Understanding Spring MVC: From Servlets to DispatcherServlet and Request Handling

Spring MVC simplifies server‑side development by abstracting low‑level servlet handling and providing a two‑level controller model that separates request processing from business logic.

1. The Former King – Servlet

Early Java web development relied on raw Servlets, which required manual handling of request and response objects, URL mapping in web.xml , and explicit forwarding or redirection.

2. Wanting to Go Further

Using plain Servlets leads to problems such as a proliferation of servlet classes, repetitive parameter extraction, hard‑coded URL mappings, and tight coupling with view technologies.

3. Spring MVC – Two‑Level Controller

Spring introduces a front‑controller called DispatcherServlet that delegates requests to secondary controllers (Handlers). The secondary controller contains only business logic, while the front‑controller manages common concerns.

4. DispatcherServlet – Front‑Controller

The DispatcherServlet orchestrates request processing: it obtains request parameters, performs validation, selects the appropriate handler, and finally renders the response.

5. HandlerMapper – Request Mapping Expert

Spring uses HandlerMapping implementations to map URLs to Handlers. Common strategies include SimpleUrlHandlerMapping , ControllerClassNameHandlerMapping , BeanNameUrlHandlerMapping , and annotation‑based @RequestMapping via DefaultAnnotationHandlerMapping .

6. HandlerInterceptor – Intercepting Requests

Before a Handler executes, one or more HandlerInterceptor instances can inspect or reject the request (e.g., for authentication), providing finer‑grained control than servlet Filter .

7. Secondary Controller – Handler

The Handler (often a Spring @Controller ) contains the business logic and returns a logical view name.

8. HandlerExecutionChain – Bridging Handler and Interceptors

Spring wraps a Handler together with its interceptors in a HandlerExecutionChain , forming a chain that processes the request sequentially.

9. ModelAndView – Decoupling Data and View

Handlers populate a ModelAndView with data and a logical view name, leaving view rendering to separate components.

10. ViewResolver – Finding the View

Based on the logical view name, a ViewResolver selects an appropriate View implementation (e.g., InternalResourceViewResolver , FreeMarkerViewResolver , VelocityViewResolver , etc.).

11. View – Rendering Data

The chosen View renders the model data into the final response (HTML, JSON, etc.) and returns it to the DispatcherServlet , which sends it back to the client.

Below are typical configuration snippets used in a Spring MVC application.

<web-app xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd" version="2.4">
    <servlet>
        <servlet-name>ShoppingServlet</servlet-name>
        <servlet-class>com.myTest.ShoppingServlet</servlet-class>
    </servlet>
    
    <servlet-mapping>
        <servlet-name>ShoppingServlet</servlet-name>
        <url-pattern>/shop/ShoppingServlet</url-pattern>
    </servlet-mapping>
</web-app>

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.0"
    xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
    http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
    <servlet>
        <servlet-name>Spring MVC</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <!-- Initialize on startup -->
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>classpath:Spring-servlet.xml</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>Spring MVC</servlet-name>
        <url-pattern>/*</url-pattern>
    </servlet-mapping>
</web-app>

In summary, Spring MVC’s core lies in the DispatcherServlet , which coordinates request handling through HandlerMappings, interceptors, Handlers, ModelAndView, ViewResolvers, and Views, providing a clean separation of concerns and greatly improving development efficiency.