How Bean Validation and @Valid Keep Bad Data Out of Your Spring App

Bean Validation in Spring Boot allows developers to declare data rules directly on fields using annotations like @NotBlank, @Min, and @Email, eliminating repetitive manual checks in controller methods. Adding the spring-boot-starter-validation dependency enables Spring to automatically wire a validator that runs these constraints at the request boundary. The @Valid annotation instructs Spring to validate an incoming request object immediately after binding, before any business logic executes. If validation fails, Spring either throws a MethodArgumentNotValidException or populates a BindingResult object, depending on whether the controller method declares one as a parameter. A common pitfall is omitting the starter dependency, which causes annotations to be silently ignored, and confusing @NotNull, @NotEmpty, and @NotBlank, which each handle empty input differently.