IOC实现原理：反射

通过反射实现注入

1. 创建UserService, UserController类

   public class UserController {
   
       private UserService userService;
   
       public UserService getUserService() {
           return userService;
       }
   }
   

   public class UserService {
   }
   

2. 通过反射将UserService对象注入到UserController类的对象中

   public class IOC_01 {
   
       public static void main(String[] args) throws NoSuchFieldException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
           Class<? extends UserController> clazz = UserController.class;
           Field userServiceFiled = clazz.getDeclaredField("userService");
           userServiceFiled.setAccessible(true);
           UserController userController = (UserController) clazz.getConstructor().newInstance();
           UserService userService = new UserService();
           userServiceFiled.set(userController, userService);
           System.out.println(userController.getUserService() == userService);
       }
   
   }
   

加上注解

1. 创建注解类AutoWried

   @Retention(RetentionPolicy.RUNTIME)
   @Target(ElementType.FIELD)
   public @interface AutoWried {
       
       
   }
   

2. 在UserController类加上注解

   public class UserController {
   
       @AutoWried
       private UserService userService;
   
       public UserService getUserService() {
           return userService;
       }
   }
   

3. 先解析出被注解的属性，然后通过反射将UserService对象注入到UserController类的对象中

   public class IOC_02 {
   
       public static void main(String[] args) throws NoSuchFieldException, NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
           Class<? extends UserController> clazz = UserController.class;
           UserController userController = clazz.getConstructor().newInstance();
           Stream.of(clazz.getDeclaredFields()).forEach(field -> {
               AutoWried annotation = field.getAnnotation(AutoWried.class);
               if (annotation != null) {
                   field.setAccessible(true);
                   Class<?> declaringClass = field.getType();
                   try {
                       Object o = declaringClass.getConstructor().newInstance();
                       field.set(userController, o);
                       System.out.println(o);
                   } catch (InstantiationException | IllegalAccessException | InvocationTargetException | NoSuchMethodException e) {
                       e.printStackTrace();
                   }
               }
           });
           System.out.println(userController.getUserService());
       }
   
   }