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Spring MVC Controllers, REST, and Thymeleaf

Spring MVC request handling with @Controller and @RestController, Thymeleaf template engine, form processing, internationalization, and the PRG (Post-Redirect-Get) pattern.

Key Facts

  • @Controller returns view names (templates); @RestController returns data (JSON/XML)
  • @GetMapping, @PostMapping, @PutMapping, @DeleteMapping map HTTP methods to handler methods
  • Model object carries data from controller to view
  • @ModelAttribute binds form fields to an object
  • PRG pattern: POST handler redirects to GET to prevent duplicate form submission
  • Thymeleaf templates go in src/main/resources/templates/
  • BindingResult must immediately follow @Valid parameter in method signature

Patterns

MVC Controller

@Controller
public class UserController {
    private final UserService userService;

    @GetMapping("/register")
    public String showRegisterForm(Model model) {
        model.addAttribute("user", new UserDto());
        return "register";  // -> templates/register.html
    }

    @PostMapping("/register")
    public String register(@ModelAttribute @Valid UserDto dto,
                           BindingResult result, Model model) {
        if (result.hasErrors()) {
            return "register";  // re-render with errors
        }
        userService.saveUser(dto);
        return "redirect:/login";  // PRG pattern
    }
}

Admin Panel with Status Filtering

@Controller @RequestMapping("/admin")
public class AdminController {
    @GetMapping
    public String showOrders(@RequestParam(required = false) String status,
                              Model model) {
        List<Order> orders = (status != null)
            ? orderService.getByStatus(OrderStatus.valueOf(status))
            : orderService.getByStatus(OrderStatus.NEW);
        model.addAttribute("orders", orders);
        model.addAttribute("statuses", OrderStatus.values());
        return "admin";
    }

    @PostMapping("/update-status")
    public String updateStatus(@RequestParam Long orderId,
                                @RequestParam String newStatus) {
        orderService.updateStatus(orderId, OrderStatus.valueOf(newStatus));
        return "redirect:/admin";
    }
}

Thymeleaf Expressions

Expression Syntax Purpose
Variable ${variable} Model attributes
Selection *{field} Fields of th:object
Message #{key} i18n message lookup
Link @{/path} URL generation

Thymeleaf Form

<form th:action="@{/register}" th:object="${user}" method="post">
    <input type="text" th:field="*{name}" />
    <span th:if="${#fields.hasErrors('name')}"
          th:errors="*{name}" class="error"></span>
    <button type="submit" th:text="#{register.submit}">Submit</button>
</form>

Iteration and Conditionals

<div th:each="item : ${menuItems}">
    <span th:text="${item.name}"></span>
    <span th:text="${item.price}"></span>
</div>
<div th:if="${user != null}">Welcome, <span th:text="${user.name}"></span></div>
<div th:unless="${user != null}">Please log in</div>

Internationalization (i18n)

# messages_en.properties
register.title=Registration
register.submit=Register

# messages_ru.properties
register.title=Registratsiya
register.submit=Zaregistrirovat'sya

@Configuration
public class LocaleConfig implements WebMvcConfigurer {
    @Bean
    public LocaleResolver localeResolver() {
        SessionLocaleResolver r = new SessionLocaleResolver();
        r.setDefaultLocale(new Locale("en"));
        return r;
    }
    @Bean
    public LocaleChangeInterceptor localeInterceptor() {
        LocaleChangeInterceptor i = new LocaleChangeInterceptor();
        i.setParamName("lang");  // switch with ?lang=ru
        return i;
    }
    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        registry.addInterceptor(localeInterceptor());
    }
}

Session-Scoped Data (Shopping Cart)

@Component
@Scope(value = "session", proxyMode = ScopedProxyMode.TARGET_CLASS)
public class SessionInfo {
    private User currentUser;
    private List<MenuItem> selectedItems = new ArrayList<>();
    private Integer totalPrice = 0;
}

Gotchas

  • BindingResult must be the parameter immediately after @Valid - any other order throws exception
  • redirect:/path sends HTTP 302; returning a view name renders the template directly
  • th:field automatically sets name, id, and value attributes - don't set them manually
  • Session-scoped bean in singleton controller requires proxyMode = TARGET_CLASS
  • @RequestParam(required = false) with primitive types causes NPE - use wrapper types

See Also

  • [[spring-validation]] - DTO validation annotations
  • [[spring-ioc-beans]] - Bean scopes (session, request)
  • [[spring-security]] - Securing controller endpoints