ReentrantReadWriteLock源码分析

ReentrantReadWriteLock是基于AQS实现的读写锁，读锁与读锁不互斥、读锁与写锁互斥、写锁与写锁互斥。

类的继承关系

AQS提供了共享和排它两种模式，acquire/release、acquireShared/releaseShared 是AQS里面的两对模板方法。写锁是排它模式基于acquire/release模板方法实现的，读锁是共享模式基于acquireShared/releaseShared这对模板方法实现的。

public class ReentrantReadWriteLock
        implements ReadWriteLock, java.io.Serializable {
    //读锁
    private final ReentrantReadWriteLock.ReadLock readerLock;
    //写锁
    private final ReentrantReadWriteLock.WriteLock writerLock;
    //AQS的实现
    final Sync sync;

    public ReentrantReadWriteLock() {
        this(false);
    }

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

    public static class ReadLock implements Lock, java.io.Serializable {
        private final Sync sync;
        protected ReadLock(ReentrantReadWriteLock lock) {
            sync = lock.sync;
        }
    }

    public static class WriteLock implements Lock, java.io.Serializable {
        private final Sync sync;
        protected WriteLock(ReentrantReadWriteLock lock) {
            sync = lock.sync;
        }
    }
}

从表面上看ReentrantReadWriteLock内部定义了readerLock和writerLock两把锁，实际上他们公用一个sync对象，可以理解为一把锁。线程分为两类：读线程和写线程。读线程和读线程之间不互斥，可以同时拿到这把锁。读线程和写线程互斥，写线程和写线程互斥。

从构造函数中可以看出，sync对象和互斥锁ReentrantLock一样有两种实现模式：公平和非公平模式。

abstract static class Sync extends AbstractQueuedSynchronizer {
    static final int SHARED_SHIFT   = 16;
    //左移运算符（<<）将一个数的二进制表示向左移动指定的位数，右侧空出的位置补0。
    //SHARED_UNIT的二进制为1 0000 0000 0000 00