Singletons工廠使用泛型和反射

Question

OrderDBManager，UserDBManager，ProductDBManager擴展DBManager。 我正在嘗試創建一個單例DBManager Factory 以獲取不同DBManager類型類的實例。

但我有一個問題，使用泛型返回確切DBManager類型類用戶想要的實例。現在它只適用於DBManager本身。 我將不勝感激您的幫助！

public class DBManagerFactory { 

    private static final Map<String, DBManager> instances = new HashMap<>(); 

    private static final String DB_MANAGER_PACKAGE_NAME = "ua.Test.db."; 

    public static DBManager getInstance(Class<? extends DBManager> dbManagerClass) { 
      String dbManagerClassName = dbManagerClass.getSimpleName(); 
      DBManager dbManager = instances.get(dbManagerClassName); 

      if(dbManager == null){ 
       Class clazz = Class.forName(DB_MANAGER_PACKAGE_NAME + dbManagerClassName); 
       dbManager = (DBManager)clazz.newInstance(); 
       instances.put(dbManagerClassName, dbManager);  
      } 
      return dbManager;            
    }}                 

Answer 1

如果你需要得到明確的返回類型，那麼你可以使用generic factory method，如下：

class Base {} 

class A extends Base {} 

class B extends Base {} 

class InitializationFailedException extends RuntimeException { 
    public InitializationFailedException(Throwable cause) { 
     super(cause); 
    } 
} 

class Factory { 

    private static Logger log = LoggerFactory.getLogger(Factory.class); 

    private static final ConcurrentHashMap<Class<? extends Base>, Base> instances = new ConcurrentHashMap<>(); 

    public static <T extends Base> T getInstance(Class<T> tClass) { 
     if (tClass == null) { 
      throw new NullPointerException(); 
     } 
     return (T) instances.computeIfAbsent(tClass, (tClass1) -> { 
      try { 
       return tClass1.newInstance(); 
      } catch (InstantiationException | IllegalAccessException e) { 
       log.error("Unable to create singleton value for " + tClass1.getCanonicalName(), e); 
       throw new InitializationFailedException(e); 
      } 
     }); 
    } 

} 

用這種方法你可以得到你的子類型引用創建：

A aInstance = Factory.getInstance(A.class); 
B bInstance = Factory.getInstance(B.class); 

爲您的問題方法簽名可能看起來像：

public static <T extends DBManager> T getInstance(Class<T> dbManagerClass); 

種

情況下，地圖應該看起來像：

private static final Map<Class<? extends DBManager>, DBManager> instances = new HashMap<>(); 

Answer 2

但我使用泛型返回的確切 DBManager型類用戶想要的實例有問題。

要返回方法中的特定類，應該指定範圍方法參數化類型，該類型允許類的客戶端指定方法應該返回的類型。

你可以這樣聲明方法：

public static <T extends DBManager> T getInstance(Class<T> dbManagerClass) { 

爲了避免不安全的類型轉換，你可以通過使用Class#cast(Object)方法爲您執行從Map<String, DBManager>檢索到的DBManager實例轉換爲T extends DBManager類型鑄造DBManager實例之前，類型檢查：

T dbManager = dbManagerClass.cast(instances.get(dbManagerClassName)); 

最後，順便檢索類：

Class clazz = Class.forName(DB_MANAGER_PACKAGE_NAME + dbManagerClassName); 

不是必需的，因爲您已經將類作爲方法的參數。
只是直接使用它。

這裏是實現這種解決方案的示例代碼：

public class DBManagerFactory { 

    private static final Map<String, DBManager> instances = new HashMap<>(); 

    public static <T extends DBManager> T getInstance(Class<T> dbManagerClass) { 

     final String dbManagerClassName = dbManagerClass.getSimpleName(); 

     T dbManager = dbManagerClass.cast(instances.get(dbManagerClassName)); 

     if (dbManager == null) { 
     try { 
      dbManager = dbManagerClass.newInstance(); 
     } catch (InstantiationException | IllegalAccessException e) { 
      e.printStackTrace(); // or better log it 
      throw new RuntimeException("faillure during reflection instantiation for class " + dbManagerClass.getName(), e); 
     } 
     instances.put(dbManagerClassName, dbManager); 
     } 

     return dbManager; 
    } 

} 

有了這些改進，客戶端類可以這樣調用方法：

DBManager instance = DBManagerFactory.getInstance(DBManager.class); 
DBManagerChild child = DBManagerFactory.getInstance(DBManagerChild.class);