Implementing Data Validation

@PostMapping
public ResponseEntity<Void> create(@Valid @RequestBody NewUser body) { /* … */ }

@Constraint(validatedBy = SkuValidator.class)
@Target({ElementType.FIELD, ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
public @interface ValidSku {
  String message() default "Invalid SKU format";
  Class<?>[] groups() default {};
  Class<? extends Payload>[] payload() default {};
}

public class SkuValidator implements ConstraintValidator<ValidSku, String> {
  private static final Pattern P = Pattern.compile("^[A-Z]{3}-\\d{6}$");
  @Override public boolean isValid(String v, ConstraintValidatorContext c) {
    return v == null || P.matcher(v).matches();
  }
}

@PostMapping
public ResponseEntity<?> create(@Valid @RequestBody Form f, BindingResult br) {
  if (br.hasErrors()) {
    Map<String,String> errs = br.getFieldErrors().stream()
      .collect(Collectors.toMap(FieldError::getField, FieldError::getDefaultMessage));
    return ResponseEntity.badRequest().body(errs);
  }
  return ResponseEntity.ok().build();
}

# src/main/resources/ValidationMessages.properties
NotBlank.user.email=Email is required
Size.user.password=Password must be {min} to {max} characters
javax.validation.constraints.Email.message=Provide a valid email address

public interface OnCreate {}
public interface OnUpdate {}

public record User(
  @Null(groups=OnCreate.class) @NotNull(groups=OnUpdate.class) Long id,
  @NotBlank String email
) {}

@PostMapping
ResponseEntity<?> create(@Validated(OnCreate.class) @RequestBody User u) { /* … */ }

public record Order(
  @NotNull @Valid Customer customer,
  @NotEmpty @Valid List<@Valid LineItem> items
) {}

@Service
@Validated
public class UserService {
  public User load(@NotBlank String email) { /* … */ }
  public @NotNull User update(@Valid UserUpdate u) { /* … */ }
}