Spring boot custom gson BEGIN_OBJECT but was STRING error

Question

I have a nodejs class that uses fetch api and calls a spring web backend using POST.

fetch(this.service, {
            method: 'POST', // *GET, POST, PUT, DELETE, etc.
            mode: 'cors', // no-cors, *cors, same-origin
            cache: 'no-cache', // *default, no-cache, reload, force-cache, only-if-cached
            credentials: 'same-origin', // include, *same-origin, omit
            headers: {
                'Content-Type': 'application/json'
                // 'Content-Type': 'application/x-www-form-urlencoded',
            },
            redirect: 'follow', // manual, *follow, error
            referrerPolicy: 'no-referrer', // no-referrer, *no-referrer-when-downgrade, origin, origin-when-cross-origin, same-origin, strict-origin, strict-origin-when-cross-origin, unsafe-url
            body: JSON.stringify(this.request) // body data type must match "Content-Type" header
        }).then(res => res.json())
        .then((result) => {
            if(result.responseStatus === 'OK'){
                resolve(result);
            }else{
                console.log("failed response");
                console.log(result);
            }
        }, (error) => {
            //handle error here
            console.log("errored response");
            console.log(error);
        });

On the backend I have this -

@Controller
@CrossOrigin(origins = "http://localhost:3000")
@RequestMapping(value = "/user", method = { RequestMethod.GET,
        RequestMethod.POST }, produces = MediaType.APPLICATION_JSON_VALUE, headers = "Accept="
                + MediaType.APPLICATION_JSON_VALUE)
public class SomeController {

    private final SomeDALImpl repository;
    private SomeResponse response;

    @Autowired
    public SomeController(SomeDALImpl repo) {
        this.repository = repo;
    }

    @RequestMapping("/abcd")
    @ResponseBody
    public SomeResponse getSome(@RequestBody @Valid SomeGetRequest request) {
        response = new SomeResponse();
        //does something
        return response;
    }
}

SomeGetRequest is a class that looks like this -

public class SomeGetRequest{
    public ObjectId someId;
    //other getter setters
}

I am trying to use gson as my default in spring instead of Jackson. When I am sending a request from front-end it is not deserializing the request coming from front-end for ObjectId.

From front end this goes in body of fetch after JSON.stringify - "{"someId":"507f1f77bcf86cd799439011"}"

and on backend this is the error -

 org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver 
[http-nio-8080-exec-6] Resolved [org.springframework.http.converter.HttpMessageNotReadableException: 
Could not read JSON: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was 
STRING at line 1 column 12 path $.userId; nested exception 
is com.google.gson.JsonSyntaxException: 
java.lang.IllegalStateException: Expected BEGIN_OBJECT but 
was STRING at line 1 column 12 path $.someId]

I have this in application.properties - spring.http.converters.preferred-json-mapper = gson I have removed Jackson dependencies in pom.xml -

<dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
            <!-- Exclude the default Jackson dependency -->
            <exclusions>
                <exclusion>
                    <groupId>org.springframework.boot</groupId>
                    <artifactId>spring-boot-starter-json</artifactId>
                </exclusion>
            </exclusions>
        </dependency>

I also added this class, but still it does not work for ObjectIds -

@Configuration
public class GsonConfig implements WebMvcConfigurer {

    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        converters.add(customGsonHttpMessageConverter());

        extendMessageConverters(converters);
    }

    private GsonHttpMessageConverter customGsonHttpMessageConverter() {
        GsonBuilder builder = new GsonBuilder().registerTypeAdapter(ObjectId.class, new JsonSerializer<ObjectId>() {
            @Override
            public JsonElement serialize(ObjectId src, Type typeOfSrc, JsonSerializationContext context) {
                return new JsonPrimitive(src.toHexString());
            }
        }).registerTypeAdapter(ObjectId.class, new JsonDeserializer<ObjectId>() {
            @Override
            public ObjectId deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context)
                    throws JsonParseException {
                return new ObjectId(json.getAsString());
            }
        });
        Gson gson = builder.create();
        GsonHttpMessageConverter gsonMessageConverter = new GsonHttpMessageConverter();
        gsonMessageConverter.setGson(gson);

        return gsonMessageConverter;
    }
}

Or, maybe I am not sending the request body properly from front-end. What should I do to correct this. Thanks, I am new to Spring.

PS - It was working fine with Jackson as default in spring.

Answer 1

The HTTP request param {"someId":"507f1f77bcf86cd799439011"}" means it's a string field. Depends on your ObjectId class structure, the correct JSON would be like {"someId":{"id":"507f1f77bcf86cd799439011"}}, nested class JSON format.

Answer 2

From your error msg:

Expected BEGIN_OBJECT but was STRING at line 1 column 12 path $.someId

And your Object SomeGetRequest doesn't have String type someId.

Convert error may be because you passing a string for `someId, but in the class, it's an object (ObjectId), Could you just change ObjectId -> String, and try it again.