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();

     

    先启动读取线程，再启动写入线程，看结果 

    14718739 get :begin 2011-04-16 18:54:46 14718739 get :end 2011-04-16 18:54:51 14737862 set:begin 2011-04-16 18:54:51 14737862 set:end 2011-04-16 18:54:56

    可以看到读取线程工作时，写入线程是不能访问数据的