What is @Valid and Bean Validation in Spring Boot?

Bean Validation (JSR-380) with Hibernate Validator provides declarative validation for Java beans. Spring Boot integrates it with spring-boot-starter-validation. Constraint annotations on entity/DTO fields: public class CreateUserRequest { @NotBlank(message = "Name is required") @Size(min = 2, max = 50) private String name; @NotBlank @Email(message = "Invalid email format") private String email; @NotNull @Min(18) @Max(120) private Integer age; @NotNull @Pattern(regexp = "^\d{10}$", message = "Must be 10 digits") private String phone; @Future private LocalDate expiryDate; @NotEmpty private List<@Valid AddressDTO> addresses; }. Triggering validation: add @Valid or @Validated to controller parameters: @PostMapping public ResponseEntity<User> createUser(@Valid @RequestBody CreateUserRequest request) { ... }. Spring automatically validates and returns 400 with error details if validation fails. Handling validation errors: @ExceptionHandler(MethodArgumentNotValidException.class) public ResponseEntity<Map<String,String>> handleValidation(MethodArgumentNotValidException ex) { Map<String,String> errors = new HashMap<>(); ex.getBindingResult().getFieldErrors().forEach(e -> errors.put(e.getField(), e.getDefaultMessage())); return ResponseEntity.badRequest().body(errors); }. Custom validators: implement ConstraintValidator interface for custom business rules. Groups: validate different constraints for different scenarios (Create group vs Update group) using validation groups.