Could not write JSON JsonObject (default message converter of spring)

Error

2024-04-15 19:58:43 [WARN] [http-nio-8085-exec-1] o.s.w.s.m.s.DefaultHandlerExceptionResolver - Resolved [org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: JsonObject]

spring was 환경에서 API를 통해 클라이언트에게 응답을 하던 중 오류가 발생

Default Converter

기본적으로 아래 컨버터들을 지원한다.

• ByteArrayHttpMessageConverter – converts byte arrays
• StringHttpMessageConverter – converts Strings
• ResourceHttpMessageConverter – converts org.springframework.core.io.Resource for any type of octet stream
• SourceHttpMessageConverter – converts javax.xml.transform.Source
• FormHttpMessageConverter – converts form data to/from a MultiValueMap<String, String>
• Jaxb2RootElementHttpMessageConverter – converts Java objects to/from XML (added only if JAXB2 is present on the classpath)
• MappingJackson2HttpMessageConverter – converts JSON (added only if Jackson 2 is present on the classpath)
• MappingJacksonHttpMessageConverter – converts JSON (added only if Jackson is present on the classpath)
• AtomFeedHttpMessageConverter – converts Atom feeds (added only if Rome is present on the classpath)
• RssChannelHttpMessageConverter – converts RSS feeds (added only if Rome is present on the classpath)

Error Cause

Gson을 위한 Converter 가 기본적으로 내장되어있지 않아 발생한 오류이다. Gson데이터를 응답하기 위해서는 Converter에 대한 설정을 Override가 필요.

Configuration

@Configuration
public class WebMvcConfig implements WebMvcConfigurer {

    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        // Create Gson instance
        Gson gson = new Gson();

        // Create GsonHttpMessageConverter with Gson instance
        GsonHttpMessageConverter gsonConverter = new GsonHttpMessageConverter();

        // Add GsonHttpMessageConverter to the list of converters
        converters.add(6, gsonConverter);
    }
}

인덱스를 추가해주지 않을경우, 맨 끝순서에 append되며, 이 경우 gson을 여전히 converting할 수 없다.
Jackson 순서 앞단에 추가해주면 우선적으로 사용하여 gson을 converting 할 수 있음

Before

0 = {ByteArrayHttpMessageConverter@12871}
1 = {StringHttpMessageConverter@12872}
2 = {StringHttpMessageConverter@12873}
3 = {ResourceHttpMessageConverter@12874}
4 = {ResourceRegionHttpMessageConverter@12875}
5 = {AllEncompassingFormHttpMessageConverter@12876}
6 = {MappingJackson2HttpMessageConverter@12877}
7 = {MappingJackson2HttpMessageConverter@12878}
8 = {Jaxb2RootElementHttpMessageConverter@12879}

After

0 = {ByteArrayHttpMessageConverter@12902}
1 = {StringHttpMessageConverter@12903}
2 = {StringHttpMessageConverter@12904}
3 = {ResourceHttpMessageConverter@12905}
4 = {ResourceRegionHttpMessageConverter@12906}
5 = {AllEncompassingFormHttpMessageConverter@12907}
6 = {GsonHttpMessageConverter@12908} « 
7 = {MappingJackson2HttpMessageConverter@12909}
8 = {Jaxb2RootElementHttpMessageConverter@12910}

references

https://www.baeldung.com/spring-httpmessageconverter-rest