Form Handling & Validation

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-validation</artifactId>
</dependency>

public class User {
    
    @NotBlank(message = "Name is required")
    private String name;
    
    @Email(message = "Invalid email format")
    @NotBlank(message = "Email is required")
    private String email;
    
    @Min(value = 18, message = "Age must be at least 18")
    @Max(value = 100, message = "Age must be less than 100")
    private int age;
    
    // getters and setters
}

@Controller
public class UserController {
    
    // Show registration form
    @GetMapping("/register")
    public String showForm(Model model) {
        model.addAttribute("user", new User());
        return "register";
    }
    
    // Process registration form with validation
    @PostMapping("/register")
    public String processForm(
            @Valid @ModelAttribute("user") User user,
            BindingResult bindingResult) {
        
        if (bindingResult.hasErrors()) {
            // If validation fails, go back to form
            return "register";
        }
        
        // Save user to database (not shown)
        return "success";
    }
}

<form action="/register" method="post">
    <label>Name:</label>
    <input type="text" name="name" value="${user.name}"/>
    <errors path="name"/><br/>
    
    <label>Email:</label>
    <input type="text" name="email" value="${user.email}"/>
    <errors path="email"/><br/>
    
    <label>Age:</label>
    <input type="number" name="age" value="${user.age}"/>
    <errors path="age"/><br/>
    
    <button type="submit">Register</button>
</form>

• @ModelAttribute binds form data to Java object.
• Use validation annotations like @NotBlank, @Email, @Min.
• @Valid enables validation on the model object.
• BindingResult captures validation errors.
• Always check hasErrors() before processing data.