标記（停止休息）
/*
* This node has already set status asking a release
* to signal it, so it can safely park.
*/
return true;
if (ws > 0) { // 辨別前節點取消或者放棄擷取鎖
/*
* Predecessor was cancelled. Skip over predecessors and indicate retry.
* 如果前節點放棄了，那就一直往前找，直到找到最近一個正常等待的狀态，并排在它的後邊。
* 注意：那些放棄的結點，由于被自己“加塞”到它們前邊，它們相當于形成一個無引用鍊，稍後就會被保安大叔趕走了(GC回收)！
*/
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.
* 如果前節點正常，那就把前節點的狀态設定成SIGNAL，告訴它拿完号後通知自己一下。有可能失敗，人家說不定剛剛釋放完呢！
*/
compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
return false;
}
// 真正處于等待狀态
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this); // 調用park()使線程進入waiting狀态
return Thread.interrupted();
}
// 釋放鎖
public void unlock() {
sync.release(1);
}
// 先進行狀态修改再進行頭節點的下一個節點喚醒操作
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
// 鎖狀态設定為 狀态 - 1（重入鎖多次釋放）
protected final boolean tryRelease(int releases) {
int c = getState() - releases;
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
boolean free = false;
if (c == 0) {
free = true;
setExclusiveOwnerThread(null);
}
setState(c);
return free;
}
// waitStatus頭結點等待設定為0 頭節點的下一個節點unpark 被喚醒。
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);
}
// 出隊在 入隊操作時 發現目前節點的頭結點 并能成功搶鎖 則頭結點出隊。p.next = null; // help GC
// 鎖釋放頭結點的下一個節點被喚醒。