七.ReadWriteLock

ReadWriteLock读写锁实现类为ReentrantReadWriteLock。用于控制读写数据，一次只有一个线程（ 写入线程）可以修改共享数据，任何数量的线程都可以同时读取数据。

通过readLock()控制读数据，writeLock()控制写数据。

public class ReadWrite {
    public static void main(String[] args) {
        Cache cache = new Cache();
        for (int i = 0; i < 3; i++) {
            final String t = String.valueOf(i);
            new Thread(()->{
                try {
                    cache.write(t,t);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }).start();
        }
        for (int i = 0; i < 3; i++) {
            final String t = String.valueOf(i);
            new Thread(()->{
                cache.read(t);
            }).start();
        }
    }
}

class Cache{

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

    public void write(String key,Object value) throws InterruptedException {
        // 写，同一时刻只有一个线程写
        readWriteLock.writeLock().lock();
        try {
            System.out.println(Thread.currentThread().getName()+" writing...");
            map.put(key,value);
            TimeUnit.MILLISECONDS.sleep(100);
            System.out.println(Thread.currentThread().getName()+" write over");
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            readWriteLock.writeLock().unlock();
        }
    }

    public void read(String key){
        // 读，多个线程可以同时读
        readWriteLock.readLock().lock();
        try {
            System.out.println(Thread.currentThread().getName()+" reading...");
            System.out.println(Thread.currentThread().getName()+" value:"+map.get(key));
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            readWriteLock.readLock().unlock();
        }
    }
}

Thread-1 writing...
Thread-1 write over
Thread-0 writing...
Thread-0 write over
Thread-2 writing...
Thread-2 write over
Thread-4 reading...
Thread-4 value:1
Thread-5 reading...
Thread-3 reading...
Thread-5 value:2
Thread-3 value:0