Validate Your Endpoints Using Spring

@RequiredArgsConstructor @Getter  
public class ShipmentComponentDTO {  
    private final String productCode;  
}

@RequiredArgsConstructor @Getter  
public class ShipmentDTO {  
    private final String productCode;  
    private final int count;  
    private final List<ShipmentComponentDTO> shipmentComponentDTOs;  
}

@RestController  
public class ShipmentController {  
    @PostMapping("/shipment/add")  
    public ResponseEntity<String> addShipment(@RequestBody ShipmentDTO shipmentDTO){  
        if (shipmentDTO.getProductCode() == null)  
            return ResponseEntity.badRequest().body("Product code can't be null");  

        if (shipmentDTO.getCount() <= 0)  
            return ResponseEntity.badRequest().body("Count can't be negative or zero");  

        if (!allComponentsAreValid(shipmentDTO))  
            return ResponseEntity.badRequest().body("Component product code can't be null");  

        return ResponseEntity.ok(":)");  
    }  

    private boolean allComponentsAreValid(ShipmentDTO shipmentDTO) {  
        List<ShipmentComponentDTO> shipmentComponentDTOs = shipmentDTO.getShipmentComponentDTOs();  

        return shipmentComponentDTOs != null &&  
                shipmentComponentDTOs.size() > 0 &&  
                shipmentComponentDTOs.  
                stream().  
                allMatch(shipmentComponentDTO -> shipmentComponentDTO.getProductCode() != null);  
    }  
}

@RequiredArgsConstructor @Getter  
public class ShipmentComponentDTO {  
    @NotBlank(message = "Component product code can't be null")  
    private final String productCode;  
    // required by the javax validation code  
  public ShipmentComponentDTO() {  
        this.productCode = "";  
    }  
}

@RequiredArgsConstructor @Getter  
public class ShipmentDTO {  
    @NotBlank(message = "Product code can't be null")  
    private final String productCode;  
    @Min(1)  
    private final int count;  

    @Size(min = 1, message = "Component product code can't be empty")  
    @NotNull(message = "Component product code can't be null")  
    @Valid  
  private final List<ShipmentComponentDTO> shipmentComponentDTOs;  
}

@RestController  
public class ShipmentController_Validation {  
    @PostMapping("validation/shipment/add")  
    public ResponseEntity<String> addShipment(@RequestBody @Validated ShipmentDTO shipmentDTO){  
        return ResponseEntity.ok(":)");  
    }  
}

@Order(Ordered.HIGHEST_PRECEDENCE)  
@ControllerAdvice  
public class ValidationAdvice{  
    @ResponseStatus(BAD_REQUEST)  
    @ResponseBody  
 @ExceptionHandler(MethodArgumentNotValidException.class)  
    public ResponseEntity<?> methodArgumentNotValidException(MethodArgumentNotValidException ex) {  
        BindingResult result = ex.getBindingResult();  
        List<FieldError> fieldErrors = result.getFieldErrors();  
        String errorMessage = fieldErrors.get(0).getDefaultMessage();  
        return ResponseEntity.badRequest().body(errorMessage);  
    }  
}