读写锁ReentrantReadWriteLock：读读共享，读写互斥，写写互斥-蒲公英云

读写锁ReentrantReadWriteLock：读读共享，读写互斥，写写互斥

JDK1.5之后，提供了读写锁ReentrantReadWriteLock，读写锁维护了一对锁，一个读锁，一个写锁，通过分离读锁和写锁，使得并发性相比一般的排他锁有了很大提升。在读多写少的情况下，读写锁能够提供比排他锁更好的并发性和吞吐量。

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从源码中可以看出，读写锁中同样依赖队列同步器Sync(AQS)实现同步功能，而读写状态就是其同步器的同步状态。下面从例子中来说明：读读共享，读写互斥，写写互斥。

代码如下：

public class ReentrantWriteReadLockTest {
    ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
    ReadLock readLock = lock.readLock();
    WriteLock writeLock = lock.writeLock();
    public void read(){
        try {
            readLock.lock();
            System.out.println("线程"+Thread.currentThread().getName()+"进入。。。");
            Thread.sleep(3000);
            System.out.println("线程"+Thread.currentThread().getName()+"退出。。。");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }finally{
            readLock.unlock();
        }
    }
    public void write(){
        try {
            writeLock.lock();
            System.out.println("线程"+Thread.currentThread().getName()+"进入。。。");
            Thread.sleep(3000);
            System.out.println("线程"+Thread.currentThread().getName()+"退出。。。");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }finally{
            writeLock.unlock();
        }
    }
    public static void main(String[] args) {
        final ReentrantWriteReadLockTest wr = new ReentrantWriteReadLockTest();
        Thread t1 = new Thread(new Runnable() {
            public void run() {
                wr.read();
            }
        }, "t1");
        Thread t2 = new Thread(new Runnable() {
            public void run() {
                wr.read();
            }
        }, "t2");
        Thread t3 = new Thread(new Runnable() {
            public void run() {
                wr.write();
            }
        }, "t3");
        Thread t4 = new Thread(new Runnable() {
            public void run() {
                wr.write();
            }
        }, "t4");
        t1.start();
        t2.start();
        //t3.start();
        //t4.start();
    }
}