读写锁：ReadWriteLock

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当我们在做加锁处理共享资源时，对于读远大于写的场景，可以选择ReadWriteLock作为同步控制的工具。下面是读写锁的互斥关系：

读写锁的互斥关系读写读非阻塞阻塞写阻塞阻塞从上面互斥关系可以看出，只有当多线程同时加读锁时，线程可以不用进入阻塞状态。

public class ReadWriteLockDemo {private static Lock lock = new ReentrantLock();private static ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();private static Lock readLock = readWriteLock.readLock();private static Lock writeLock = readWriteLock.writeLock();private int value;public Object handleRead(Lock lock) throws InterruptedException {try {lock.lock(); // 模拟读操作Thread.sleep(1000); // 读操作的耗时越多，读写锁的优势就越明显return value;} finally {lock.unlock();}}public void handleWrite(Lock lock, int index) throws InterruptedException {try {lock.lock();Thread.sleep(1000);value = index;} finally {lock.unlock();}}public static void main(String[] args) {final ReadWriteLockDemo demo = new ReadWriteLockDemo();Runnable readRunnale = new Runnable() {@Overridepublic void run() {try {demo.handleRead(readLock);//demo.handleRead(lock);} catch (Exception e) {e.printStackTrace();}}};Runnable writeRunnale = new Runnable() {@Overridepublic void run() {try {demo.handleWrite(writeLock, new Random().nextInt());//demo.handleWrite(lock, new Random().nextInt());} catch (Exception e) {e.printStackTrace();}}};for (int i = 0; i < 18; i++) {new Thread(readRunnale).start();}for (int i = 18; i < 20; i++) {new Thread(writeRunnale).start();}}}

当使用读写锁时，本案例大概耗时4S; 使用普通重入锁时，耗时20S左右。

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