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Explanation

Spring Boot integrates Jakarta Bean Validation to validate incoming request data. By using annotations such as @NotNull, @NotBlank, @Email, and @Size, invalid requests are rejected before reaching the business logic.

Code:

public class StudentDTO {

    @NotBlank
    private String name;

    @Email
    private String email;

    @Min(18)
    private int age;
}

Explanation

The @Valid annotation tells Spring Boot to validate the request object before executing the controller method. If validation fails, Spring automatically throws a validation exception instead of calling the service layer.

Code:

@PostMapping
public Student createStudent(@Valid @RequestBody StudentDTO student) {
    return service.save(student);
}

Explanation

Custom validators are useful when built-in validation annotations cannot satisfy business requirements. You can create your own validation annotation and implement validation logic using the ConstraintValidator interface.

Code:

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = PhoneValidator.class)
public @interface ValidPhone {
    String message() default "Invalid phone number";
}

Explanation

A class annotated with @RestControllerAdvice can catch validation exceptions from every controller. This provides a centralized place to return consistent and user-friendly error messages for invalid requests.

Code:

@RestControllerAdvice
public class GlobalExceptionHandler {

    @ExceptionHandler(MethodArgumentNotValidException.class)
    public ResponseEntity<String> handleValidationException() {
        return ResponseEntity.badRequest().body("Validation Failed");
    }
}

Explanation

Instead of returning a generic error message, collect all validation errors and return them in a structured JSON response. This helps API consumers identify exactly which fields failed validation and why.

Code:

@ExceptionHandler(MethodArgumentNotValidException.class)
public Map<String, String> handleErrors(MethodArgumentNotValidException ex) {
    Map<String, String> errors = new HashMap<>();
    ex.getBindingResult().getFieldErrors()
      .forEach(error -> errors.put(error.getField(), error.getDefaultMessage()));
    return errors;
}