6.读写锁ReadWriterLockh

ReadWriterLock

• 读锁，共享锁，
• 写锁，独占锁，排他锁

public class Test {
    public static void main(String[] args) {
        MyCache myCache = new MyCache();
        for (int i = 1; i <= 5; i++) {
            int finalI = i;
            new Thread(() -> {
                myCache.put(finalI + "", finalI);
            }, i + "").start();
        }

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


/*
* 1开始写入，key=1
3开始写入，key=3
3写入完成
2开始写入，key=2
2写入完成
1写入完成
4开始写入，key=4
4写入完成
* 可以看到，1写入时，3插队了，
* */
//不使用读写锁，会有插队问题
class MyCache {
    private volatile Map<String, Object> map = new HashMap<>();

    public void put(String key, Object value) {
        System.out.println(Thread.currentThread().getName() + "开始写入，key=" + key);
        map.put(key, value);
        System.out.println(Thread.currentThread().getName() + "写入完成");
    }

    public void get(String key) {
        System.out.println(Thread.currentThread().getName() + "开始读取，key=" + key);
        map.get(key);
        System.out.println(Thread.currentThread().getName() + "读取完成");
    }
}

//使用读写锁，也可使用synchronized或者ReentrantLock()解决，但读写锁粒度更高
class MyCacheLock {
    private volatile Map<String, Object> map = new HashMap<>();
    //private ConcurrentHashMap<String, Object> map = new ConcurrentHashMap<>();
    private ReadWriteLock readWriteLock = new ReentrantReadWriteLock();

    public void put(String key, Object value) {
        Lock lock = readWriteLock.writeLock();
        try {
            lock.lock();
            System.out.println(Thread.currentThread().getName() + "开始写入，key=" + key);
            map.put(key, value);
            System.out.println(Thread.currentThread().getName() + "写入完成");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            lock.unlock();
        }

    }

    public void get(String key) {
        System.out.println(Thread.currentThread().getName() + "开始读取，key=" + key);
        map.get(key);
        System.out.println(Thread.currentThread().getName() + "读取完成");
    }
}