读写锁

读写锁

ReentrantReadWriteLock

读，可多线程读

写，只允许一个写

• 读读共存

• 读写不共存

• 写写不共存

写操作：原子+独占，整个过程必须是一个完整的统一体，中间不需被分割打断

// 资源类
class Mycache{
    // 缓存的一般加volatile
    private volatile Map<String,Object> map = new HashMap<>();
    //private Lock lock = new ReentrantLock();
    private ReentrantReadWriteLock rwLock = new ReentrantReadWriteLock();
    
    // 写
    public void put(String key,Object value){
        //System.out.println(Thread.currentThread.getName()+"写入中");
        //TimeUnit.MILLISECONDS.sleep(300);
        //map.put(key,value);
        //System.out.println(Thread.currentThread.getName()+"写入完成");
        rwLock.writeLock().lock();
        try{
            map.put(key,value);
        }catch (Exception e){
            e.printStackTrace();
        }finally{
            rwLock.writeLock().unlock();
        }
        
    }
    // 读
    public Object get(String key){
       // System.out.println(Thread.currentThread.getName()+"读取中");
       // TimeUnit.MILLISECONDS.sleep(300);
       // Object result = map.get(key);
       // System.out.println(Thread.currentThread.getName()+"读取完成");
        rwLock.readLock().lock();
        try{
            return map.get(key);
        }catch (Exception e){
            e.printStackTrace();
        }finally{
            rwLock.readLock().unlock();
        }
        
    }
}

public class ReadWriteLockTest{
    public static void main(String[] args){
        Mycache mycache = new Mycache();
        // 五个线程写
        for(int i=0; i<5; i++){
            final int tempInt = i;
            new Thread(()->{
                mycache.put(tempInt+"",tempInt+"");
            },String.valueOf(i)).start();
        }
        // 五个线程读
        for(int i=0; i<5; i++){
            final int tempInt = i;
            new Thread(()->{
                mycache.get(tempInt+"");
            },String.valueOf(i)).start();
        }
    }
}

结果显示：写锁会独占写，读锁则是共享读