Interceptor是SpringMVC中的拦截器，类似于Servlet中的Filter，主要针对Controller层进行拦截请求。

Interceptor的注册过程

在DispatcherServlet#doDispatch方法中的会调用getHandler方法（可接Controller查找部分），在这个方法里面会调用getHandlerExecutionChain方法，之后会遍历AbstractHandlerMapping.adaptedInterceptors，添加到chain中：

拦截器的调用主要在DispatcherServlet#doDispatch方法中，见applyPreHandle方法：

在这个方法中遍历HandlerExecutionChain.interceptorList中的拦截器对请求进行处理：

动态添加Interceptor

只要把自定义的Interceptor加到AbstractHandlerMapping.adaptedInterceptors中就完成了Interceptor的动态添加。

通过Context可以获取到RequestMappingHandlerMapping的Bean，而它又继承AbstractHandlerMapping类：

所以通过反射可以很方便地获取到adaptedInterceptors这个成员变量，然后直接加入即可：

   @RequestMapping("/add")
    public String addInterceptor() throws Exception {
        ServletRequestAttributes attributes = (ServletRequestAttributes) RequestContextHolder.getRequestAttributes();
        assert attributes != null;
        ServletContext servletContext = attributes.getRequest().getServletContext();
        WebApplicationContext context = WebApplicationContextUtils.getWebApplicationContext(servletContext);
        // 从IoC容器中获取bean
        assert context != null;
        RequestMappingHandlerMapping handlerMapping = context.getBean(RequestMappingHandlerMapping.class);

        Class<?> clazz = Class.forName("org.springframework.web.servlet.handler.AbstractHandlerMapping");
        Field field = clazz.getDeclaredField("adaptedInterceptors");
        field.setAccessible(true);
        List interceptors = (List) field.get(handlerMapping);
        interceptors.add(new MyInterceptor());

        return "Add success";
    }
}

public class MyInterceptor implements HandlerInterceptor {

    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        PrintWriter writer = response.getWriter();
        writer.write("This is MyInterceptor\n");
        writer.flush();
        writer.close();

        return true;
    }
    
}