Java中已知的锁有两种，一种是synchronized，另一种是Lock；这两种的基本原理在之前的文章中已经做了分析：
这次我们从最常用的Lock也就是ReentrantLock的Acquire和Release锁的过程入手，一步一步的跟源代码，探究获取锁和释放锁的步骤。
获取锁
ReentrantLock lock=new ReentrantLock();
lock.lock();
1.lock()
上面代码是我们常用的获取锁的方式：调用ReentrantLock 的lock()方法，默认情况下ReentrantLock是一个非公平锁，类名NonfairSync，属于ReentrantLock的内部类，我们来看源码：
static final class NonfairSync extends Sync {
private static final long serialVersionUID = 7316153563782823691L;
/**
* Performs lock. Try immediate barge, backing up to normal
* acquire on failure.
*/
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
}
这里的lock方法先去通过CAS操作将state的值从0变为1，（注：ReentrantLock用state表示“持有锁的线程已经重复获取该锁的次数”。当state等于0时，表示当前没有线程持有锁），如果成功，就设置ExclusiveOwnerThread的值为当前线程（Exclusive是独占的意思，ReentrantLock用exclusiveOwnerThread表示“持有锁的线程”）。
如果设置失败，说明state>0已经有线程持有了锁，此时执行acquire(1)再请求一次锁。
2.acquire()
/**
* Acquires in exclusive mode, ignoring interrupts. Implemented
* by invoking at least once {@link #tryAcquire},
* returning on success. Otherwise the thread is queued, possibly
* repeatedly blocking and unblocking, invoking {@link
* #tryAcquire} until success. This method can be used
* to implement method {@link Lock#lock}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
*/
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
这里我们注意一下acquire方法上面的注释，已经说得很清楚了，这里我大概说下我的理解：
请求独占锁，忽略所有中断，至少执行一次tryAcquire，如果成功就返回，否则线程进入阻塞--唤醒两种状态切换中，直到tryAcquire成功。
我们对里面的tryAcquire()，、addWaiter()、acquireQueued()挨个分析。
在这个方法里先执行tryAcquire(arg):
3.tryAcquire()
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;
}
先判断state是否为0，如果为0就执行上面提到的lock方法的前半部分，通过CAS操作将state的值从0变为1，否则判断当前线程是否为exclusiveOwnerThread，然后把state++，也就是重入锁的体现，我们注意前半部分是通过CAS来保证同步，后半部分并没有同步的体现，原因是：
后半部分是线程重入，再次获得锁时才触发的操作，此时当前线程拥有锁，所以对ReentrantLock的属性操作是无需加锁的。
如果tryAcquire()获取失败，则要执行addWaiter()向等待队列中添加一个独占模式的节点
4.addWaiter()
/**
* 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;
}
这个方法的注释：创建一个入队node为当前线程，Node.EXCLUSIVE 是独占锁, Node.SHARED 是共享锁。
先找到等待队列的tail节点pred，如果pred！=null，就把当前线程添加到pred后面进入等待队列，如果不存在tail节点执行enq()
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;
}
}
}
}
这里进行了循环，如果此时存在了tail就执行同上一步骤的添加队尾操作，如果依然不存在，就把当前线程作为head结点。
插入节点后，调用acquireQueued()进行阻塞
5.acquireQueued()
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);
}
}
先获取当前节点的前一节点p，如果p是head的话就再进行一次tryAcquire(arg)操作，如果成功就返回，否则就执行shouldParkAfterFailedAcquire、parkAndCheckInterrupt来达到阻塞效果；
6.shouldParkAfterFailedAcquire()
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;
}
addWaiter()构造的新节点，waitStatus的默认值是0。此时，进入最后一个if判断，CAS设置pred.waitStatus为SIGNAL==-1。最后返回false。
回到第五步acquireQueued()中后，由于shouldParkAfterFailedAcquire()返回false，会继续进行循环。假设node的前继节点pred仍然不是头结点或锁获取失败，则会再次进入shouldParkAfterFailedAcquire()。上一轮循环中，已经将pred.waitStatus设置为SIGNAL==-1，则这次会进入第一个判断条件，直接返回true，表示应该阻塞。
7.parkAndCheckInterrupt()
private final boolean parkAndCheckInterrupt() {
LockSupport.park(this);
return Thread.interrupted();
}
很显然，一旦shouldParkAfterFailedAcquire返回true也就是应该阻塞，就会执行parkAndCheckInterrupt()来达到阻塞效果，此时线程阻塞在这里，需要其它线程来唤醒，唤醒后就会再次循环第5步acquireQueued里的请求逻辑。
我们回到第6步，看一个留下的逻辑片段
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;
}
什么时候会遇到ws > 0的case呢？当pred所维护的获取请求被取消时（也就是node的waitStatus 值为CANCELLED），这时就会循环移除所有被取消的前继节点pred，直到找到未被取消的pred。移除所有被取消的前继节点后，直接返回false。
8.cancelAcquire()
到这里我们回到第5步可以看到主体逻辑基本走完了，在该方法的finally里有一个cancelAcquire()方法
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
}
}
也就是在第5步的执行过程中，如果出现异常或者出现中断，就会执行finally的取消线程的请求操作，核心代码是node.waitStatus = Node.CANCELLED;将线程的状态改为CANCELLED。
释放锁
1.release()
/**
* Releases in exclusive mode. Implemented by unblocking one or
* more threads if {@link #tryRelease} returns true.
* This method can be used to implement method {@link Lock#unlock}.
*
* @param arg the release argument. This value is conveyed to
* {@link #tryRelease} but is otherwise uninterpreted and
* can represent anything you like.
* @return the value returned from {@link #tryRelease}
*/
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
我们还是通过方法上面的注释来理解一下：
释放独占锁，如果tryRelease成功返回true的话就会解开阻塞等待的线程
显然，tryRelease方法来释放锁，如果释放成功，先判断head节点是否有效，最后unparkSuccessor启动后续等待的线程。
2.tryRelease()
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;
}
先获取state减去释放的一次，然后判断当前线程是否和持有锁线程一致，如果不一致，抛出异常，继续判断state的值，只有当值为0时，free标志才置为true，否则说明是重入锁，需要多次释放直到state为0。
3.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.
*/
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);
}
这个方法名：启动后续线程，先拿到head节点的waitStatus并清空，然后获取next节点，并做检查，如果next节点失效，就从等待队列的尾部进行轮询，拿到第一个有效的节点，然后通过LockSupport.unpark(s.thread);唤醒，令该线程重新进入到获取锁的第5步循环去acquire锁。
疑惑点
1.我们在获取锁的很多步骤中看到tryAcquire的操作，原因是当获取一次失败后，程序会去执行失败后的逻辑代码，但是在执行过程中有可能锁的状态也同时发生了变化（释放锁、pred节点失效等情况），这时候需要去tryAcquire一下，省去了阻塞再唤醒的成本。
2.等待队列的waitStatus属性使用很多，在这里我们先读一下源码注释：
/** 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;
SIGNAL：后续线程正在阻塞，所以当前node在释放锁时必须启动后续线程，为了避免竞争激烈，acquire 方法第一次执行需要一个信号，也就是这个启动信号。
CANCELLED：这个node失效了，因为超时或者被中断等原因。
CONDITION：这个node当前是属于条件锁
PROPAGATE：这个node是共享锁节点，他需要进行唤醒传播
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