java 多线程 读写互斥锁ReentrantReadWriteLock：读读不互斥，读写互斥，写写互斥

ReentrantReadWriteLock：

类ReentrantLock具有相互互斥的排他效果，也就是说，同一时间，只有一个线程执行lock()方法后面的任务。这样做虽然可以解决问题，但是效率非常低。使用ReentrantReadWriterLock可以加快运行效率，某些不需要操作实例变量的方法中，完全可以使用它来提升代码运行效率。

为什么不需要操作实例变量变量方法才可以用它来提升效率呢？

• (1),读读不互斥
• (2),读写互斥
• (3),写读互斥
• (4),写写互斥

读读不互斥：

import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * @ClassName ReentrantReadWriteLockRRExample
 * @projectName: object1
 * @author: Zhangmingda
 * @description： XXX
 * date: 2021/4/25.
 */
public class ReentrantReadWriteLockRRExample {
    public static void main(String[] args) {
        ReentrantReadWriteLock  rWlock = new ReentrantReadWriteLock();
        Runnable r = () ->{
            rWlock.readLock().lock();
            try {
                System.out.println("当前时间戳：" + System.currentTimeMillis());
                Thread.sleep(1000);
                System.out.println("当前时间戳：" + System.currentTimeMillis());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }finally {
                rWlock.readLock().unlock();
            }
        };
        Thread t1 = new Thread(r,"t1");
        Thread t2 = new Thread(r,"t2");
        t1.start();
        t2.start();
    }
}

 读写互斥：

import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * @ClassName ReentrantReadWriteLockRW
 * @projectName: object1
 * @author: Zhangmingda
 * @description： XXX
 * date: 2021/4/25.
 */
public class ReentrantReadWriteLockRW {
    public static void main(String[] args) {
        ReentrantReadWriteLock rWLock = new ReentrantReadWriteLock(true);
        Runnable rr = () -> {
            String tName = Thread.currentThread().getName();
            rWLock.readLock().lock();
            System.out.println(tName + "开始读...");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }finally {
                System.out.println(tName + "读完了!");
                rWLock.readLock().unlock();
            }
        };
        Runnable rw = () -> {
            String tName = Thread.currentThread().getName();
            rWLock.writeLock().lock();
            System.out.println(tName + "开始写...");
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }finally {
                System.out.println(tName + "写完了!");
                rWLock.writeLock().unlock();
            }
        };
        Thread reader = new Thread(rr,"读者1");
        Thread reader1 = new Thread(rr,"读者2");
        Thread writer = new Thread(rw,"作家1");
        Thread writer1 = new Thread(rw,"作家2");
        reader.start();
        reader1.start();
        writer.start();
        writer1.start();
    }
}