Validation & Exception Handling

🌿 Spring BootLesson 8Intermediate

Spring Boot integrates with the Bean Validation API (JSR-380) via Hibernate Validator. Combined with a global @ControllerAdvice exception handler, you can produce consistent, informative error responses across your entire API.

1 Bean Validation Annotations

Java — Request DTO with Validation

import jakarta.validation.constraints.*;
import lombok.Data;

@Data
public class CreateUserRequest {

    @NotBlank(message = "Name is required")
    @Size(min = 2, max = 100, message = "Name must be 2-100 characters")
    private String name;

    @NotBlank(message = "Email is required")
    @Email(message = "Must be a valid email address")
    private String email;

    @NotBlank(message = "Password is required")
    @Size(min = 8, message = "Password must be at least 8 characters")
    @Pattern(regexp = ".*[A-Z].*", message = "Password must contain at least one uppercase letter")
    private String password;

    @Min(value = 0, message = "Age must be positive")
    @Max(value = 120, message = "Age must be realistic")
    private Integer age;

    @NotNull(message = "Role is required")
    private Role role;
}

2 Global Exception Handler

Java — GlobalExceptionHandler.java

@RestControllerAdvice
public class GlobalExceptionHandler {

    // Handle validation errors (400)
    @ExceptionHandler(MethodArgumentNotValidException.class)
    public ResponseEntity<ErrorResponse> handleValidation(MethodArgumentNotValidException ex) {
        Map<String, String> fieldErrors = new LinkedHashMap<>();
        ex.getBindingResult().getFieldErrors().forEach(err ->
            fieldErrors.put(err.getField(), err.getDefaultMessage())
        );
        return ResponseEntity.badRequest().body(
            new ErrorResponse("VALIDATION_ERROR", "Input validation failed", fieldErrors)
        );
    }

    // Handle not found (404)
    @ExceptionHandler(ResourceNotFoundException.class)
    public ResponseEntity<ErrorResponse> handleNotFound(ResourceNotFoundException ex) {
        return ResponseEntity.status(HttpStatus.NOT_FOUND)
            .body(new ErrorResponse("NOT_FOUND", ex.getMessage(), null));
    }

    // Handle conflict (409)
    @ExceptionHandler(ConflictException.class)
    public ResponseEntity<ErrorResponse> handleConflict(ConflictException ex) {
        return ResponseEntity.status(HttpStatus.CONFLICT)
            .body(new ErrorResponse("CONFLICT", ex.getMessage(), null));
    }

    // Catch-all (500)
    @ExceptionHandler(Exception.class)
    public ResponseEntity<ErrorResponse> handleGeneral(Exception ex) {
        return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR)
            .body(new ErrorResponse("INTERNAL_ERROR", "An unexpected error occurred", null));
    }
}

// ErrorResponse DTO
@Data @AllArgsConstructor
public class ErrorResponse {
    private String              code;
    private String              message;
    private Map<String, String> fieldErrors;
    private String              timestamp = Instant.now().toString();
}

3 Code Challenge

Challenge: Create a custom annotation @UniqueEmail using ConstraintValidator that checks the database to ensure the email doesn't already exist during registration. Apply it to the CreateUserRequest.email field.