Understanding Spring MVC Request Flow: Controllers, Handlers, and Adapters

Environment: Spring Boot 2.4.11

Overview

When a request arrives, Spring processes it through a series of core components.

Spring MVC processing flow

DispatcherServlet : entry point for all requests.

HandlerMapping : maps request URLs to handler methods.

HandlerAdapter : invokes the matched handler.

HandlerMethodArgumentResolver : resolves method arguments.

HandlerMethodReturnValueHandler : processes the return value.

ViewResolver : resolves a view when a ModelAndView is returned.

Typical @RestController definition

@RestController
@RequestMapping("/users")
public class UsersController {
    @GetMapping("/save")
    public Object save(Users users) {
        return users;
    }
}

This is the most common way to define a controller in Spring MVC.

Using HttpRequestHandler

@Component("/others/chrh")
public class ControllerHttpRequestHandler implements HttpRequestHandler {
    @Override
    public void handleRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=utf8");
        PrintWriter out = response.getWriter();
        out.print("<h1>Hello, HttpRequestHandler</h1>");
        out.close();
    }
}

Implementing HttpRequestHandler requires a bean name that starts with “/”. BeanNameUrlHandlerMapping registers this bean with the URL as the key.

Implementing the Controller interface

@Component("/others/custom")
public class CustomController implements Controller {
    @Override
    public ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response) throws Exception {
        response.setContentType("text/html;charset=utf8");
        PrintWriter out = response.getWriter();
        out.print("<h1>Controller interface</h1>");
        out.close();
        return null;
    }
}

The bean name also starts with “/”, so BeanNameUrlHandlerMapping associates it.

Extending HttpServlet

@Component("/others/servlet")
public class ControllerServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    @Override
    protected void service(HttpServletRequest req, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=utf8");
        PrintWriter out = response.getWriter();
        out.print("<h1>Hello HttpServlet</h1>");
        out.close();
    }
}

To make this work you must register a SimpleServletHandlerAdapter:

@Configuration
public class WebConfig implements WebMvcConfigurer {
    @Bean
    public SimpleServletHandlerAdapter simpleServletHandlerAdapter() {
        return new SimpleServletHandlerAdapter();
    }
}

You can also annotate the servlet with @WebServlet("/others/servlet") and enable scanning with @ServletComponentScan on a configuration class.

These are the main ways Spring MVC supports controller definitions. The next article will analyze the source code to show how a request selects a HandlerMapping and which HandlerAdapter processes it.