2. 当前链表中的 tail 节点是否为空，如果不为空，则通过 cas 操作把当前线程的 node添加到AQS队列
3. 如果为空或者cas失败，调用enq将节点添加到AQS队列
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;
/**
* Status field, taking on only the values:
* SIGNAL: The successor of this node is (or will soon be)
* blocked (via park), so the current node must
* unpark its successor when it releases or
* cancels. To avoid races, acquire methods must
* first indicate they need a signal,
* then retry the atomic acquire, and then,
* on failure, block.
* CANCELLED: This node is cancelled due to timeout or interrupt.
* Nodes never leave this state. In particular,
* a thread with cancelled node never again blocks.
* CONDITION: This node is currently on a condition queue.
* It will not be used as a sync queue node
* until transferred, at which time the status
* will be set to 0. (Use of this value here has
* nothing to do with the other uses of the
* field, but simplifies mechanics.)
* PROPAGATE: A releaseShared should be propagated to other
* nodes. This is set (for head node only) in
* doReleaseShared to ensure propagation
* continues, even if other operations have
* since intervened.
* 0: None of the above
*
* The values are arranged numerically to simplify use.
* Non-negative values mean that a node doesn't need to
* signal. So, most code doesn't need to check for particular
* values, just for sign.
*
* The field is initialized to 0 for normal sync nodes, and
* CONDITION for condition nodes. It is modified using CAS
* (or when possible, unconditional volatile writes).
*/
volatile int waitStatus;
/**
* Link to predecessor node that current node/thread relies on
* for checking waitStatus. Assigned during enqueuing, and nulled
* out (for sake of GC) only upon dequeuing. Also, upon
* cancellation of a predecessor, we short-circuit while
* finding a non-cancelled one, which will always exist
* because the head node is never cancelled: A node becomes
* head only as a result of successful acquire. A
* cancelled thread never succeeds in acquiring, and a thread only
* cancels itself, not any other node.
*/
volatile Node prev;
/**
* Link to the successor node that the current node/thread
* unparks upon release. Assigned during enqueuing, adjusted
* when bypassing cancelled predecessors, and nulled out (for
* sake of GC) when dequeued. The enq operation does not
* assign next field of a predecessor until after attachment,
* so seeing a null next field does not necessarily mean that
* node is at end of queue. However, if a next field appears
* to be null, we can scan prev's from the tail to
* double-check. The next field of cancelled nodes is set to
* point to the node itself instead of null, to make life
* easier for isOnSyncQueue.
*/
volatile Node next;
/**
* The thread that enqueued this node. Initialized on
* construction and nulled out after use.
*/
volatile Thread thread;
/**
* Link to next node waiting on condition, or the special
* value SHARED. Because condition queues are accessed only
* when holding in exclusive mode, we just need a simple
* linked queue to hold nodes while they are waiting on
* conditions. They are then transferred to the queue to
* re-acquire. And because conditions can only be exclusive,
* we save a field by using special value to indicate shared
* mode.
*/
Node nextWaiter;
/**
* Returns true if node is waiting in shared mode.
*/
final boolean isShared() {
return nextWaiter == SHARED;
}
/**
* 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;
}
Node() { // Used to establish initial head or SHARED marker
}
Node(Thread thread, Node mode) { // Used by addWaiter
this.nextWaiter = mode;
this.thread = thread;
}
Node(Thread thread, int waitStatus) { // Used by Condition
this.waitStatus = waitStatus;
this.thread = thread;
}
}
/**
* 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;
private Node addWaiter(Node mode) {
Node node = new Node(Thread.currentThread(), mode); // 将当前线程封装成Node
// Try the fast path of enq; backup to full enq on failure
Node pred = tail; // 将AQS当前的 tail (尾节点赋值给 pred) 相当于中介节点
if (pred != null) {
// 1. 当前尾节点不为 null 证明已经存在队列
// 2. 将当前节点的前置节点设置为 原队列的尾节点
node.prev = pred;
if (compareAndSetTail(pred, node)) // 将当前节点设置为尾部节点 {
// 原尾节点的下一节点为当前节点 也就是现在的尾结点
pred.next = node;
// 这样就给当前节点添加到了已有的 AQS 队列中
return node;
}
}
// 如果为Null 证明当前队列不存在 所以调用 enq 进行队列初始化
enq(node);
return node;
}
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
//获取当前节点的等待标识
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.
*/
// 证明当前为等待 可以放心的挂起线程
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.
*/
//修改当前的状态 将状态从初始状态修改为 -1
compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
return false;
}
protected final boolean tryRelease(int releases) {
int c = getState() - releases; // 当前状态 - 1
if (Thread.currentThread() != getExclusiveOwnerThread())
// 如果当前线程不是锁线程 抛出异常
throw new IllegalMonitorStateException();
boolean free = false;
if (c == 0) {
// 如果当前状态修改为 0 锁释放成功
free = true;
// 将锁线程设置为 NULL
setExclusiveOwnerThread(null);
}
// 设置状态
setState(c);
return free;
}
unparkSuccessor
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.
*/
// 获取当前头结点的状态标识
int ws = node.waitStatus;
if (ws < 0)
// 挂起状态
// 修改当前头结点的状态标识
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;
// 当前头结点的后续节点 也就是真正需要恢复的线程
if (s == null || s.waitStatus > 0) {
// 如果当前线程为销毁线程 从尾部进行寻找 寻找一个为 挂起状态的 线程
s = null;
for (Node t = tail; t != null && t != node; t = t.prev)
if (t.waitStatus <= 0)
s = t;
}
if (s != null)
// 恢复当前线程
LockSupport.unpark(s.thread);
}
为什么在释放锁的时候是从 tail 进行扫描
回到 enq 那个方法、。在标注为红色部分的代码来看一个新的节点是如何加入到链表中的
1. 将新的节点的prev指向tail
2. 通过cas将tail设置为新的节点，因为cas是原子操作所以能够保证线程安全性
3. t.next=node；设置原tail的next节点指向新的节点
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;
// ------ 注意 ------
if (compareAndSetTail(t, node)) {
// 这里并不是一个原子操作
t.next = node;
return t;
}
}
}
}
在cas操作之后， t.next=node操作之前。 存在其他线程调用unlock方法从head 开始往后遍历，由于 t.next=node 还没执行意味着链表的关系还没有建立完整。 就会导致遍历到 t 节点的时候被中断。所以从后往前遍历，一定不会存在这个问题
