【JUC并发编程】读写锁：ReadWriteLock

一、介绍

二、代码演示

1. 不使用读写锁

package readwritelock;

import java.util.HashMap;
import java.util.Map;

/**
 * @author swaggyhang
 * @create 2023-07-09 11:16
 */
public class Test01 {
    public static void main(String[] args) {
        MyCache myCache = new MyCache();

        for (int i = 1; i <= 5; i++) {
            final int temp = i;
            new Thread(() -> {
                myCache.put(temp + "", temp);
            }, String.valueOf(i)).start();
        }

        for (int i = 1; i <= 5; i++) {
            final int temp = i;
            new Thread(() -> {
                myCache.get(temp + "");
            }, String.valueOf(i)).start();
        }
    }
}

class MyCache {

    private volatile Map<String, Object> map = new HashMap<>();

    // 存入
    public void put(String key, Object value) {
        System.out.println("线程" + Thread.currentThread().getName() + "：写入 => " + key);
        map.put(key, value);
        System.out.println("线程" + Thread.currentThread().getName() + "：写入OK ");
    }

    // 取出
    public void get(String key) {
        System.out.println("线程" + Thread.currentThread().getName() + "：取出 => " + key);
        Object o = map.get(key);
        System.out.println("线程" + Thread.currentThread().getName() + "：取出OK");
    }
}

2. 使用读写锁

package readwritelock;

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * @author swaggyhang
 * @create 2023-07-09 11:27
 */
public class Test02 {
    public static void main(String[] args) {
        MyCacheLock myCacheLock = new MyCacheLock();

        for (int i = 1; i <= 5; i++) {
            final int temp = i;
            new Thread(() -> {
                myCacheLock.put(temp + "", temp);
            }, String.valueOf(i)).start();
        }

        for (int i = 1; i <= 5; i++) {
            final int temp = i;
            new Thread(() -> {
                myCacheLock.get(temp + "");
            }, String.valueOf(i)).start();
        }
    }
}

class MyCacheLock {

    private volatile Map<String, Object> map = new HashMap<>();

    // 读写锁，更加细粒度的控制
    private ReadWriteLock readWriteLock = new ReentrantReadWriteLock();

    // 存入，写入的时候，只希望同时只有一个线程写入
    public void put(String key, Object value) {
        readWriteLock.writeLock().lock();
        try {
            System.out.println("线程" + Thread.currentThread().getName() + "：写入 => " + key);
            map.put(key, value);
            System.out.println("线程" + Thread.currentThread().getName() + "：写入OK ");
        } finally {
            readWriteLock.writeLock().unlock();
        }
    }

    // 取出，读取的时候，所有人都可以读取
    public void get(String key) {
        readWriteLock.readLock().lock();
        try {
            System.out.println("线程" + Thread.currentThread().getName() + "：取出 => " + key);
            Object o = map.get(key);
            System.out.println("线程" + Thread.currentThread().getName() + "：取出OK");
        } finally {
            readWriteLock.readLock().unlock();
        }
    }
}

三、总结

• ReadWriteLock读写锁特点
  ① 写锁是独占锁，一次只能被一个线程占有
  ② 读锁是共享锁，多个线程可以同时占有

• 读-读：可以共存

• 读-写：不能共存

• 写-写：不能共存