反例 1: SpringContext 作为静态变量
UserController 和 SpringContextUtils 类没有依赖关系， SpringContextUtils.getApplication() 可能返回空。并且 Spring 非依赖关系的 Bean 之间的初始化顺序是不确定的，虽然可能当前初始化顺序恰好符合期望，但后续可能发生变化。

@Component
public class SpringContextUtils {

    @Getter
    private static ApplicationContext applicationContext;

    public SpringContextUtils(ApplicationContext context) {
        applicationContext = context;
    }
}

@Component
public class UserController {

    public void handle(){
        MyService bean = SpringContextUtils.getApplicationContext().getBean(MyService.class);
    }
}

反例 2: Switch 在 Spring Bean 中注册, 但通过静态方式读取

@Component
public class SwitchConfig {

    @PostConstruct
    public void init() {
        SwitchManager.register("appName", MySwitch.class);
    }

    public static class MySwitch {
        @AppSwitch(des = "config", level = Switch.Level.p1)
        public static String config;
    }
}

@Component
public class UserController{

    public String getConfig(){
        // UserController 和 SwitchConfig 类没有依赖关系, MySwitch.config 可能还没有初始化
        return MySwitch.config;
    }
}

通过 SpringBeanFactory 保证初始化顺序：

public class PreInitializer implements BeanFactoryPostProcessor, PriorityOrdered {
  @Override
  public int getOrder() {
    return Ordered.HIGHEST_PRECEDENCE;
  }
  @Override
  public void postProcessBeanFactory(
    ConfigurableListableBeanFactory beanFactory) throws BeansException {
       try {
        SwitchManager.init(应用名, 开关类.class);
      } catch (SwitchCenterException e) {
        // 此处抛错最好阻断程序启动，避免开关读不到持久值引发问题
    } catch (SwitchCenterError e) {
        System.exit(1);
    }
    }
}
@Component
public class SpringContextUtilPostProcessor implements BeanFactoryPostProcessor, PriorityOrdered, ApplicationContextAware {

    private ApplicationContext applicationContext;

    @Override
    public int getOrder() {
        return Ordered.HIGHEST_PRECEDENCE;
    }

    @Override
    public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory)
            throws BeansException {
        SpringContextUtils.setApplicationContext(applicationContext);
    }

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.applicationContext = applicationContext;
    }
}