Validation

Introduction

Bean Validation (Jakarta Validation) declares constraints on DTO fields and Spring enforces them automatically when you add @Valid. Invalid requests fail fast with a 400 — your service code stays clean.

Informative example

public record SignupRequest(
    @NotBlank @Email String email,
    @NotBlank @Size(min = 12, max = 72) String password,
    @NotBlank @Pattern(regexp = "[A-Za-z][A-Za-z '-]{1,49}") String name,
    @Min(13) @Max(120) int age
) {}

@PostMapping("/signup")
public UserDto signup(@Valid @RequestBody SignupRequest req) { ... }

Best practices

• Validate at the boundary, not deep inside services.
• Custom rules → custom annotation + ConstraintValidator.
• Group validation (@Validated(OnCreate.class)) when create vs update differ.