读锁与写锁

记住这3条：“读锁-读锁可以共存，读锁-写锁不可共存，写锁-写锁不可共存”。

用多线程来展示读锁和写锁的使用

public class ReadWriteLockDemo {

    private static volatile Map<Integer, Object> map = new ConcurrentHashMap<>();
    private static ReentrantReadWriteLock lock = new ReentrantReadWriteLock();

    public void get(Integer key) {
        Lock readLock = null;
        try {
            readLock = lock.readLock();
            readLock.lock();
            System.out.println(Thread.currentThread().getName() + "\t 开始读取key:" + key);
            Object value = map.get(key);
            System.out.println(Thread.currentThread().getName() + "\t 结束读取key:" + key + ", value:" + value);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (readLock != null) {
                readLock.unlock();
            }
        }
    }

    private void put(Integer key, Object value) {
        Lock writeLock = null;
        try {
            writeLock = lock.writeLock();
            writeLock.lock();
            System.out.println(Thread.currentThread().getName() + "\t 开始写入key:" + key + ", value:" + value);
            map.put(key, value);
            System.out.println(Thread.currentThread().getName() + "\t 结束写入key:" + key + ", value:" + value);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            writeLock.unlock();
        }
    }

    public static void main(String[] args) throws InterruptedException {
        System.out.println("------写锁不可与写锁共存-----");
        for (int i = 0; i < 5; i++) {
            final int finalI = i;
            new Thread(() -> {
                ReadWriteLockDemo readWriteLockDemo = new ReadWriteLockDemo();
                readWriteLockDemo.put(finalI, finalI);
            }, "A").start();
        }

        TimeUnit.SECONDS.sleep(3);

        System.out.println("-----读锁与读锁可以共存-----");
        for (int i = 0; i < 5; i++) {
            final int finalI = i;
            new Thread(() -> {
                ReadWriteLockDemo readWriteLockDemo = new ReadWriteLockDemo();
                readWriteLockDemo.get(finalI);
            }, "B").start();
        }
    }
}

控制台输出如下：

------写锁不可与写锁共存-----
A	 开始写入key:0, value:0
A	 结束写入key:0, value:0
A	 开始写入key:3, value:3
A	 结束写入key:3, value:3
A	 开始写入key:4, value:4
A	 结束写入key:4, value:4
A	 开始写入key:1, value:1
A	 结束写入key:1, value:1
A	 开始写入key:2, value:2
A	 结束写入key:2, value:2
-----读锁与读锁可以共存-----
B	 开始读取key:0
B	 结束读取key:0, value:0
B	 开始读取key:2
B	 结束读取key:2, value:2
B	 开始读取key:1
B	 结束读取key:1, value:1