Spring Boot @autowired 不起作用，不同包中的类

使用Spring Boot的开发者可能会遇到一种情况，即在不同包中的类中，使用@Autowired注解无法自动装配所需的依赖。这个问题可能会让开发者感到困惑，因为按照Spring Boot的规则，只需要在需要自动装配的字段上加上@Autowired注解，就可以完成依赖的自动注入。

java
package com.example.service;
import com.example.repository.UserRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
@Service
public class UserService {
    @Autowired
    private UserRepository userRepository;
    // 其他业务逻辑方法...
}

java
package com.example.controller;
import com.example.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class UserController {
    @Autowired
    private UserService userService;
    // 其他请求处理方法...
}

java
package com.example;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.ComponentScan;
@SpringBootApplication
@ComponentScan(basePackages = {"com.example.service", "com.example.controller"})
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

java
package com.example.config;
import com.example.controller.UserController;
import com.example.service.UserService;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
@Configuration
public class AppConfig {
    @Bean
    public UserService userService() {
        return new UserService();
    }
    @Bean
    public UserController userController() {
        return new UserController();
    }
}