概述
ReentrantReadWriteLock维护了一对相关的锁，它们分别是共享readLock和独占writeLock。关于共享读锁和排他写锁的概念其实很好理解。所谓共享读锁就是一个线程读的时候，其它线程也可以来读（共享），但是不能来写。排他写锁是指一个线程在写的时候，其它线程不能来写或读（排他）。除了这个特点之外，ReentrantReadWriteLock还有一个特点就是可重入的。它和ReentrantLock一样都是支持Condition的。而且ReentrantReadWerite还支持锁降级，即允许将写锁降级为读锁。
简单使用
最最基础的用法如下：
ReentrantReadWriteLock lock=new ReentrantReadWriteLock();
public void read(){
lock.readLock().lock();
//需要加读锁的操作
lock.readLock().unlock();
}
public void write(){
lock.writeLock().lock();
//需要加写锁的操作
lock.writeLock().unlock();
}
ReentrantReadWriteLock无非就是这几种情况，读读共享，写写互斥，读写互斥，写读互斥。
下面我们就以这个最基础的用法，来分析一下其内部的原理
源码分析
继承体系
共享读锁的实现原理分析
lock方法
- 首先进入调用具体的实现
public void lock() {
sync.acquireShared(1);
}
- 然后调用了这个方法
public final void acquireShared(int arg) {
if (tryAcquireShared(arg) < 0)
doAcquireShared(arg);
}
其中 int tryAcquireShared(int unused)的具体实现如下：
protected final int tryAcquireShared(int unused) {
/*
* Walkthrough:
* 1. If write lock held by another thread, fail.
* 2. Otherwise, this thread is eligible for
* lock wrt state, so ask if it should block
* because of queue policy. If not, try
* to grant by CASing state and updating count.
* Note that step does not check for reentrant
* acquires, which is postponed to full version
* to avoid having to check hold count in
* the more typical non-reentrant case.
* 3. If step 2 fails either because thread
* apparently not eligible or CAS fails or count
* saturated, chain to version with full retry loop.
*/
Thread current = Thread.currentThread();
int c = getState();
//持有写锁的线程可以获取读锁，如果获取锁的线程不是当前线程，则返回-1
if (exclusiveCount(c) != 0 &&
getExclusiveOwnerThread() != current)
return -1;
int r = sharedCount(c);//获取共享读锁的数量
if (!readerShouldBlock() &&
r < MAX_COUNT &&
compareAndSetState(c, c + SHARED_UNIT)) {
if (r == 0) {
//如果首次获取锁，则初始化firstReader和firstReaderHoldCount
firstReader = current;
firstReaderHoldCount = 1;
} else if (firstReader == current) {
//如果当前线程是首次获取读锁的线程
firstReaderHoldCount++;
} else {
//更新HoldCounter
HoldCounter rh = cachedHoldCounter;
if (rh == null || rh.tid != getThreadId(current))
cachedHoldCounter = rh = readHolds.get();
else if (rh.count == 0)
readHolds.set(rh);
rh.count++;
}
return 1;
}
return fullTryAcquireShared(current);
}
整个函数的工作流程如下：
- 如果写锁已经被持有了，但是持有写锁的不是当前写出，那么就直接返回-1（体现写锁的排他性）.
- 如果在尝试获取锁是不需要阻塞等待（由锁的公平性决定），并且读锁的共享计数小于最大值，那么就直接通过CAS更新读锁数量，获取读锁。
- 如果第二步执行失败了，那么就会调用
fullTryAcquireShared(current)
fullTryAcquireShared(current)的具体实现如下：
final int fullTryAcquireShared(Thread current) {
/*
* This code is in part redundant with that in
* tryAcquireShared but is simpler overall by not
* complicating tryAcquireShared with interactions between
* retries and lazily reading hold counts.
*/
HoldCounter rh = null;
for (;;) { //自旋
int c = getState();
if (exclusiveCount(c) != 0) { //写锁已经被持有了
if (getExclusiveOwnerThread() != current) //持有写锁的不是单线程
return -1; //其它线程持有读锁后，就不能在获取写锁了
// else we hold the exclusive lock; blocking here
// would cause deadlock.
} else if (readerShouldBlock()) {//由公平性决定需要阻塞
// Make sure we're not acquiring read lock reentrantly
if (firstReader == current) {
// assert firstReaderHoldCount > 0;
} else {
//更新锁计数（可重入的体现）
if (rh == null) {
rh = cachedHoldCounter;
if (rh == null || rh.tid != getThreadId(current)) {
rh = readHolds.get();
if (rh.count == 0)
//如果当前线程的持有读锁数为0，那么就没必要使用计数器，直接移除
readHolds.remove();
}
}
if (rh.count == 0)
return -1;
}
}
if (sharedCount(c) == MAX_COUNT) //如果读锁的数量超过最大值了
throw new Error("Maximum lock count exceeded");
if (compareAndSetState(c, c + SHARED_UNIT)) { //CAS更新读锁数量
if (sharedCount(c) == 0) {
//首次获取读锁
firstReader = current;
firstReaderHoldCount = 1;
} else if (firstReader == current) {
//当前线程是首次获取读锁的线程，直接更新持有数
firstReaderHoldCount++;
} else {
//当前线程是后来共享读锁的线程
if (rh == null)
rh = cachedHoldCounter;
if (rh == null || rh.tid != getThreadId(current))
rh = readHolds.get();//更新为当前线程的计数器
else if (rh.count == 0)
readHolds.set(rh);
rh.count++;
cachedHoldCounter = rh; // cache for release
}
return 1;
}
}
}
可以看出其实int fullTryAcquireShared(Thread current) 也每什么特别，它的代码和int tryAcquireShared(int unused)差不多。只不过是增加了自旋重试，和“持有读锁数的延迟读取”
- 我们回到
void acquireShared(int arg)方法，如果tryAcquireShared(arg)获取读锁失败后，它调用的doAcquireShared(arg)又做了什么呢？
它的具体实现如下：
private void doAcquireShared(int arg) {
final Node node = addWaiter(Node.SHARED); //添加一个共享模式的Node到等待队列尾部
boolean failed = true;
try {
boolean interrupted = false; //获取前驱节点
for (;;) {
final Node p = node.predecessor();
if (p == head) {
//如果前驱节点，尝试获取资源
int r = tryAcquireShared(arg);
if (r >= 0) {
//获取成功，更新等待队列，并唤醒下一个等待的节点
setHeadAndPropagate(node, r);
p.next = null; // help GC
if (interrupted)
selfInterrupt();
failed = false;
return;
}
}
if (shouldParkAfterFailedAcquire(p, node) && //检查获取失败后是否可以阻塞
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
其实整个获取共享读锁的源码看下来，我们可以发现，AQS框架下，获取锁一般的流程就是首先尝试去直接获取，如果获取不到了，那么尝试自旋获取，如果还是获取不到，那么就去等待队列排队，排队的时候，如果发现自己是第二个那么就再次尝试获取锁，如果还是没获取到，那么就老老实实的在等待队列中park阻塞等待了。
我们通过源码，也可发现AQS框架下的锁，其实如果线程之间对锁的争用很低的时候，大多数时候直接就能拿到锁，几乎不需要排队，阻塞之类的，性能非常之高。
unlock方法
- 第一步还是调用具体的实现
public void unlock() {
sync.releaseShared(1);
}
- 具体的实现如下
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
- 首先来看
tryReleaseShared(arg)
protected final boolean tryReleaseShared(int unused) {
Thread current = Thread.currentThread();
if (firstReader == current) { //如过当前线程是第一获取到读锁的线程
// assert firstReaderHoldCount > 0;
//直接更新线程持有数
if (firstReaderHoldCount == 1)
firstReader = null;
else
firstReaderHoldCount--;
} else {
HoldCounter rh = cachedHoldCounter;
if (rh == null || rh.tid != getThreadId(current))
rh = readHolds.get(); //获取当前线程的计数器
int count = rh.count;
if (count <= 1) {
readHolds.remove();
if (count <= 0)
throw unmatchedUnlockException();
}
--rh.count;
}
for (;;) { //自旋
int c = getState();
int nextc = c - SHARED_UNIT;
if (compareAndSetState(c, nextc)) //更新state
// Releasing the read lock has no effect on readers,
// but it may allow waiting writers to proceed if
// both read and write locks are now free.
return nextc == 0;
}
}
我们从tryReleaseShared(arg)的实现中可以看出，它的主要是去更新锁计数器和state。如果state为0的话，就返回true，否则就返回false。
- 我们回过头看，如果
tryReleaseShared(arg)返回true，即锁释放后state为0了，那么它会执行doReleaseShared();方法，它的具体实现如下：
private void doReleaseShared() {
/*
* Ensure that a release propagates, even if there are other
* in-progress acquires/releases. This proceeds in the usual
* way of trying to unparkSuccessor of head if it needs
* signal. But if it does not, status is set to PROPAGATE to
* ensure that upon release, propagation continues.
* Additionally, we must loop in case a new node is added
* while we are doing this. Also, unlike other uses of
* unparkSuccessor, we need to know if CAS to reset status
* fails, if so rechecking.
*/
for (;;) {
Node h = head;
if (h != null && h != tail) {
int ws = h.waitStatus;
if (ws == Node.SIGNAL) {
if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue; // loop to recheck cases
unparkSuccessor(h);
}
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
if (h == head) // loop if head changed
break;
}
}
这个方法的作用就是唤醒等待队列中线程，现在资源已经空闲了，等待的线程可以唤醒来获取锁了。
排他写锁的实现原理分析
排他写锁的实现原理其实和ReentrantLock一致。我们只看几处和共享读锁不同的地方。
//公平锁实现
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (!hasQueuedPredecessors() && //判断当前线程是否还有前节点
compareAndSetState(0, acquires)) {//CAS修改state
//获取锁成功，设置锁的持有线程为当前线程
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {//该线程之前已经拿到锁
int nextc = c + acquires; //重入的体现
if (nextc < 0)
throw new Error("Maximum lock count exceeded");
setState(nextc); //更新State
return true;
}
return false;
}
其实非公平锁的实现也差不多，只不过少了!hasQueuedPredecessors() 它不会去判断当前线程是否还有前驱节点，直接就开始获取锁了。
unlock方法也差不多我就不赘述了。