@Import({User.class, DBHelper.class})
    @Configuration(proxyBeanMethods = false) //告诉SpringBoot这是一个配置类 == 配置文件
    public class MyConfig {
    }

    //1、返回我们IOC容器
    ConfigurableApplicationContext run = SpringApplication.run(MainApplication.class, args);
    //...
    //5、获取组件
    String[] beanNamesForType = run.getBeanNamesForType(User.class);
    for (String s : beanNamesForType) {
        //com.xiaozhi.bean.User
        //user01
        System.out.println(s);
    }
    DBHelper bean1 = run.getBean(DBHelper.class);
    //ch.qos.logback.core.db.DBHelper@188cbcde
    System.out.println(bean1);

@Configuration(proxyBeanMethods = false) //告诉SpringBoot这是一个配置类 == 配置文件
//有tom名字的Bean时，MyConfig类的Bean才能生效
@ConditionalOnBean(name="tom")
public class MyConfig {

    //给容器中添加组件。以方法名作为组件的id。返回类型就是组件类型。返回的值，就是组件在容器中的实例
    @Bean
    //当容器中有tom组件的时候,才给注册user01
    //@ConditionalOnBean(name="tom")
    public User user01(){
        User zhangsan = new User("zhangsan", 18);
        //user组件依赖了Pet组件
        zhangsan.setPet(tomcatPet());
        return zhangsan;
    }
    //@Bean("tom")
    @Bean("tom2")
    public Pet tomcatPet(){
        return new Pet("tomcat");
    }
}