Understanding the Detailed Usage of @Nullable Annotation

You might think this simple, common annotation is something you already know.

The @Nullable annotation can be placed on methods, fields, and parameters. When used on a method, it indicates that the return value may be null; on a field, it signals that the field value may be null; on a parameter, it marks the argument as possibly null.

Example of using @Nullable on a method return value:

@Nullable
public ApiResult upload(@NotNull(message = "#x4E0A#x4F20#x53C2#x6570#x4E0D#x80FD#x4E3A#x7A7A;") @RequestParam("file") MultipartFile[] file) throws BaseException {
    ApiResult apiResult = new ApiResult();
    SimpleDateFormat simpleDateFormat = new SimpleDateFormat("yyyyMMdd");
    String format = simpleDateFormat.format(new Date());
    String realPath = filePath + File.separator + format;
    String returnPath = format;
    File targetFile = new File(realPath);
    if (!targetFile.exists()){
        targetFile.mkdirs();
    }
}

Annotating the method with @Nullable indicates that the method’s return value can be null.

Example of using @Nullable on a parameter:

private void checkUser(String fansid, String openid, @Nullable String op) throws BaseException{
    Consumer consumer = consumerService.selectByPrimaryKey(fansid);
    if (consumer == null) {
        throw new ParamException("#x7528#x6237#x4E0D#x5B58#x5728;");
    }
    Consumer consumer1 = consumerService.selectByPrimaryKey(openid);
    if(consumer1 == null){
        throw new ParamException("#x88AB#x5173#x6CE8#x8005#x4FE1#x606F#x5F02#x5E38;");
    }
}

Placing @Nullable before a parameter marks that argument as allowed to be null.

Example of using @Nullable on a field:

@Validated
@RestController
@RequestMapping("miniapi/follow")
public class FollowController extends BaseController {
    @Nullable
    private String isTime;

    @Autowired
    private FollowService followService;

    @Autowired
    private ConsumerService consumerService;

    private Logger logger = LoggerFactory.getLogger(this.getClass());
}

The @Nullable annotation above the isTime field indicates that this field may be null.

Beyond our own code, third‑party libraries also use @Nullable. In the Spring toolkit, the org.springframework.util.StringUtils class employs @Nullable in its empty‑check method.

Spring Toolkit Source Code Usage Example

Spring’s StringUtils.isEmpty method is defined as follows:

public static boolean isEmpty(@Nullable Object str) {
   return (str == null || "".equals(str));
}

That concludes our brief overview of @Nullable usage. Feel free to leave comments or discuss further.