内容简介：java中锁是个很重要的概念，当然这里的前提是你会涉及并发编程。除了语言提供的锁关键字 synchronized和volatile之外，jdk还有其他多种实用的锁。不过这些锁大多都是基于AQS队列同步器。ReadWriteLock 读写锁就是其中一个。
java中锁是个很重要的概念，当然这里的前提是你会涉及并发编程。
除了语言提供的锁关键字 synchronized和volatile之外，jdk还有其他多种实用的锁。
不过这些锁大多都是基于AQS队列同步器。ReadWriteLock 读写锁就是其中一个。
读写锁的含义是，将读锁与写锁分开对待，读锁可以任意个一起读，因为读并不涉及数据变更，而遇到写锁后，所有后续的读写都将被阻塞。这特性有什么用呢？比如我们有一个缓存，我们可以用它来提高访问速度，但是当数据变更时，怎样能保证能读到准确的数据？
在没有读写锁之前，我们可以使用wait/notify机制，我们可以以写锁作为一个同步介质，当写锁被占用时，读只能等待，写操作完成后，通知所有读继续。这看起来不那么好实现！
当有了读写锁后，我们就不需要这么麻烦了，只需要读操作使用读锁，写操作获取写锁操作。大家可能会想，既然都要获取锁，那和其他锁有什么差别呢，一般看到锁咱们都会想到串行，阻塞。但其实读写锁不是这样的。看起来你是每次都获取读锁，但其实单纯的读锁并不会阻塞线程，所以同样是并行无阻，读锁只有在一种情况下会阻塞，那就是写锁被某线程占用时。因为写锁被占用则意味着，数据可能马上发生变化，如果此再允许读操作任意进行的话，多半可能读到写了一半或者是老数据，而这简直太糟了。而写锁则只每次都会真正进行后续操作的阻塞动作，使写操作保证强一致性。
好了，以上就是咱们从概念上来理解读写锁。
而实际上呢？ReadWriteLock只是一个接口，而其实现则可能是n多的。我们就以jdk实现的 ReentrantReadWriteLock 为契机，看一下读写锁的实现吧。
在介绍 ReetrantReadWriteLock 之前，我们要先简单说下 ReentrantLock 重入锁，从字面意思理解，就是可重新进入的锁。那么，到底是什么意思呢？我们想一下，如果我们有2个资源锁可用，那么，如果我在本线程上上锁两次，是不是资源就没有了呢，那第三次进行锁获取的时候，是不是就把自己给锁死了呢？想想应该是这样的，但是为啥平时咱们都遇不到这种情况呢？原因就在于可重入性。可重入的意思是说，如果当前线程进行多次加锁操作，那么无论如何它自己都是可以进入的。简单从实现来说就是，锁会排除当前线程，从而避免自身阻塞。这些需求看起来很理所当然，但是咱们自己实现的时候可能会因为场景不一样，从而不一定需要这种特性呢。syncronized也是一种重入锁。好了，说了这么多，还是没有看到 ReetrantLock是怎么实现的！
用个不恰当的图描绘下：
我们来看下源码就一目了然了。
/**
* Fair version of tryAcquire
*/
protected final boolean tryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
if (!hasQueuedPredecessors() &&
compareAndSetState(0, acquires)) {
// 第一次进入获取到锁后，标记获得锁的线程，后续判定重入
setExclusiveOwnerThread(current);
return true;
}
}
// 重入锁判定，否则失败
else if (current == getExclusiveOwnerThread()) {
// 最多可重入 int 次
int nextc = c + acquires;
if (nextc < 0)
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
return false;
}
}
重入锁介绍完后，咱们可以安心的来说说 ReentrantReadWriteLock了。该读写锁也是一种可重入锁。它要实现的特性就是，读读锁无阻塞，写锁必阻塞（包括写读锁/写写锁），读写锁阻塞（需等待读锁释放后才能获取写锁从而保证无脏读）。
从上面可以看出，读和写是两个锁，但是他们的状态却是互相关联的，那怎样设计其数据结构呢？用两个变量去推导往往不太可行，因为其本身就是锁，如果再用两个变量去判定锁状态，那么又如何保证变量自身的可靠性呢？ReentrantReadWriteLock 是通过一个状态变量来控制的，具体为 高16位保存读锁状态，低16位保存写锁状态，而在改变状态时，使用cas保证写入的可靠性。（其实这里可以看出，锁个数不应该超过16位即65536个，这种锁数量已经完全被忽略掉了）。有了数据结构，咱们再看下怎么控制读写互联。读锁的获取，写锁没被占用时，即低位为0时，高位大于0即可代表获取了读锁，所以，读锁是n个可用的。而写锁的获取，则要依赖高低位判定了，高位大于0，即代表还有读锁存在，不能进入，如果高位为0，也不一定可进入，低位不为0则代表有写锁在占用，所以只有高低位都为0时，写锁才可用。
下面，来看下读写锁的具体实现！
来个例子先:
public class ReadWriteLockTest {
private ReentrantReadWriteLock reentrantReadWriteLock = new ReentrantReadWriteLock();
/**
* 读锁
*/
private Lock r = reentrantReadWriteLock.readLock();
/**
* 写锁
*/
private Lock w = reentrantReadWriteLock.writeLock();
/**
* 执行线程池
*/
private ExecutorService executorService = Executors.newCachedThreadPool();
@Test
public void testReadLock() {
for (int i = 0; i < 10; i++) {
Thread readWorker = new ReadWorker();
executorService.submit(readWorker);
}
waitForExecutorFinish();
}
@Test
public void testWriteLock() {
for (int i = 0; i < 10; i++) {
Thread writeWorker = new WriteWorker();
executorService.submit(writeWorker);
}
waitForExecutorFinish();
}
@Test
public void testReadWriteLock() {
for (int i = 0; i < 10; i++) {
Thread readWorker = new ReadWorker();
Thread writeWorker = new WriteWorker();
executorService.submit(readWorker);
executorService.submit(writeWorker);
}
waitForExecutorFinish();
}
/**
* 线程模拟完成后，关闭线程池
*/
private void waitForExecutorFinish() {
executorService.shutdown();
try {
executorService.awaitTermination(100, TimeUnit.SECONDS);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
private final class ReadWorker extends Thread {
@Override
public void run() {
r.lock();
try {
SleepUtils.second(1);
System.out.println(System.currentTimeMillis() + ": " + Thread.currentThread().getName() + " reading...");
SleepUtils.second(1);
}
finally {
r.unlock();
}
}
}
private final class WriteWorker extends Thread {
@Override
public void run() {
w.lock();
try {
SleepUtils.second(1);
System.out.println(System.currentTimeMillis() + ": " + Thread.currentThread().getName() + " writing...");
SleepUtils.second(1);
}
finally {
w.unlock();
}
}
}
}
可以看到 testReadLock(), 无阻塞，立即完成10个读任务！
而 testWriteLock()，则是全部阻塞执行，20秒完成串行10个任务！
而 testReadWriteLock(), 则是 读锁与写锁交替执行，在执行写锁时，所有锁等待，在执行读锁时，可能存在多个锁同时运行！执行结果样例如下:
1543816105277: pool-1-thread-1 reading... 1543816107278: pool-1-thread-2 writing... 1543816109278: pool-1-thread-20 writing... 1543816111278: pool-1-thread-16 writing... 1543816113279: pool-1-thread-12 writing... 1543816115279: pool-1-thread-8 writing... 1543816117280: pool-1-thread-19 reading... 1543816117280: pool-1-thread-15 reading... 1543816119280: pool-1-thread-4 writing... 1543816121280: pool-1-thread-18 writing... 1543816123281: pool-1-thread-3 reading... 1543816123281: pool-1-thread-7 reading... 1543816125287: pool-1-thread-14 writing... 1543816127290: pool-1-thread-6 writing... 1543816129290: pool-1-thread-10 writing... 1543816131290: pool-1-thread-11 reading... 1543816131290: pool-1-thread-13 reading... 1543816131290: pool-1-thread-9 reading... 1543816131290: pool-1-thread-5 reading... 1543816131290: pool-1-thread-17 reading...
ok, 现象已经展示了，是时候透过现象看本质了！
1. 读锁的获取过程 r.lock(), 其实现为 ReadLock！
public void lock() {
// 调用 AQS 的 acquireShared() 方法，进行统一调度
sync.acquireShared(1);
}
// AQS 获取共享读锁
public final void acquireShared(int arg) {
// 调用 ReentrantReadWriteLock.Sync.tryAcquireShared(), 定义锁获取方式
if (tryAcquireShared(arg) < 0)
doAcquireShared(arg);
}
// 获取读锁，unused 传参未使用，直接使用内置的高位加1方式处理
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();
// 写锁使用中，则直接获取失败
if (exclusiveCount(c) != 0 &&
getExclusiveOwnerThread() != current)
return -1;
int r = sharedCount(c);
// 读锁任意获取，除了超过最大限制
if (!readerShouldBlock() &&
r < MAX_COUNT &&
compareAndSetState(c, c + SHARED_UNIT)) {
if (r == 0) {
firstReader = current;
firstReaderHoldCount = 1;
} else if (firstReader == current) {
firstReaderHoldCount++;
} else {
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;
}
// 对读锁阻塞情况，进行处理
return fullTryAcquireShared(current);
}
// 获取低位数，即写锁状态值
static int exclusiveCount(int c) {
return c & EXCLUSIVE_MASK;
}
// 获取高位数，即读锁状态值
static int sharedCount(int c) {
return c >>> SHARED_SHIFT;
}
/**
* Full version of acquire for reads, that handles CAS misses
* and reentrant reads not dealt with in tryAcquireShared.
*/
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 (;;) {
int c = getState();
if (exclusiveCount(c) != 0) {
if (getExclusiveOwnerThread() != current)
return -1;
// else we hold the exclusive lock; blocking here
// would cause deadlock.
} else if (readerShouldBlock()) {
// Make sure we're not acquiring read lock reentrantly
if (firstReader == current) {
// assert firstReaderHoldCount > 0;
} else {
if (rh == null) {
rh = cachedHoldCounter;
if (rh == null || rh.tid != getThreadId(current)) {
rh = readHolds.get();
if (rh.count == 0)
readHolds.remove();
}
}
if (rh.count == 0)
return -1;
}
}
if (sharedCount(c) == MAX_COUNT)
throw new Error("Maximum lock count exceeded");
// 验证通过，cas更新锁状态，使用 SHARED_UNIT 进行高位加1
if (compareAndSetState(c, c + SHARED_UNIT)) {
if (sharedCount(c) == 0) {
firstReader = current;
firstReaderHoldCount = 1;
} else if (firstReader == current) {
firstReaderHoldCount++;
} else {
if (rh == null)
rh = cachedHoldCounter;
if (rh == null || rh.tid != getThreadId(current))
rh = readHolds.get();
else if (rh.count == 0)
readHolds.set(rh);
rh.count++;
cachedHoldCounter = rh; // cache for release
}
return 1;
}
}
}
以上是获取读锁的过程，其实际控制很简单，只是多了很多的状态统计，所以看起来复杂！
2. 下面，来看写锁的获取过程，WriteLock.lock()
public void lock() {
// AQS获取独占锁
sync.acquire(1);
}
// AQS 锁调度
public final void acquire(int arg) {
// 如果获取锁失败，则加入到等待队列中
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
// ReentrantReadWriteLock.Sync.tryAcquire(), 写锁获取过程
protected final boolean tryAcquire(int acquires) {
/*
* Walkthrough:
* 1. If read count nonzero or write count nonzero
* and owner is a different thread, fail.
* 2. If count would saturate, fail. (This can only
* happen if count is already nonzero.)
* 3. Otherwise, this thread is eligible for lock if
* it is either a reentrant acquire or
* queue policy allows it. If so, update state
* and set owner.
*/
Thread current = Thread.currentThread();
int c = getState();
int w = exclusiveCount(c);
// 如果是0，则说明不存在读写锁，直接成功
// 否则分有读锁和有写锁两种情况判断
if (c != 0) {
// (Note: if c != 0 and w == 0 then shared count != 0)
// 存在读锁，或者不是当前线程（重入），则直接失败
if (w == 0 || current != getExclusiveOwnerThread())
return false;
if (w + exclusiveCount(acquires) > MAX_COUNT)
throw new Error("Maximum lock count exceeded");
// Reentrant acquire
setState(c + acquires);
return true;
}
// cas 更新 state
if (writerShouldBlock() ||
!compareAndSetState(c, c + acquires))
return false;
setExclusiveOwnerThread(current);
return true;
}
/**
* 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;
}
// AQS 的锁入队列操，从队列中进行锁获取，如果获取失败，则产线一个中断标志
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
// 这里是公平锁的实现方式，只会从队列头获取锁
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return interrupted;
}
// 阻塞判定，响应中断
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
ok, 读写锁的获取已经完成，再来看一下释放的过程！
3. 读锁的释放 ReadLock.unlock()
public void unlock() {
// AQS 的释放控制
sync.releaseShared(1);
}
// AQS 释放锁
public final boolean releaseShared(int arg) {
if (tryReleaseShared(arg)) {
doReleaseShared();
return true;
}
return false;
}
// ReentrantReadWriteLock.Sync.tryReleaseShared() 自定义释放
protected final boolean tryReleaseShared(int unused) {
Thread current = Thread.currentThread();
if (firstReader == current) {
// assert firstReaderHoldCount > 0;
if (firstReaderHoldCount == 1)
firstReader = null;
else
firstReaderHoldCount--;
} else {
HoldCounter rh = cachedHoldCounter;
if (rh == null || rh.tid != getThreadId(current))
rh = readHolds.get();
int count = rh.count;
if (count <= 1) {
readHolds.remove();
if (count <= 0)
throw unmatchedUnlockException();
}
--rh.count;
}
for (;;) {
int c = getState();
int nextc = c - SHARED_UNIT;
// cas更新状态，每次减1，直到为0，锁才算真正释放
if (compareAndSetState(c, nextc))
// 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;
}
}
/**
* Release action for shared mode -- signals successor and ensures
* propagation. (Note: For exclusive mode, release just amounts
* to calling unparkSuccessor of head if it needs signal.)
*/
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;
}
}
4. 读锁的释放, WriteLock.unlock()
public void unlock() {
// AQS 释放控制
sync.release(1);
}
// AQS
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
// 释放锁
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
// Sync.tryRelease()
protected final boolean tryRelease(int releases) {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
int nextc = getState() - releases;
// 如果写锁状态为0，则意味着当前线程完全释放锁，将 owner 线各设置为null
boolean free = exclusiveCount(nextc) == 0;
if (free)
setExclusiveOwnerThread(null);
setState(nextc);
return free;
}
/**
* Wakes up node's successor, if one exists.
*
* @param node the node
*/
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;
}
// 调用 LockSupport 释放锁
if (s != null)
LockSupport.unpark(s.thread);
}
综上，读写锁的简要解析就算完成了。 其主要使用 AQS 的基础组件，进行锁调度！ 使用CAS进行状态的安全设置！ 而锁的阻塞，则是使用 LockSupport 工具组件进行实际阻塞！
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实用Common Lisp编程
Peter Seibel / 田春 / 人民邮电出版社 / 2011-10 / 89.00元
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