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

public class StudentRequestDto {

    // 1. @NotBlank: Rejects null, "", and whitespace "   "
    @NotBlank(message = "Name is mandatory and cannot be blank")
    @Size(min = 2, max = 50, message = "Name must be 2 to 50 characters")
    private String name;

    // 2. @Email: Enforces valid email pattern
    @NotBlank(message = "Email is mandatory")
    @Email(message = "Email must be a valid email format")
    private String email;

    // 3. @NotNull: Rejects null (Used for numbers/dates where String logic does not apply)
    @NotNull(message = "Age is mandatory")
    @Min(value = 18, message = "Age must be at least 18")
    @Max(value = 60, message = "Age must not exceed 60")
    private Integer age;
}

@RestController
@RequestMapping("/api/students")
public class StudentController {

    private final StudentService service;

    public StudentController(StudentService service) {
        this.service = service;
    }

    // @Valid triggers Spring to evaluate DTO constraints before invoking createStudent
    @PostMapping
    public StudentResponseDto createStudent(@Valid @RequestBody StudentRequestDto requestDto) {
        return service.createStudent(requestDto);
    }
}

Constraint Annotations (@NotBlank, @Min) ──► Rule Book (Defines validation constraints)
@Valid Annotation                        ──► Security Guard (Triggers execution of rules)