ReentrantReadWriteLock

笔试被问了一个pthread+mutex实现读写锁，回来一看原来跟java 里的Thread，lock ， condition是完全一致的概念，faint。

当然，知道也没用，像我这总弱鸡也就用用读写锁，连人家代码都没看过。

参考

http://yhjhappy234.blog.163.com/blog/static/3163283220135178183769/

http://developer.51cto.com/art/201103/249288.htm

简要介绍

读写锁是concurrent包里比较使用广泛的锁了，实现逻辑是读可重入，写操作无法重入且与读操作互斥。当然，还有很多其他调度特性，降级特性，我觉得我们可以先从基本实现来看

分析1-用例

学东西先从例子看起吧，先看个JDK官方给的例子：使用ReentrantReadWriteLock做cache的例子，不过感觉讲的是读写锁之间怎么转换。

class CachedData {
Object data;
volatile boolean cacheValid;
final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

void processCachedData() {
rwl.readLock().lock();
if (!cacheValid) {
// Must release read lock before acquiring write lock
rwl.readLock().unlock();
rwl.writeLock().lock();
try {
// Recheck state because another thread might have
// acquired write lock and changed state before we did.
if (!cacheValid) {
data = ...
cacheValid = true;
}
// Downgrade by acquiring read lock before releasing write lock
rwl.readLock().lock();
} finally {
rwl.writeLock().unlock(); // Unlock write, still hold read
}
}

try {
use(data);
} finally {
rwl.readLock().unlock();
}
}
}

分析2-基本实现

public class ReentrantReadWriteLock implements ReadWriteLock, java.io.Serializable

public static class ReadLock implements Lock, java.io.Serializable
public static class WriteLock implements Lock, java.io.Serializable

/** Inner class providing readlock */
private final ReentrantReadWriteLock.ReadLock readerLock;
/** Inner class providing writelock */
private final ReentrantReadWriteLock.WriteLock writerLock;
/** Performs all synchronization mechanics */
private final Sync sync;

public ReentrantReadWriteLock.WriteLock writeLock() { return writerLock; }
public ReentrantReadWriteLock.ReadLock  readLock()  { return readerLock; }

在ReentrantReadWriteLock内部实现了ReadLock和WriteLock两个类，但仔细看这两个类的实现发现他们全部由一个Sync来具体实现的

public ReentrantReadWriteLock() {
this(false);
}
public ReentrantReadWriteLock(boolean fair) {
sync = (fair)? new FairSync() : new NonfairSync();
readerLock = new ReadLock(this);
writerLock = new WriteLock(this);
}

这是两个lock的lock方法

public void lock() {//write Lock
sync.acquire(1);
}
public void lock() {//read Lock
sync.acquireShared(1);
}

所以我们具体看一下Sync的实现吧，

abstract static class Sync extends AbstractQueuedSynchronizer

Sync是AQS的子类，