Implement a simple caching mechanism based on synchronized or Readwritelock

1  Packagecn.xxx.xxx;2 3 ImportJava.util.HashMap;4 ImportJava.util.Map;5 ImportJava.util.concurrent.locks.ReadWriteLock;6 ImportJava.util.concurrent.locks.ReentrantReadWriteLock;7 8  Public classCachedemo_my {9 Ten      Public Static voidMain (string[] args) { One         //internal class instantiation requires the static keyword in front of the inner class A         FinalCacheclass Cacheclass =NewCacheclass (); -          for(inti = 0; I < 10; i++) {  -             NewThread (NewRunnable () { the   - @Override -                  Public voidrun () { -Object valueobject = Cacheclass.getdata ("1"); +System.out.println (Thread.CurrentThread (). GetName () + ":" +valueobject); -                 } + }). Start (); A         }  at     } -      -  -     Static classCacheclass { -         Privatemap<string, object> cachemap =NewHashmap<string, object>(); -  in         /** - * 1.0 does not consider concurrency issues: two times from the database to * Query data from the database!  + * Query data from the database!  - * Thread-1: null Why is NULL, concurrent, process as shown in Thread-0 the * Thread-2: AAA * * Thread-0: null why is NULL because no value in the first read map returns NULL, and Cachemap.put (Key, "AAA") $ * is not re-assigned to object so it is nullPanax Notoginseng * solution is obtained directly from Cachemap.get (key), do not intermediate object, here I will not change - * Thread-3: AAA the * Thread-4: AAA + * Thread-5: AAA A * Thread-6: AAA the * Thread-7: AAA + * Thread-8: AAA - * Thread-9: AAA $          *  $          * @paramKey -          * @return -          */ the         //Public Object GetData (String key) { -         //Object value = Cachemap.get (key);Wuyi         //if (value = = null) { the         //System.out.println ("Querying data from a database! "); -         //cachemap.put (Key, "AAA"); Wu         // } -         //return value; About         // } $  -         /** - * 2.0 using synchronized Synchronous code block to solve concurrency problem implementation simple directly add synchronized keyword -          *  A * Query data from the database!  + * Thread-4: BBB the * Thread-1: BBB - * Thread-2: BBB $ * Thread-0: BBB the * Thread-3: BBB the * Thread-8: BBB the * Thread-7: BBB the * Thread-6: BBB - * Thread-9: BBB in * Thread-5: BBB the          *  the          * @paramKey About          * @return the          */ the //Public synchronized Object getData (String key) { the //          + //if (Cachemap.get (key) ==null) { - //System.out.println ("Querying data from a database! "); the //cachemap.put (Key, "BBB");Bayi //         } the //return Cachemap.get (key); the //         } -  -         /** the * 3.0 using read/write lock the          *  the query data from the database!  the Thread-1: CCC - Thread-3: CCC the Thread-4: CCC the Thread-2: CCC the Thread-0: CCC94 Thread-5: CCC the Thread-7: CCC the Thread-8: CCC the Thread-9: CCC98 Thread-6: CCC About          */ -         PrivateReadwritelock RWL =NewReentrantreadwritelock ();101          PublicObject getData (String key) {102             //Lock to try Finally, avoid the exception of the code has been locked103 Rwl.readlock (). Lock ();104             Try { the                 if(Cachemap.get (key) = =NULL) { 106                     Try{107                         //the read lock was removed to write the lock and the lock.108 rwl.readlock (). Unlock ();109                         //Lock to try Finally, avoid the exception of the code has been locked the Rwl.writelock (). Lock ();111                         //prevent the first thread from writing the data, followed by the thread to write the                         if(Cachemap.get (key) = =NULL) {113SYSTEM.OUT.PRINTLN ("Querying data from a database! "); theCachemap.put (Key, "CCC"); the                         } the                     }117                     finally{118 rwl.writelock (). Unlock ();119                     }  - Rwl.readlock (). Lock ();121                 }122}finally {123 rwl.readlock (). Unlock ();124             } the 126             returnCachemap.get (key);127         } -     }129}

Implement a simple caching mechanism based on synchronized or Readwritelock