Spring MVC 详解

Spring MVC 是 Spring Framework 的一个模块，专门用于开发 Web 应用程序。它实现了 MVC（Model-View-Controller）设计模式，使得开发 Web 应用变得简单高效。本文将详细介绍 Spring MVC 的核心概念、工作原理以及实际应用。

什么是 Spring MVC

Spring MVC 是一个基于 Java 的实现了 MVC 设计模式的请求驱动类型的轻量级 Web 框架，通过把 Model、View、Controller 分离，将 Web 层进行职责解耦，把复杂的 Web 应用分成逻辑清晰的几部分，简化开发，减少出错，方便组内开发人员之间的配合。

MVC 设计模式

Model（模型）：负责存储数据和处理用户请求的业务逻辑
View（视图）：负责展示数据给用户
Controller（控制器）：负责接收用户请求并调用模型和视图完成用户请求

Spring MVC 的核心组件

Spring MVC 框架由以下几个核心组件组成：

1. DispatcherServlet：前端控制器，是整个 Spring MVC 的核心。负责接收请求、分发请求、处理响应。

2. HandlerMapping：处理器映射器，根据请求的 URL 找到对应的处理器（Handler）。

3. HandlerAdapter：处理器适配器，按照特定的规则去执行处理器。

4. Handler：处理器，也称为 Controller，负责具体的业务处理逻辑。

5. ViewResolver：视图解析器，负责将处理结果生成 View 视图。

6. View：视图，负责将模型数据通过页面展示给用户。

Spring MVC 的工作流程

Spring MVC 的工作流程可以用以下步骤来描述：

1. 用户发送请求至前端控制器 DispatcherServlet

2. DispatcherServlet 收到请求后，调用 HandlerMapping 处理器映射器

3. 处理器映射器根据请求 URL 找到具体的处理器，生成处理器对象及处理器拦截器（如果有），一并返回给 DispatcherServlet

4. DispatcherServlet 通过 HandlerAdapter 处理器适配器调用处理器

5. 执行处理器（Controller，也叫后端控制器）

6. Controller 执行完成返回 ModelAndView

7. HandlerAdapter 将 Controller 执行结果 ModelAndView 返回给 DispatcherServlet

8. DispatcherServlet 将 ModelAndView 传给 ViewResolver 视图解析器

9. ViewResolver 解析后返回具体 View

10. DispatcherServlet 对 View 进行渲染视图（即将模型数据填充至视图中）

11. DispatcherServlet 响应用户

![Spring MVC 工作流程图](https://cdn.jsdelivr.net/gh/doocs/advanced-java@main/images/spring-mvc-process.png)

Spring MVC 的配置方式

Spring MVC 提供了两种配置方式：XML 配置和 Java 配置。

XML 配置方式

<!-- web.xml 配置 DispatcherServlet -->

<servlet>

    <servlet-name>dispatcher</servlet-name>

    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>

    <init-param>

        <param-name>contextConfigLocation</param-name>

        <param-value>classpath:spring-mvc.xml</param-value>

    </init-param>

    <load-on-startup>1</load-on-startup>

</servlet>

<servlet-mapping>

    <servlet-name>dispatcher</servlet-name>

    <url-pattern>/</url-pattern>

</servlet-mapping>

<!-- spring-mvc.xml 配置视图解析器等 -->

<beans xmlns="http://www.springframework.org/schema/beans"

       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

       xmlns:context="http://www.springframework.org/schema/context"

       xmlns:mvc="http://www.springframework.org/schema/mvc"

       xsi:schemaLocation="http://www.springframework.org/schema/beans

       http://www.springframework.org/schema/beans/spring-beans.xsd

       http://www.springframework.org/schema/context

       http://www.springframework.org/schema/context/spring-context.xsd

       http://www.springframework.org/schema/mvc

       http://www.springframework.org/schema/mvc/spring-mvc.xsd">

    <!-- 开启注解扫描 -->

    <context:component-scan base-package="com.example.controller"/>

    <!-- 开启 SpringMVC 注解驱动 -->

    <mvc:annotation-driven/>

    <!-- 配置视图解析器 -->

    <bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">

        <property name="prefix" value="/WEB-INF/views/"/>

        <property name="suffix" value=".jsp"/>

    </bean>

</beans>

Java 配置方式

// WebConfig.java

@Configuration
@EnableWebMvc

@ComponentScan("com.example.controller")

public class WebConfig implements WebMvcConfigurer {

    

    @Override

    public void configureViewResolvers(ViewResolverRegistry registry) {

        registry.jsp("/WEB-INF/views/", ".jsp");

    }

}

// WebAppInitializer.java

public class WebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {

    

    @Override

    protected Class<?>[] getRootConfigClasses() {

        return new Class[] { RootConfig.class };

    }

    

    @Override

    protected Class<?>[] getServletConfigClasses() {

        return new Class[] { WebConfig.class };

    }

    

    @Override

    protected String[] getServletMappings() {

        return new String[] { "/" };

    }

}

Spring MVC 的常用注解

Spring MVC 提供了丰富的注解，使得开发更加简便：

1. @Controller：标识一个类为控制器

2. @RequestMapping：映射请求路径

3. @GetMapping：映射 GET 请求

4. @PostMapping：映射 POST 请求

5. @PutMapping：映射 PUT 请求

6. @DeleteMapping：映射 DELETE 请求

7. @PathVariable：获取 URL 中的动态参数

8. @RequestParam：获取请求参数

9. @RequestBody：获取请求体内容

10. @ResponseBody：将返回值直接写入响应体

11. @RestController：@Controller 和 @ResponseBody 的组合注解

12. @ModelAttribute：绑定请求参数到模型对象

示例代码

@Controller

@RequestMapping("/user")

public class UserController {

    

    @GetMapping("/{id}")

    public String getUserById(@PathVariable("id") Long id, Model model) {

        User user = userService.findById(id);

        model.addAttribute("user", user);

        return "userDetail";

    }

    

    @PostMapping

    public String createUser(@RequestParam String username, @RequestParam String password) {

        userService.createUser(username, password);

        return "redirect:/user/list";

    }

    

    @GetMapping("/json/{id}")

    @ResponseBody

    public User getUserJson(@PathVariable("id") Long id) {

        return userService.findById(id);

    }

}

Spring MVC 的数据绑定

Spring MVC 提供了强大的数据绑定功能，可以将请求参数绑定到 Java 对象上。

简单类型绑定

@GetMapping("/hello")

public String hello(@RequestParam String name) {

    return "Hello, " + name;

}

对象绑定

@PostMapping("/register")

public String register(User user) {

    userService.register(user);

    return "redirect:/login";

}

集合类型绑定

@PostMapping("/batch")

public String batchUpdate(@RequestParam("ids") List<Long> ids) {

    userService.batchDelete(ids);

    return "redirect:/user/list";

}

日期类型绑定

@InitBinder

public void initBinder(WebDataBinder binder) {

    SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");

    dateFormat.setLenient(false);

    binder.registerCustomEditor(Date.class, new CustomDateEditor(dateFormat, true));

}

@PostMapping("/event")

public String createEvent(@RequestParam("eventDate") Date eventDate, @RequestParam String name) {

    eventService.createEvent(name, eventDate);

    return "redirect:/event/list";

}

Spring MVC 的视图解析

Spring MVC 支持多种视图技术，包括 JSP、Thymeleaf、FreeMarker 等。

JSP 视图

@GetMapping("/hello")

public String hello(Model model) {

    model.addAttribute("message", "Hello, Spring MVC!");

    return "hello"; // 返回视图名称，会被解析为 /WEB-INF/views/hello.jsp

}

Thymeleaf 视图

// 配置 Thymeleaf 视图解析器

@Bean

public ViewResolver thymeleafViewResolver() {

    ThymeleafViewResolver resolver = new ThymeleafViewResolver();

    resolver.setTemplateEngine(templateEngine());

    resolver.setCharacterEncoding("UTF-8");

    return resolver;

}

@Bean

public SpringTemplateEngine templateEngine() {

    SpringTemplateEngine engine = new SpringTemplateEngine();

    engine.setTemplateResolver(templateResolver());

    return engine;

}

@Bean

public ITemplateResolver templateResolver() {

    SpringResourceTemplateResolver resolver = new SpringResourceTemplateResolver();

    resolver.setPrefix("/WEB-INF/templates/");

    resolver.setSuffix(".html");

    resolver.setTemplateMode(TemplateMode.HTML);

    resolver.setCharacterEncoding("UTF-8");

    return resolver;

}

Spring MVC 的异常处理

Spring MVC 提供了多种异常处理机制：

@ExceptionHandler 注解

@Controller

public class UserController {

    

    @ExceptionHandler(UserNotFoundException.class)

    public ModelAndView handleUserNotFoundException(UserNotFoundException ex) {

        ModelAndView mav = new ModelAndView("error");

        mav.addObject("message", ex.getMessage());

        return mav;

    }

    

    @GetMapping("/user/{id}")

    public String getUser(@PathVariable Long id) {

        User user = userService.findById(id);

        if (user == null) {

            throw new UserNotFoundException("User not found with id: " + id);

        }

        return "userDetail";

    }

}

全局异常处理器

@ControllerAdvice

public class GlobalExceptionHandler {

    

    @ExceptionHandler(Exception.class)

    public ModelAndView handleException(Exception ex) {

        ModelAndView mav = new ModelAndView("error");

        mav.addObject("message", "An error occurred: " + ex.getMessage());

        return mav;

    }

    

    @ExceptionHandler(UserNotFoundException.class)

    public ModelAndView handleUserNotFoundException(UserNotFoundException ex) {

        ModelAndView mav = new ModelAndView("error");

        mav.addObject("message", ex.getMessage());

        return mav;

    }

}

SimpleMappingExceptionResolver

@Bean

public SimpleMappingExceptionResolver exceptionResolver() {

    SimpleMappingExceptionResolver resolver = new SimpleMappingExceptionResolver();

    Properties mappings = new Properties();

    mappings.put("java.lang.Exception", "error");

    mappings.put("com.example.exception.UserNotFoundException", "user-not-found");

    resolver.setExceptionMappings(mappings);

    resolver.setDefaultErrorView("default-error");

    return resolver;

}

Spring MVC 的拦截器

Spring MVC 的拦截器（Interceptor）类似于 Servlet 的过滤器（Filter），用于在请求处理的前后执行一些操作。

定义拦截器

public class AuthInterceptor implements HandlerInterceptor {

    

    @Override

    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {

        // 在请求处理之前执行，返回 true 表示继续处理，返回 false 表示中断处理

        String token = request.getHeader("Authorization");

        if (token == null || !tokenService.isValid(token)) {

            response.sendRedirect("/login");

            return false;

        }

        return true;

    }

    

    @Override

    public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {

        // 在请求处理之后，视图渲染之前执行

    }

    

    @Override

    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {

        // 在整个请求处理完毕后执行

    }

}

注册拦截器

@Configuration

public class WebConfig implements WebMvcConfigurer {

    

    @Autowired

    private AuthInterceptor authInterceptor;

    

    @Override

    public void addInterceptors(InterceptorRegistry registry) {

        registry.addInterceptor(authInterceptor)

                .addPathPatterns("/admin/**")

                .excludePathPatterns("/admin/login");

    }

}

Spring MVC 与 RESTful API

Spring MVC 对 RESTful API 的开发提供了良好的支持。

RESTful 控制器

@RestController

@RequestMapping("/api/users")

public class UserRestController {

    

    @Autowired

    private UserService userService;

    

    @GetMapping

    public List<User> getAllUsers() {

        return userService.findAll();

    }

    

    @GetMapping("/{id}")

    public ResponseEntity<User> getUserById(@PathVariable Long id) {

        User user = userService.findById(id);

        if (user == null) {

            return ResponseEntity.notFound().build();

        }

        return ResponseEntity.ok(user);

    }

    

    @PostMapping

    public ResponseEntity<User> createUser(@RequestBody User user) {

        User savedUser = userService.save(user);

        return ResponseEntity.status(HttpStatus.CREATED).body(savedUser);

    }

    

    @PutMapping("/{id}")

    public ResponseEntity<User> updateUser(@PathVariable Long id, @RequestBody User user) {

        if (!userService.exists(id)) {

            return ResponseEntity.notFound().build();

        }

        user.setId(id);

        User updatedUser = userService.update(user);

        return ResponseEntity.ok(updatedUser);

    }

    

    @DeleteMapping("/{id}")

    public ResponseEntity<Void> deleteUser(@PathVariable Long id) {

        if (!userService.exists(id)) {

            return ResponseEntity.notFound().build();

        }

        userService.delete(id);

        return ResponseEntity.noContent().build();

    }

}

Spring MVC 的文件上传

Spring MVC 提供了简单的文件上传功能。

配置 MultipartResolver

@Bean

public CommonsMultipartResolver multipartResolver() {

    CommonsMultipartResolver resolver = new CommonsMultipartResolver();

    resolver.setMaxUploadSize(5242880); // 5MB

    return resolver;

}

处理文件上传

@Controller

public class FileUploadController {

    

    @PostMapping("/upload")

    public String handleFileUpload(@RequestParam("file") MultipartFile file, RedirectAttributes redirectAttributes) {

        if (file.isEmpty()) {

            redirectAttributes.addFlashAttribute("message", "请选择一个文件上传");

            return "redirect:/uploadForm";

        }

        

        try {

            byte[] bytes = file.getBytes();

            Path path = Paths.get("uploads/" + file.getOriginalFilename());

            Files.write(path, bytes);

            

            redirectAttributes.addFlashAttribute("message", "文件上传成功: " + file.getOriginalFilename());

        } catch (IOException e) {

            e.printStackTrace();

            redirectAttributes.addFlashAttribute("message", "文件上传失败: " + e.getMessage());

        }

        

        return "redirect:/uploadForm";

    }

    

    @GetMapping("/uploadForm")

    public String showUploadForm() {

        return "uploadForm";

    }

}

Spring MVC 的实际应用案例

下面是一个简单的用户管理系统的示例：

实体类

public class User {

    private Long id;

    private String username;

    private String email;

    private String password;

    

    // 构造函数、getter 和 setter 方法

}

服务层

@Service

public class UserService {

    

    private Map<Long, User> users = new ConcurrentHashMap<>();

    private AtomicLong idGenerator = new AtomicLong();

    

    public List<User> findAll() {

        return new ArrayList<>(users.values());

    }

    

    public User findById(Long id) {

        return users.get(id);

    }

    

    public User save(User user) {

        Long id = idGenerator.incrementAndGet();

        user.setId(id);

        users.put(id, user);

        return user;

    }

    

    public User update(User user) {

        users.put(user.getId(), user);

        return user;

    }

    

    public void delete(Long id) {

        users.remove(id);

    }

    

    public boolean exists(Long id) {

        return users.containsKey(id);

    }

}

控制器

@Controller

@RequestMapping("/users")

public class UserController {

    

    @Autowired

    private UserService userService;

    

    @GetMapping

    public String listUsers(Model model) {

        List<User> users = userService.findAll();

        model.addAttribute("users", users);

        return "userList";

    }

    

    @GetMapping("/{id}")

    public String viewUser(@PathVariable Long id, Model model) {

        User user = userService.findById(id);

        if (user == null) {

            throw new RuntimeException("User not found with id: " + id);

        }

        model.addAttribute("user", user);

        return "userDetail";

    }

    

    @GetMapping("/new")

    public String newUserForm(Model model) {

        model.addAttribute("user", new User());

        return "userForm";

    }

    

    @PostMapping

    public String createUser(@ModelAttribute User user, BindingResult result) {

        if (result.hasErrors()) {

            return "userForm";

        }

        userService.save(user);

        return "redirect:/users";

    }

    

    @GetMapping("/{id}/edit")

    public String editUserForm(@PathVariable Long id, Model model) {

        User user = userService.findById(id);

        if (user == null) {

            throw new RuntimeException("User not found with id: " + id);

        }

        model.addAttribute("user", user);

        return "userForm";

    }

    

    @PostMapping("/{id}")

    public String updateUser(@PathVariable Long id, @ModelAttribute User user, BindingResult result) {

        if (result.hasErrors()) {

            return "userForm";

        }

        user.setId(id);

        userService.update(user);

        return "redirect:/users";

    }

    

    @GetMapping("/{id}/delete")

    public String deleteUser(@PathVariable Long id) {

        userService.delete(id);

        return "redirect:/users";

    }

}

视图模板（使用 JSP）

<!-- userList.jsp -->

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>

<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>

<!DOCTYPE html>

<html>

<head>

    <meta charset="UTF-8">

    <title>用户列表</title>

</head>

<body>

    <h1>用户列表</h1>

    <a href="<c:url value='/users/new'/>">添加用户</a>

    <table border="1">

        <tr>

            <th>ID</th>

            <th>用户名</th>

            <th>邮箱</th>

            <th>操作</th>

        </tr>

        <c:forEach items="${users}" var="user">

            <tr>

                <td>${user.id}</td>

                <td>${user.username}</td>

                <td>${user.email}</td>

                <td>

                    <a href="<c:url value='/users/${user.id}'/>">查看</a>

                    <a href="<c:url value='/users/${user.id}/edit'/>">编辑</a>

                    <a href="<c:url value='/users/${user.id}/delete'/>">删除</a>

                </td>

            </tr>

        </c:forEach>

    </table>

</body>

</html>

参考资料

1. Spring 官方文档：https://docs.spring.io/spring-framework/docs/current/reference/html/web.html

2. Spring MVC 教程：https://www.baeldung.com/spring-mvc-tutorial

3. Spring Boot 与 Spring MVC：https://spring.io/guides/gs/serving-web-content/