Singletons工厂使用泛型和反射

Question

OrderDBManager，UserDBManager，ProductDBManager扩展DBManager。 我正在尝试创建一个单例DBManager Factory 以获取不同DBManager类型类的实例。

但我有一个问题，使用泛型返回确切DBManager类型类用户想要的实例。现在它只适用于DBManager本身。 我将不胜感激您的帮助！

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

如果你需要得到明确的返回类型，那么你可以使用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); 
      } 
     }); 
    } 

} 

用这种方法你可以得到你的子类型引用创建：

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

为您的问题方法签名可能看起来像：

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

种

情况下，地图应该看起来像：

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

Answer 2

但我使用泛型返回的确切 DBManager型类用户想要的实例有问题。

要返回方法中的特定类，应该指定范围方法参数化类型，该类型允许类的客户端指定方法应该返回的类型。

你可以这样声明方法：

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

为了避免不安全的类型转换，你可以通过使用Class#cast(Object)方法为您执行从Map<String, DBManager>检索到的DBManager实例转换为T extends DBManager类型铸造DBManager实例之前，类型检查：

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

最后，顺便检索类：

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

不是必需的，因为您已经将类作为方法的参数。
只是直接使用它。

这里是实现这种解决方案的示例代码：

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; 
    } 

} 

有了这些改进，客户端类可以这样调用方法：

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