[Spring] Cannot construct instance... (Jackson, POJO)

오류 내용

Postman을 이용하여 Controller의 CreateResponse(회원 생성) 테스트를 진행하던 중

 

포스트맨에서는 아래와 같은 오류 메시지가,

{
"timestamp": "2023-11-02T03:58:01.298+00:00",
"status": 400,
"error": "Bad Request",
"trace": "org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Cannot construct instance of com.example.demo.model.dto.CreateRequest (although at least one Creator exists): cannot deserialize from Object value (no delegate- or property-based Creator); nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of com.example.demo.model.dto.CreateRequest (although at least one Creator exists): cannot deserialize from Object value (no delegate- or property-based Creator)\n at [Source: (org.springframework.util.StreamUtils$NonClosingInputStream); line: 2, column: 5]\r\n\tat org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.readJavaType(AbstractJackson2HttpMessageConverter.java:391)\r\n\tat org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.read(AbstractJackson2HttpMessageConverter.java:343)\r\n\tat org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodArgumentResolver.readWithMessageConverters(AbstractMessageConverterMethodArgumentResolver.java:185)\r\n\tat org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.readWithMessageConverters(RequestResponseBodyMethodProcessor.java:160)\r\n\tat org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.resolveArgument(RequestResponseBodyMethodProcessor.java:133)\r\n\tat org.springframework.web.method.support.HandlerMethodArgumentResolverComposite.resolveArgument(HandlerMethodArgumentResolverComposite.java:122)\r\n\tat org.springframework.web.method.support.InvocableHandlerMethod.getMethodArgumentValues(InvocableHandlerMethod.java:179)\r\n\tat org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:146)\r\n\tat org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:117)\r\n\tat org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:895)\r\n\tat org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:808)\r\n\tat org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87)\r\n\tat org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1072)\r\n\tat org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:965)\r\n\tat org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006)\r\n\tat org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:909)\r\n\tat javax.servlet.http.HttpServlet.service(HttpServlet.java:555)\r\n\tat org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883)\r\n\tat javax.servlet.http.HttpServlet.service(HttpServlet.java:623)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:209)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:153)\r\n\tat org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:178)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:153)\r\n\tat org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100)\r\n\tat org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:178)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:153)\r\n\tat org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93)\r\n\tat org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:178)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:153)\r\n\tat org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201)\r\n\tat org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:178)\r\n\tat org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:153)\r\n\tat org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:168)\r\n\tat org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:90)\r\n\tat org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:481)\r\n\tat org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:130)\r\n\tat org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:93)\r\n\tat org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74)\r\n\tat org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)\r\n\tat org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:390)\r\n\tat org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:63)\r\n\tat org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:928)\r\n\tat org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1794)\r\n\tat org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:52)\r\n\tat org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191)\r\n\tat org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659)\r\n\tat org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)\r\n\tat java.base/java.lang.Thread.run(Thread.java:1589)\r\nCaused by: com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of com.example.demo.model.dto.CreateRequest (although at least one Creator exists): cannot deserialize from Object value (no delegate- or property-based Creator)\n at [Source: (org.springframework.util.StreamUtils$NonClosingInputStream); line: 2, column: 5]\r\n\tat com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:63)\r\n\tat com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1728)\r\n\tat com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1353)\r\n\tat com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:1415)\r\n\tat com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:352)\r\n\tat com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:185)\r\n\tat com.fasterxml.jackson.databind.deser.DefaultDeserializationContext.readRootValue(DefaultDeserializationContext.java:323)\r\n\tat com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4674)\r\n\tat com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3682)\r\n\tat org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.readJavaType(AbstractJackson2HttpMessageConverter.java:380)\r\n\t... 51 more\r\n",
"message": "JSON parse error: Cannot construct instance of com.example.demo.model.dto.CreateRequest (although at least one Creator exists): cannot deserialize from Object value (no delegate- or property-based Creator); nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of com.example.demo.model.dto.CreateRequest (although at least one Creator exists): cannot deserialize from Object value (no delegate- or property-based Creator)\n at [Source: (org.springframework.util.StreamUtils$NonClosingInputStream); line: 2, column: 5]",
"path": "/api/users/"
}

 

서버에서는 아래와 같은 오류 메시지가 떴다.

2023-11-02 12:58:01.192  WARN 16296 --- [nio-8080-exec-3] .w.s.m.s.DefaultHandlerExceptionResolver : Resolved [org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Cannot construct instance of com.example.demo.model.dto.CreateRequest (although at least one Creator exists): cannot deserialize from Object value (no delegate- or property-based Creator); nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of com.example.demo.model.dto.CreateRequest (although at least one Creator exists): cannot deserialize from Object value (no delegate- or property-based Creator)<EOL> at [Source: (org.springframework.util.StreamUtils$NonClosingInputStream); line: 2, column: 5]]

 

@Getter
@AllArgsConstructor
public class CreateRequest {

    String name;
}

-> 기존의 코드

 

 

오류 원인 및 해결

Cannot construct instance... 라는 메시지가 주요 원인인 것 같아서 찾아보니 기본 생성자가 없는 것이 문제였다.

Jackson 라이브러리는 기본적으로 POJO(Plain Old Java Object) 스타일의 객체를 다루기 때문에, 기본 생성자와 setter가 필요하다. -> getter/setter 메소드를 이용해 객체의 필드 값을 읽고 쓰는 방식을 사용하기 때문

Jackson Library?

Jackson 라이브러리는 Java 객체와 JSON 형태의 데이터를 상호 변환하는데 사용되는 라이브러리이다.

Java 객체 -> JSON 문자열로 변환:  직렬화(Serialization)

JSON 문자열 -> Java 객체로 변환:  역직렬화(Deserialization)

POJO (Plain Old Java Object)?

"단순한 오래된 자바 객체"라는 뜻으로, 이 용어는 특정한 규약이나 클래스, 인터페이스를 따르지 않는, '단순한' 자바 객체를 가리킨다.

 

POJO는 복잡한 엔터프라이즈 시스템을 구축하는 데 있어서 투명성을 높이고, 객체 지향 디자인을 깨끗하게 유지하도록 돕는다.
POJO는 다른 프레임워크에 종속적이지 않으므로, 코드의 재사용성이 높고 유지보수가 쉽다. 이런 특징 때문에, 스프링 프레임워크 같은 모던한 자바 기술에서 POJO를 적극적으로 활용하고 있다.

 

-> CreateRequest 클래스에 @NoArgsConstructor 어노테이션을 추가함으로 기본 생성자를 만들어 주어서 오류를 해결했다.