Understanding the Four Types of Return Values in Spring MVC

Spring MVC is a fundamental Java web framework, and knowing the possible return types of controller methods is essential for both traditional and modern front‑end/back‑end separated architectures.

1. ModelAndView

Before front‑end separation, ModelAndView was the most common return type. It allows you to set a view name and bind data to the model.

@RequestMapping("/book")
public ModelAndView getAllBook() {
    ModelAndView mv = new ModelAndView();
    List
books = new ArrayList<>();
    Book b1 = new Book();
    b1.setId(1);
    b1.setName("三国演义");
    b1.setAuthor("罗贯中");
    books.add(b1);
    Book b2 = new Book();
    b2.setId(2);
    b2.setName("红楼梦");
    b2.setAuthor("曹雪芹");
    books.add(b2);
    mv.addObject("bs", books);
    mv.setViewName("book"); // specify view name
    return mv;
}

The two most common operations are setting the data model and specifying the view name.

2. Void

When a method returns void , it may truly have no data to return, or it may use other mechanisms to send a response.

2.1 No value

@RequestMapping("/test2")
@ResponseBody
public void test2() {
    // your code
}

2.2 Redirect

@RequestMapping("/test1")
@ResponseBody
public void test1(HttpServletResponse resp) {
    resp.setStatus(302);
    resp.addHeader("Location", "/aa/index");
}
// or using sendRedirect
public void test1(HttpServletResponse resp) {
    resp.sendRedirect("/aa/index");
}

2.3 Server‑side forward

@GetMapping("/test5")
public void test5(HttpServletRequest req, HttpServletResponse resp) {
    req.getRequestDispatcher("/WEB-INF/jsp/index.jsp").forward(req, resp);
}

2.4 Write string directly

@RequestMapping("/test2")
@ResponseBody
public void test2(HttpServletResponse resp) throws IOException {
    resp.setContentType("application/json;charset=utf-8");
    PrintWriter out = resp.getWriter();
    List
books = new ArrayList<>();
    // populate books ...
    String s = new Gson().toJson(books);
    out.write(s);
    out.flush();
    out.close();
}

Even though the method returns void , data can still be sent to the client via the response object.

3. String

Returning a String can represent several different meanings.

3.1 Logical view name

@RequestMapping("/hello")
public String aaa(Model model) {
    model.addAttribute("username", "张三");
    return "hello"; // logical view name
}

3.2 Redirect

@RequestMapping("/test4")
public String test4() {
    return "redirect:/aa/index";
}

3.3 Forward

@RequestMapping("/test3")
public String test3() {
    return "forward:/WEB-INF/jsp/order.jsp";
}

3.4 Raw string response

@RestController
public class HelloController {
    @GetMapping("/hello")
    public String hello() {
        return "hello provider!"; // returned directly as response body
    }
}
// or using @Controller with @ResponseBody on the method

4. JSON

Returning JSON is the most common case in a front‑end/back‑end separated project. Objects, lists, or maps are automatically converted to JSON by HttpMessageConverter (Jackson, Gson, etc.).

@GetMapping("/user")
@ResponseBody
public User getUser() {
    User user = new User();
    List
favorites = new ArrayList<>();
    favorites.add("足球");
    favorites.add("篮球");
    user.setFavorites(favorites);
    user.setUsername("zhagnsan");
    user.setPassword("123");
    return user;
}

@GetMapping("/users")
@ResponseBody
public List
getAllUser() {
    List
users = new ArrayList<>();
    for (int i = 0; i < 10; i++) {
        User e = new User();
        e.setUsername("zhangsan:" + i);
        e.setPassword("pwd:" + i);
        users.add(e);
    }
    return users;
}

Summary

The four return types— ModelAndView , void , String , and JSON—cover most scenarios you will encounter when developing Spring MVC applications. Choose the appropriate type based on whether you need to render a view, perform a redirect/forward, return raw data, or produce JSON for a front‑end client.