多线程-读写锁

1.读写锁：一个资源可以被多个读线程访问，或者可以被一个写线程访问，但是不能同时存在读写线程，读写互斥，读读共享的。

代码如下:

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

    //放数据
    public void put(String key, Object value) {

        try {
            rwLock.writeLock().lock();
            System.out.println(Thread.currentThread().getName() +" 正在写操作" + key);
            TimeUnit.MICROSECONDS.sleep(300);
            map.put(key,value);
            System.out.println(Thread.currentThread().getName() +" 写完了" + key);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            rwLock.writeLock().unlock();
        }

    }

    //取数据
    public Object get(String key) {
        Object result = null;
        try {
            rwLock.readLock().lock();
            System.out.println(Thread.currentThread().getName() +" 正在读取操作" + key);
            TimeUnit.MICROSECONDS.sleep(300);
            result = map.get(key);
            System.out.println(Thread.currentThread().getName() +" 取完了" + key);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            rwLock.readLock().unlock();
        }


        return result;
    }
}

public class ReadWriteLockDemo {

    public static void main(String[] args) throws InterruptedException {
        MyCache myCache = new MyCache();
        //创建线程放数据
        for(int i = 0; i <= 5; i++) {
            final int num = i;
            new Thread(() -> {
                myCache.put(num +"",num +"");
            },String.valueOf(i)).start();
        }

        //创建线程读数据
        for(int i = 0; i <= 5; i++) {
            final int num = i;
            new Thread(() -> {
                myCache.get(num +"");
            },String.valueOf(i)).start();
        }
    }


}

结果如下:

---

 

 代码如下:

public class ReadWriteLockDemo1 {
    public static void main(String[ ]args) {
        ReentrantReadWriteLock rwLock = new ReentrantReadWriteLock();
        ReentrantReadWriteLock.ReadLock readLock = rwLock.readLock();
        ReentrantReadWriteLock.WriteLock writeLock = rwLock.writeLock();

        //锁降级
        writeLock.lock();
        System.out.println("---开始write操作");

        readLock.lock();
        System.out.println("---开始read操作");

        writeLock.unlock();
        System.out.println("---write操作完成");

        readLock.unlock();
        System.out.println("---read操作完成");
    }

}

正常结果如下:

---

异常代码如下：

public class ReadWriteLockDemo1 {
    public static void main(String[ ]args) {
        ReentrantReadWriteLock rwLock = new ReentrantReadWriteLock();
        ReentrantReadWriteLock.ReadLock readLock = rwLock.readLock();
        ReentrantReadWriteLock.WriteLock writeLock = rwLock.writeLock();

        //锁降级

        readLock.lock();
        System.out.println("---开始read操作");

        writeLock.lock();
        System.out.println("---开始write操作");


//        writeLock.unlock();
//        System.out.println("---write操作完成");
//
//        readLock.unlock();
//        System.out.println("---read操作完成");
    }
    
}

结果如下：会一直阻塞。