介绍：
ReentrantLock 是一个互斥锁，在基本行为和机制上与synchonized一样，只不过synchonized用方法和声明访问了隐式的锁监视器，但是ReentrantLock 做了功能上的扩展。
ReentrantLock 被最后一个成功lock，但是还没unlock的线程拥有。当锁不被其他线程拥有，一个线程会成功的申请锁资源并立即返回。如果当前线程已经拥有了锁，再申请时也会立即返回。通过调用方法isHeldByCurrentThread()获取是否当前线程获得了锁，getHoldCount()得到获得几次锁资源。
此类的构造方法接受一个可选的公平 参数。当设置为 true 时，在多个线程的争用下，这些锁倾向于将访问权授予等待时间最长的线程。否则此锁将无法保证任何特定访问顺序。与采用默认设置（使用不公平锁）相比，使用公平锁的程序在许多线程访问时表现为很低的总体吞吐量（即速度很慢，常常极其慢），但是在获得锁和保证锁分配的均衡性时差异较小。****不过要注意的是，公平锁不能保证线程调度的公平性。因此，使用公平锁的众多线程中的一员可能获得多倍的成功机会，这种情况发生在其他活动线程没有被处理并且目前并未持有锁时。还要注意的是，未定时的 tryLock 方法并没有使用公平设置。因为即使其他线程正在等待，只要该锁是可用的，此方法就可以获得成功。
推荐使用用try-catch 块代码去调用lock(),如下：
class X {
private final ReentrantLock lock = new ReentrantLock();
public void m() {
lock.lock();
// block until condition holds try {
// ... method body
} finally { lock.unlock() }
}
}
}
除了实现lock接口，这个类还定义了一些public和protected方法去检查锁的状态。有些方法仅仅用来监控和维护。
这个类的序列化行为与内建的锁一样：反序列化的锁是没有获取锁状态，无论当它序列化时是否获取锁。
这个锁支持最大2147483647次的重入次数，超过这个数会报错。
类关系图：
444.png
从这个图可以看到ReentrantLock类实现了接口Lock和Serializable。
public class ReentrantLock implements Lock, java.io.Serializable {
ReentrantLock类的API调用都委托给一个内部类 Sync ,而该类继承AbstractQueuedSynchronizer类；
/**
* Base of synchronization control for this lock. Subclassed
* into fair and nonfair versions below. Uses AQS state to
* represent the number of holds on the lock.
*/
abstract static class Sync extends AbstractQueuedSynchronizer
而Sync又分为两个子类：公平锁和非公平锁，默认为非公平锁
/**
* Sync object for fair locks
*/
static final class FairSync extends Sync
//Sync object for Nonfair locks
static final class NonfairSync extends Sync
ReentrantLock调用lock()方法时的调用关系图
非公平锁类调用lock()方法时的调用关系：
555.png
代码解析：
1.nofairTryAcquire：
/* Performs non-fair tryLock. tryAcquire is implemented in * subclasses, but both need nonfair try for trylock method.
*/
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;
}
首先获取当前状态（初始化为0），当它等于0的时候，代表还没有任何线程获得该锁，然后通过CAS（底层是通过CompareAndSwapInt实现）改变state，如果设置成功设置当前线程为持有锁的线程；其他线程会直接返回false;当该线程重入的时候，state已经不等于0，这个时候并不需要CAS，因为该线程已经持有锁，然后会重新通过setState设置state的值，这里就实现了一个偏向锁的功能，即锁偏向该线程；
2.acquireQueued
只有当锁被一个线程持有，另外一个线程请求获得该锁的时候才会进入这个方法。
调用前首先调用addWaiter
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;
}
首先，new一个节点，这个时候模式为：mode为 Node.EXCLUSIVE，默认为null即排它锁；
然后：
如果该队列已经有node即tail!=null，则将新节点的前驱节点置为tail，再通过CAS将tail指向当前节点，前驱节点的后继节点指向当前节点，然后返回当前节点；
如果队列为空或者CAS失败，则通过enq入队：
/** * 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;
}
}
}
}
进队的时候，要么是第一个入队并且设置head节点并且循环设置tail，要么是add tail，如果CAS不成功，则会无限循环，直到设置成功，即使高并发的场景，也最终能够保证设置成功，然后返回包装好的node节点；
acquireQueued:
/** * 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);
}
}
该方法的主要作用就是真正让node入队，同时将已经进入虚拟队列的节点进行阻塞，我们看到，如果当前节点的前驱节点是head并且尝试获取锁的时候成功了，则直接返回，不需要阻塞；同时把当前节点设为头结点，原头结点则释放。
如果前驱节点不是头节点或者获取锁的时候失败了，则进行判定是否需要阻塞：
/** * 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;
}
这段代码对该节点的前驱节点的状态进行判断，如果前驱节点已经处于signal状态，则返回true，表明当前节点可以进入阻塞状态；否则，将前驱节点状态CAS置为signal状态，然后通过上层的for循环进入parkAndCheckInterrupt代码块park：
/** * Convenience method to park and then check if interrupted * * @return {@code true} if interrupted */
private final boolean parkAndCheckInterrupt(){
LockSupport.park(this);
return Thread.interrupted();
}
这个时候将该线程交给操作系统内核进行阻塞；
总体来讲，acquireQueued就是依靠前驱节点的状态来决定当前线程是否应该处于阻塞状态，如果前驱节点处于cancel状态，则丢弃这些节点，重新构建队列；
公平锁类调用lock()方法时的调用关系：
666.png
非公平锁类和公平锁类调用lock()时的区别：
1.非公平锁类调用lock（）时，不排队先尝试获取锁资源，修改状态，修改不成功再入队。具体实现先调用AbstractQueuedSynchronizer的方法
protected final boolean compareAndSetState(int expect, int update) ，而公平锁类是直接入队，不给插队的机会，当直接插队失败才会入队。
2.调用tryAcquire()时也不同，
1.公平锁类的tryAcquire()
/**
* Fair version of tryAcquire. Don't grant access unless
* recursive call or no waiters or is first.
*/
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (!hasQueuedPredecessors() &&
compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
int nextc = c + acquires;
if (nextc < 0)
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
2.非公平锁的tryAcquire()
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
/**
* Performs non-fair tryLock. tryAcquire is implemented in
* subclasses, but both need nonfair try for trylock method.
*/
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;
}
公平锁类先调用!hasQueuedPredecessors()检查此节点前面有没有非头节点的节点。这样保证了顺序的获得锁资源。非公平锁不调用!hasQueuedPredecessors(）直接CAS
再来看unlock():
1.调用流程图：
777.png
1.Reentantlock方法unlock()：
/**
* Attempts to release this lock.
*
* <p>If the current thread is the holder of this lock then the hold
* count is decremented. If the hold count is now zero then the lock
* is released. If the current thread is not the holder of this
* lock then {@link IllegalMonitorStateException} is thrown.
*
* @throws IllegalMonitorStateException if the current thread does not
* hold this lock
*/
public void unlock() {
sync.release(1);
}
unlock()调用AbstractQueuedSynchronizer的release。
2.AbstractQueuedSynchronizer的release(int arg):
/**
* 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;
}
tryRelease(arg)为true则调用unparkSucessor(h)，否则直接返回false。
3.我们再来看tryRelease(arg)：
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;
}
当释放锁的thread与当前获得锁的线程不一致时，抛出异常，参数releasese是要释放的重入锁的个数，c是释放后还剩几个。如果c==0则返回true。无论返回是true还是false都会更新state的值。state为零说明锁资源已经可以竞争了，非零说明锁资源还在某个线程没有释放。如果为零，下一步应该是唤醒一个线程，使这个线程获得竞争锁的权利。
4.unparkSucessor(h):
/**
* 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;
}
这个作用即：当头结点的状态小于0，则将头结点的状态CAS为0，然后通过链表获取下一个节点，如果下一个节点为null或者不符合要求的状态，则从队尾遍历整个链表，直到遍历到离head节点最近的一个节点并且等待状态符合预期，则将头结点的后继节点置为该节点；
对刚刚筛出来的符合要求的节点唤醒，也就是该节点获得 争夺 锁的权利。
最后让我们回到调用lock()时，线程被park的那段代码：
/**
* 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);
}
}
也就是说for循环又可以继续跑了，去做一系列的判断，并尝试获得锁，上面已经讲了，不再说了。这样，线程从lock（）到unlock()发生的事情都大体讲清楚了。下一节，会分析await()和singnal()。
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