22.ReadWriteLock读写锁

import java.util.Random;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * ReadWriteLock读写锁:允许多个线程同时读
 * 读-读不互斥：读写之间不阻塞
 * 读-写互斥：读阻塞写，写阻塞读
 * 写-写互斥：写写阻塞
 */
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;
    //读
    public Object handleRead(Lock lock) throws InterruptedException{
        try {
            lock.lock();
            Thread.sleep(1000);//读操作的耗时越多读写的优势越明显
            return value;
        } finally {
            lock.unlock();
        }
    }
    //写
    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 readRunnable = () -> {
            try {
                demo.handleRead(readLock);//读写锁
//                    demo.handleRead(lock);//重入锁
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        };
        Runnable writeRunnable = () -> {
            try {
                demo.handleWrite(writeLock,new Random().nextInt());//读写锁
//                    demo.handleWrite(lock,new Random().nextInt());//重入锁
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        };
        //读写分离
        for (int i = 0; i < 18; i++) {
            new Thread(readRunnable).start();
        }
        for (int i = 18; i < 20; i++) {
            new Thread(writeRunnable).start();
        }
    }
    //使用读写锁，由于使用了读写分离，读线程完全并行，而写会阻塞读(注意for循环次数，写线程之间实际是串行的),因此运行大约2秒多结束
    //而使用重入锁，所有读写线程之间都必须相互等待，因此执行时间大约20多秒
}