ReadWriteLock rwl = new ReentrantReadWriteLock();讀寫鎖

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(); 
 
//***************************************************      

對象的方法中一旦加入synchronized修飾，則任何時刻隻能有一個線程通路synchronized修飾的方法。假設有個資料對象擁有寫方法與讀方法，多線程環境中要想保證資料的安全，需對該對象的讀寫方法都要加入 synchronized同步塊。這樣任何線程在寫入時，其它線程無法讀取與改變資料；如果有線程在讀取時，其他線程也無法讀取或寫入。這種方式在寫入操作遠大于讀操作時，問題不大，而當讀取遠遠大于寫入時，會造成性能瓶頸，因為此種情況下讀取操作是可以同時進行的，而加鎖操作限制了資料的并發讀取。  

         ReadWriteLock解決了這個問題，當寫操作時，其他線程無法讀取或寫入資料，而當讀操作時，其它線程無法寫入資料，但卻可以讀取資料 。

        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. }      

兩個線程均是讀線程，結果如下

 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

兩讀線程均可同時讀取資料，下面看一個是讀線程，一個寫線程的情況

Data data = new Data();

  Worker t1 = new Worker(data,false);

  Worker t2 = new Worker(data,true);

  t2.start();

  Thread.sleep(100);

  t1.start();