Spring Validation

What is validation?

• User input may be different from the requirements from the server
• In order to prevent any invalid data from being sent to the server or DB, we need to validate the data

 

Example

1. Add dependency

• Add Gradle dependency

implementation 'org.springframework.boot:spring-boot-starter-validation:3.1.3'

2. Add validation annotations in dto

• We need to add annotations to the fields that we want to validate

// UserDto.java

public class UserDto {
    private Long id;
    @NotEmpty
    @Email
    private String email;
    @NotEmpty(message = "First Name should not be null or empty")
    private String firstName;
    @NotEmpty
    private String lastName;
}

• Below is a list of common annotations for validation

Annotation	Condition
@NotNull	Should not be null
@Null	Should be null
@NotEmpty	Should not be null or empty
@Max(n)	Should be less than or equal to n
@AssertTrue	Should be true
@Future	Should be a date in the future (based on current date)
@Past	Should be a date in the past

3. Enable validation by @Valid annotation in REST APIs

• In the REST API function that we want to validate the request body, we use @Valid annotation

// UserController.java

@PostMapping
public ResponseEntity<UserDto> createUser(@Valid @RequestBody UserDto userDto) {
    UserDto savedUserDto = userService.createUser(userDto);
    return new ResponseEntity<>(savedUserDto, HttpStatus.OK);
}

• Then we can see that a validation error response is thrown.

4. Customise validation error response

• It would be better to have a custom exception for the validation error. 
• We can use 'handleMethodArgumentNotValid' function from ResponseEntityExceptionHandler.
• Below, we get a list of validation errors and return it as a response entity.

@ControllerAdvice
public class GlobalExceptionHandler extends ResponseEntityExceptionHandler {
	...
    
    @Override
    protected ResponseEntity<Object> handleMethodArgumentNotValid(
            MethodArgumentNotValidException ex, 
            HttpHeaders headers, 
            HttpStatusCode status, 
            WebRequest request) 
    {
        Map<String, String> errors = new HashMap<>();
        List<ObjectError> errorList = ex.getBindingResult().getAllErrors();
        errorList.forEach(e -> {
            errors.put(((FieldError) e).getField(), e.getDefaultMessage());
        });

        return new ResponseEntity<>(errors, HttpStatus.BAD_REQUEST);
    }
}

The response shown when firstName is empty and email is not well formatted

• We can customize the message we want to show for the validation error
• We use the 'message' field inside each annotation

@NotEmpty(message = "First Name should not be null or empty")
private String firstName;

message for invalid firstName has changed