ReadWriteLock 读写之间互斥吗？（面试精华）

开发中遇到并发的问题一般会用到锁，Synchronized存在明显的一个性能问题就是读与读之间互斥；ReadWriteLock是JDK5中提供的读写分离锁。

读写分离锁可以有效地帮助减少锁竞争，以提升系统的性能。

ReadWriteLock管理一组锁，一个是只读的锁，一个是写锁。Java并发库中ReetrantReadWriteLock实现了ReadWriteLock接口并添加了可重入的特性。

而读写锁ReentrantReadWriteLock：读读共享，读写互斥，写写互斥；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。下面从例子中来说明：读读共享，读写互斥，写写互斥。

import java.util.concurrent.locks.ReentrantReadWriteLock;

public class ReentrantWriteReadLockTest {
    ReentrantReadWriteLock reentrantReadWriteLock = new ReentrantReadWriteLock();
    ReentrantReadWriteLock.ReadLock readLock = reentrantReadWriteLock.readLock();
    ReentrantReadWriteLock.WriteLock writeLock = reentrantReadWriteLock.writeLock();
    public void read(){
        try{
            readLock.lock();
            System.out.println("线程"+Thread.currentThread().getName()+"进入");
            Thread.sleep(1000);
            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(1000);
            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 reentrantWriteReadLockTest = new ReentrantWriteReadLockTest();
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                reentrantWriteReadLockTest.read();
            }
        },"t1");
        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                reentrantWriteReadLockTest.read();
            }
        },"t2");
        Thread t3 = new Thread(new Runnable() {
            @Override
            public void run() {
                reentrantWriteReadLockTest.write();
            }
        },"t3");
        Thread t4 = new Thread(new Runnable() {
            @Override
            public void run() {
                reentrantWriteReadLockTest.write();
            }
        },"t4");
        t1.start();
        t2.start();
        t3.start();
        t4.start();
    }
}

线程t1和t2可以同时进入，说明了读读共享！当我们启动线程t2和t3时，一个线程必须等待另一个线程退出，才能进入，说明了读写互斥！当我们启动线程t3和t4时，一个线程必须等待另一个线程退出，才能进入，说明了写写互斥！