七、ReentrantReadWriteLock读写锁

七、ReentrantReadWriteLock读写锁

/**
 * 多个线程同时读一个资源类没有任何问题， 所以为了满足并发量，读取共享资源应该可以同时进行。.
 * 但是
 * 如果有一个线程想去写共享资源来，就不应该再有其它线程可以对该资源进行读或写
 * 小总结:
 * 读-读能共存
 * 读-写不能共存
 * 写-写不能共存
 */
public class ReadWriteLockDemo {
    public static void main(String[] args) {
        MyCache myCache = new MyCache();
        for (int i = 1; i <= 5; i++) {
            final int temInt = i;
            new Thread(() -> {
                myCache.put(temInt + "", temInt + "");
            }, String.valueOf(i)).start();
        }

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

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

public class MyCache {
    private volatile Map<String, Object> map = new HashMap<>();
    private ReadWriteLock readWriteLock = new ReentrantReadWriteLock();

    public void put(String key, Object value) {
        readWriteLock.writeLock().lock();
        try {
            System.out.println(Thread.currentThread().getName() + "\t开始写入\t" + key);
            //暂停一会线程毫秒
            try { TimeUnit.MILLISECONDS.sleep(300);} catch (InterruptedException e) {e.printStackTrace();}
            map.put(key, value);
            System.out.println(Thread.currentThread().getName() + "\t写入完成");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            readWriteLock.writeLock().unlock();
        }
    }

    public void get(String key) {
        readWriteLock.readLock().lock();
        try {
            System.out.println(Thread.currentThread().getName() + "\t开始读取");
            //暂停一会线程毫秒
            try { TimeUnit.MILLISECONDS.sleep(300);} catch (InterruptedException e) {e.printStackTrace();}
            Object result = map.get(key);
            System.out.println(Thread.currentThread().getName() + "\t读取完成\t" + result);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            readWriteLock.readLock().unlock();
        }
    }
}