ReadWriteLock rwl = new ReentrantReadWriteLock();读写锁

class CachedData {
   Object data;
   volatile boolean cacheValid;
   ReadWriteLock rwl = new ReentrantReadWriteLock();

   void processCachedData() {
     rwl.readLock().lock();
     if (!cacheValid) {
        // Must release read lock before acquiring write lock
        rwl.readLock().unlock();
        rwl.writeLock().lock();
        // Recheck state because another thread might have acquired
        //   write lock and changed state before we did.
        if (!cacheValid) {
          data = ...
          cacheValid = true;
        }
        // Downgrade by acquiring read lock before releasing write lock
      
        rwl.writeLock().unlock(); // Unlock write, still hold read
 
rwl.readLock().lock(); 
 
//***************************************************      

对象的方法中一旦加入synchronized修饰，则任何时刻只能有一个线程访问synchronized修饰的方法。假设有个数据对象拥有写方法与读方法，多线程环境中要想保证数据的安全，需对该对象的读写方法都要加入 synchronized同步块。这样任何线程在写入时，其它线程无法读取与改变数据；如果有线程在读取时，其他线程也无法读取或写入。这种方式在写入操作远大于读操作时，问题不大，而当读取远远大于写入时，会造成性能瓶颈，因为此种情况下读取操作是可以同时进行的，而加锁操作限制了数据的并发读取。  

         ReadWriteLock解决了这个问题，当写操作时，其他线程无法读取或写入数据，而当读操作时，其它线程无法写入数据，但却可以读取数据 。

        且看 以下例子

1. public class
2. static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");  
3.   
4. public static void
5. new
6. new Worker(data,true);  
7. new Worker(data,true);  
8.         t1.start();  
9.         t2.start();  
10.     }  
11.   
12. static class Worker extends
13.         Data data;  
14. boolean
15.   
16. public Worker(Data data, boolean
17. this.data = data;  
18. this.read = read;  
19.         }  
20.   
21. public void
22. if
23.                 data.get();  
24. else
25.                 data.set();  
26.         }  
27.     }  
28.   
29. static class
30. new
31.         Lock read = lock.readLock();  
32.         Lock write = lock.writeLock();  
33. public  void
34.             write.lock();  
35.             System.out.println(Thread.currentThread().hashCode()  
36. " set:begin " + sdf.format(new
37. try
38. 5000);  
39. //
40. catch
41.   
42. finally
43. " set:end "
44. new
45.                 write.unlock();  
46.             }  
47.               
48.   
49.         }  
50.   
51. public  int
52.             read.lock();  
53.             System.out.println(Thread.currentThread().hashCode()  
54. " get :begin " + sdf.format(new
55. try
56. 5000);  
57. //
58. catch
59.   
60. finally
61. " get :end "
62. new
63.                 read.unlock();  
64.             }  
65.               
66.   
67. return 1;  
68.         }  
69.     }  
70. }      

两个线程均是读线程，结果如下

 22474382 get :begin 2011-04-16 18:26:13

4699264 get :begin 2011-04-16 18:26:13

22474382 get :end 2011-04-16 18:26:18

4699264 get :end 2011-04-16 18:26:18

两读线程均可同时读取数据，下面看一个是读线程，一个写线程的情况

Data data = new Data();

  Worker t1 = new Worker(data,false);

  Worker t2 = new Worker(data,true);

  t2.start();

  Thread.sleep(100);

  t1.start();