java 读写互斥lock 实现缓存问题

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java 读写互斥lock 实现缓存问题0


package cn.itcast.heima2;

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class CacheDemo {
	private static ReadWriteLock rwl = new ReentrantReadWriteLock(); //读写锁
	private static Map<String, Object> cache = new HashMap<String, Object>();
	public static void main(String[] args) {
		for(int j=0;j<10;j++){ //写入
			final String key= "key"+j;
			new Thread(new Runnable() {
				public void run() {
					 put(key,key);
				}
			}).start();
			new Thread(new Runnable() { //读取
				public void run() {
					Object data = getData(key);
					System.out.println(Thread.currentThread().getName()+"-- get data : {key:"+key+",value: "+data+"}");
				}
			}).start();
		}
	}
	//取
	public  static Object getData(String key){
		rwl.readLock().lock();
		Object value = null;
		try{
			value = cache.get(key);
			if(value == null){
				rwl.readLock().unlock();
				rwl.writeLock().lock();
				try{
					if(value==null){
						value = "no";//实际失去queryDB();
					}
				}finally{
					rwl.writeLock().unlock();
				}
				rwl.readLock().lock();
			}
		}finally{
			rwl.readLock().unlock();
		}
		return value;
	}
	//存
	public static void put(String key,Object value){
		rwl.writeLock().lock();
		System.out.println("begin of put data >>");
		try{
			Thread.sleep(1000);
			if(key != null){
				cache.put(key, value);
			}
			System.out.println(Thread.currentThread().getName()+"-- put data :  {key:"+key+",value: "+value+"}");
		} catch (InterruptedException e) {
			e.printStackTrace();
		}finally{
			rwl.writeLock().unlock();
		}
		System.out.println("end of put data >>");
	}
}

打印结果如图所示：

在向缓存中写入数据的时候，读取缓存的线程应该等待写入锁释放之后才进行读取才对。
也就是说，打印结果应该是:
     begin of put data >>
     Thread-0-- put data : {key:key0,value: key0}
     end of put data >>
     Thread-3-- get data : {key:key0,value: key0}
才对。不知道以上代码哪里出问题了。还请大家帮我看看。

谢谢~~

问题补充：程序打印结果：
begin of put data >>
Thread-0-- put data : {key:key0,value: key0}
Thread-3-- get data : {key:key1,value: no}
begin of put data >>
end of put data >>
Thread-4-- put data : {key:key2,value: key2}
Thread-7-- get data : {key:key3,value: no}
begin of put data >>
end of put data >>
Thread-8-- put data : {key:key4,value: key4}
end of put data >>
Thread-1-- get data : {key:key0,value: key0}
Thread-11-- get data : {key:key5,value: no}
begin of put data >>
Thread-12-- put data : {key:key6,value: key6}
Thread-15-- get data : {key:key7,value: no}
begin of put data >>
end of put data >>
Thread-16-- put data : {key:key8,value: key8}
end of put data >>
begin of put data >>
Thread-2-- put data : {key:key1,value: key1}
Thread-5-- get data : {key:key2,value: key2}
end of put data >>
begin of put data >>
Thread-19-- get data : {key:key9,value: no}
Thread-6-- put data : {key:key3,value: key3}
Thread-9-- get data : {key:key4,value: key4}
begin of put data >>
end of put data >>
Thread-10-- put data : {key:key5,value: key5}
Thread-13-- get data : {key:key6,value: key6}
Thread-17-- get data : {key:key8,value: key8}
begin of put data >>
end of put data >>
Thread-14-- put data : {key:key7,value: key7}
begin of put data >>
end of put data >>
Thread-18-- put data : {key:key9,value: key9}
end of put data >>

Java 缓存 Lock

2013年9月05日 16:11

javawei
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查看图片附件

2个答案按时间排序按投票排序

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打印结果不应该是楼主描述的那样。end of put data >> 和 Thread-3-- get data 没在锁的保护范围内啊，这样打印顺序就没有任何保证了

2013年9月06日 16:44

teasp
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添加评论

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没看出来问题啊，如果你的意思是为什么是先Thread-3-- get data : {key:key1,value: no}，而不是 Thread-3-- get data : {key:key0,value: key0}的问题话？
主线程main只是负责启动了读写的线程，而不是说执行，你的get和put操作都不是在主线程执行的，无法保证他们顺序执行。

2013年9月06日 08:40