안녕하십니까 선생님 얼마 전부터 선생님의 강의로 스프링부트를 배우고 있는 학생입니다. 그러나 며칠 전부터 질문 게시글을 확인하는 에러가 발생하고 있어 질문을 드리고자 글을 남깁니다. 현재까지 한 진도는 2-16까지 왔습니다.

게시물을 작성하는 것까지는 무난히 되고 있지만 내용을 보기 위해 클릭을 하게 되면 에러가 발생하고 있습니다. 아래는 클릭 시 나타나는 에러 메시지입니다.

Whitelabel Error Page
This application has no explicit mapping for /error, so you are seeing this as a fallback.

Mon May 01 14:36:15 JST 2023
There was an unexpected error (type=Internal Server Error, status=500).
An error happened during template parsing (template: "class path resource [templates/question_detail.html]")
org.thymeleaf.exceptions.TemplateInputException: An error happened during template parsing (template: "class path resource [templates/question_detail.html]")
    at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parse(AbstractMarkupTemplateParser.java:241)
    at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parseStandalone(AbstractMarkupTemplateParser.java:100)
    at org.thymeleaf.engine.TemplateManager.parseAndProcess(TemplateManager.java:666)
    at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1103)
    at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1077)
    at org.thymeleaf.spring6.view.ThymeleafView.renderFragment(ThymeleafView.java:372)
    at org.thymeleaf.spring6.view.ThymeleafView.render(ThymeleafView.java:192)
    at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1415)
    at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1159)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1098)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:974)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1011)
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:903)
    at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:564)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:885)
    at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:658)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:205)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149)
    at org.springframework.security.web.FilterChainProxy.lambda$doFilterInternal$3(FilterChainProxy.java:231)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:365)
    at org.springframework.security.web.access.intercept.AuthorizationFilter.doFilter(AuthorizationFilter.java:100)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374)
    at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:126)
    at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:120)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374)
    at org.springframework.security.web.authentication.AnonymousAuthenticationFilter.doFilter(AnonymousAuthenticationFilter.java:100)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374)
    at org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:179)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374)
    at org.springframework.security.web.savedrequest.RequestCacheAwareFilter.doFilter(RequestCacheAwareFilter.java:63)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374)
    at org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:107)
    at org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:93)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374)
    at org.springframework.security.web.csrf.CsrfFilter.doFilterInternal(CsrfFilter.java:117)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374)
    at org.springframework.security.web.header.HeaderWriterFilter.doHeadersAfter(HeaderWriterFilter.java:90)
    at org.springframework.security.web.header.HeaderWriterFilter.doFilterInternal(HeaderWriterFilter.java:75)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374)
    at org.springframework.security.web.context.SecurityContextHolderFilter.doFilter(SecurityContextHolderFilter.java:82)
    at org.springframework.security.web.context.SecurityContextHolderFilter.doFilter(SecurityContextHolderFilter.java:69)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374)
    at org.springframework.security.web.context.request.async.WebAsyncManagerIntegrationFilter.doFilterInternal(WebAsyncManagerIntegrationFilter.java:62)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374)
    at org.springframework.security.web.session.DisableEncodeUrlFilter.doFilterInternal(DisableEncodeUrlFilter.java:42)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:374)
    at org.springframework.security.web.FilterChainProxy.doFilterInternal(FilterChainProxy.java:233)
    at org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:191)
    at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:352)
    at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:268)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149)
    at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149)
    at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149)
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:174)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:149)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:166)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:90)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:482)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:115)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:93)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:341)
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:390)
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:63)
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:894)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1741)
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:52)
    at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191)
    at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.base/java.lang.Thread.run(Thread.java:833)
Caused by: org.attoparser.ParseException: Exception evaluating SpringEL expression: "#fields.hasAnyErrors()" (template: "form_errors" - line 9, col 16)
    at org.attoparser.MarkupParser.parseDocument(MarkupParser.java:393)
    at org.attoparser.MarkupParser.parse(MarkupParser.java:257)
    at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parse(AbstractMarkupTemplateParser.java:230)
    ... 86 more
Caused by: org.thymeleaf.exceptions.TemplateProcessingException: Exception evaluating SpringEL expression: "#fields.hasAnyErrors()" (template: "form_errors" - line 9, col 16)
    at org.thymeleaf.spring6.expression.SPELVariableExpressionEvaluator.evaluate(SPELVariableExpressionEvaluator.java:292)
    at org.thymeleaf.standard.expression.VariableExpression.executeVariableExpression(VariableExpression.java:166)
    at org.thymeleaf.standard.expression.SimpleExpression.executeSimple(SimpleExpression.java:66)
    at org.thymeleaf.standard.expression.Expression.execute(Expression.java:109)
    at org.thymeleaf.standard.expression.Expression.execute(Expression.java:138)
    at org.thymeleaf.standard.expression.Expression.execute(Expression.java:125)
    at org.thymeleaf.standard.processor.StandardIfTagProcessor.isVisible(StandardIfTagProcessor.java:59)
    at org.thymeleaf.standard.processor.AbstractStandardConditionalVisibilityTagProcessor.doProcess(AbstractStandardConditionalVisibilityTagProcessor.java:61)
    at org.thymeleaf.processor.element.AbstractAttributeTagProcessor.doProcess(AbstractAttributeTagProcessor.java:74)
    at org.thymeleaf.processor.element.AbstractElementTagProcessor.process(AbstractElementTagProcessor.java:95)
    at org.thymeleaf.util.ProcessorConfigurationUtils$ElementTagProcessorWrapper.process(ProcessorConfigurationUtils.java:633)
    at org.thymeleaf.engine.ProcessorTemplateHandler.handleOpenElement(ProcessorTemplateHandler.java:1314)
    at org.thymeleaf.engine.OpenElementTag.beHandled(OpenElementTag.java:205)
    at org.thymeleaf.engine.Model.process(Model.java:282)
    at org.thymeleaf.engine.ProcessorTemplateHandler.handleOpenElement(ProcessorTemplateHandler.java:1587)
    at org.thymeleaf.engine.OpenElementTag.beHandled(OpenElementTag.java:205)
    at org.thymeleaf.engine.Model.process(Model.java:282)
    at org.thymeleaf.engine.ProcessorTemplateHandler.handleOpenElement(ProcessorTemplateHandler.java:1587)
    at org.thymeleaf.engine.OpenElementTag.beHandled(OpenElementTag.java:205)
    at org.thymeleaf.engine.Model.process(Model.java:282)
    at org.thymeleaf.engine.ProcessorTemplateHandler.handleOpenElement(ProcessorTemplateHandler.java:1587)
    at org.thymeleaf.engine.OpenElementTag.beHandled(OpenElementTag.java:205)
    at org.thymeleaf.engine.Model.process(Model.java:282)
    at org.thymeleaf.engine.Model.process(Model.java:290)
    at org.thymeleaf.engine.GatheringModelProcessable.process(GatheringModelProcessable.java:78)
    at org.thymeleaf.engine.ProcessorTemplateHandler.handleCloseElement(ProcessorTemplateHandler.java:1640)
    at org.thymeleaf.engine.TemplateHandlerAdapterMarkupHandler.handleCloseElementEnd(TemplateHandlerAdapterMarkupHandler.java:388)
    at org.thymeleaf.templateparser.markup.InlinedOutputExpressionMarkupHandler$InlineMarkupAdapterPreProcessorHandler.handleCloseElementEnd(InlinedOutputExpressionMarkupHandler.java:322)
    at org.thymeleaf.standard.inline.OutputExpressionInlinePreProcessorHandler.handleCloseElementEnd(OutputExpressionInlinePreProcessorHandler.java:220)
    at org.thymeleaf.templateparser.markup.InlinedOutputExpressionMarkupHandler.handleCloseElementEnd(InlinedOutputExpressionMarkupHandler.java:164)
    at org.attoparser.HtmlElement.handleCloseElementEnd(HtmlElement.java:169)
    at org.attoparser.HtmlMarkupHandler.handleCloseElementEnd(HtmlMarkupHandler.java:412)
    at org.attoparser.MarkupEventProcessorHandler.handleCloseElementEnd(MarkupEventProcessorHandler.java:473)
    at org.attoparser.ParsingElementMarkupUtil.parseCloseElement(ParsingElementMarkupUtil.java:201)
    at org.attoparser.MarkupParser.parseBuffer(MarkupParser.java:725)
    at org.attoparser.MarkupParser.parseDocument(MarkupParser.java:301)
    ... 88 more
Caused by: java.lang.IllegalStateException: Neither BindingResult nor plain target object for bean name 'answerForm' available as request attribute
    at org.springframework.web.servlet.support.BindStatus.<init>(BindStatus.java:153)
    at org.springframework.web.servlet.support.RequestContext.getBindStatus(RequestContext.java:926)
    at org.thymeleaf.spring6.context.webmvc.SpringWebMvcThymeleafRequestContext.getBindStatus(SpringWebMvcThymeleafRequestContext.java:232)
    at org.thymeleaf.spring6.util.FieldUtils.getBindStatusFromParsedExpression(FieldUtils.java:306)
    at org.thymeleaf.spring6.util.FieldUtils.getBindStatus(FieldUtils.java:253)
    at org.thymeleaf.spring6.util.FieldUtils.getBindStatus(FieldUtils.java:240)
    at org.thymeleaf.spring6.util.FieldUtils.getBindStatus(FieldUtils.java:227)
    at org.thymeleaf.spring6.util.FieldUtils.checkErrors(FieldUtils.java:212)
    at org.thymeleaf.spring6.util.FieldUtils.hasAnyErrors(FieldUtils.java:75)
    at org.thymeleaf.spring6.expression.Fields.hasAnyErrors(Fields.java:50)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:568)
    at org.springframework.expression.spel.support.ReflectiveMethodExecutor.execute(ReflectiveMethodExecutor.java:139)
    at org.springframework.expression.spel.ast.MethodReference.getValueInternal(MethodReference.java:112)
    at org.springframework.expression.spel.ast.MethodReference$MethodValueRef.getValue(MethodReference.java:380)
    at org.springframework.expression.spel.ast.CompoundExpression.getValueInternal(CompoundExpression.java:93)
    at org.springframework.expression.spel.ast.SpelNodeImpl.getValue(SpelNodeImpl.java:114)
    at org.springframework.expression.spel.standard.SpelExpression.getValue(SpelExpression.java:338)
    at org.thymeleaf.spring6.expression.SPELVariableExpressionEvaluator.evaluate(SPELVariableExpressionEvaluator.java:265)
    ... 123 more
    ```

    다음은 문제가 있으리라고 예상되는 파일들의 소스 입니다
    AnswerFrom.class

    ```
    package com.example.sbb.answer;

import jakarta.validation.constraints.NotEmpty;
import lombok.Getter;
import lombok.Setter;

@Getter
@Setter
public class AnswerForm {
    @NotEmpty(message = "content is essential")
    private String content;
}

templates/question_detail.html

    <html layout:decorate="~{layout}">
<div layout:fragment="content" class="container my-3">
    <h2 class="border-bottom py-2" th:text="${question.subject}"></h2>
    <div class="card my-3">
        <div class="card-body">
            <div class="card-text" style="white-space: pre-line;" th:text="${question.content}"></div>
            <div class="d-flex justify-content-end">
                <div class="badge bg-light text-dark p-2 text-start">
                    <div th:text="${#temporals.format(question.createDate, 'yyyy-MM-dd HH:mm')}"></div>
                </div>
            </div>
        </div>
    </div>
    <h5 class="border-bottom my-3 py-2" 
        th:text="|${#lists.size(question.answerList)}개의 답변이 있습니다.|"></h5>
    <div class="card my-3" th:each="answer : ${question.answerList}">
        <div class="card-body">
            <div class="card-text" style="white-space: pre-line;" th:text="${answer.content}"></div>
            <div class="d-flex justify-content-end">
                <div class="badge bg-light text-dark p-2 text-start">
                    <div th:text="${#temporals.format(answer.createDate, 'yyyy-MM-dd HH:mm')}"></div>
                </div>
            </div>
        </div>
    </div>
    <form th:action="@{|/answer/create/${question.id}|}" th:object="${answerForm}" method="post" class="my-3">
        <div th:replace="~{form_errors :: formErrorsFragment}"></div>
        <textarea th:field="*{content}" rows="10" class="form-control"></textarea>
        <input type="submit" value="답변등록" class="btn btn-primary my-2">
    </form>
</div>
</html>

form_errors.html

    <!DOCTYPE html>
<html xmlns:th="http://www.thymeleaf.org">
<head>
<meta charset="UTF-8">
<title>Insert title here</title>
</head>
<body>
    <div th:fragment="formErrorsFragment" class="alert alert-danger"
        role="alert" th:if="${#fields.hasAnyErrors()}">error!</div>
    <div th:each="err : ${#fields.allErrors()}" th:text="${err}"/>
</body>
</html>

아주 기초적인 문제인 것으로 보이는데 아직 제게는 어려운 문제입니다.
끝으로 좋은 강의를 무료로 보여주시고 사후 지원도 해주셔서 정말 감사합니다. 꼭 완강하고 싶습니다.