具体报错信息：
1 Whitelabel Error Page 2 This application has no explicit mapping for /error, so you are seeing this as a fallback. 3 4 Mon Feb 01 21:33:07 CST 2021 5 There was an unexpected error (type=Internal Server Error, status=500). 6 An error happened during template parsing (template: "class path resource [templates/index.html]") 7 org.thymeleaf.exceptions.TemplateInputException: An error happened during template parsing (template: "class path resource [templates/index.html]") 8 at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parse(AbstractMarkupTemplateParser.java:241) 9 at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parseStandalone(AbstractMarkupTemplateParser.java:100) 10 at org.thymeleaf.engine.TemplateManager.parseAndProcess(TemplateManager.java:666) 11 at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1098) 12 at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1072) 13 at org.thymeleaf.spring5.view.ThymeleafView.renderFragment(ThymeleafView.java:362) 14 at org.thymeleaf.spring5.view.ThymeleafView.render(ThymeleafView.java:189) 15 at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1394) 16 at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1139) 17 at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1078) 18 at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:961) 19 at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) 20 at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898) 21 at javax.servlet.http.HttpServlet.service(HttpServlet.java:626) 22 at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883) 23 at javax.servlet.http.HttpServlet.service(HttpServlet.java:733) 24 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) 25 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) 26 at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) 27 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) 28 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) 29 at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) 30 at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) 31 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) 32 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) 33 at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) 34 at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) 35 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) 36 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) 37 at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) 38 at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) 39 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) 40 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) 41 at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202) 42 at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97) 43 at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:542) 44 at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:143) 45 at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) 46 at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78) 47 at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) 48 at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:374) 49 at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) 50 at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:888) 51 at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1597) 52 at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) 53 at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) 54 at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) 55 at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) 56 at java.lang.Thread.run(Thread.java:748) 57 Caused by: org.attoparser.ParseException: Exception evaluating SpringEL expression: "map.user.headerUrl" (template: "index" - line 123, col 13) 58 at org.attoparser.MarkupParser.parseDocument(MarkupParser.java:393) 59 at org.attoparser.MarkupParser.parse(MarkupParser.java:257) 60 at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parse(AbstractMarkupTemplateParser.java:230) 61 ... 48 more 62 Caused by: org.thymeleaf.exceptions.TemplateProcessingException: Exception evaluating SpringEL expression: "map.user.headerUrl" (template: "index" - line 123, col 13) 63 at org.thymeleaf.spring5.expression.SPELVariableExpressionEvaluator.evaluate(SPELVariableExpressionEvaluator.java:290) 64 at org.thymeleaf.standard.expression.VariableExpression.executeVariableExpression(VariableExpression.java:166) 65 at org.thymeleaf.standard.expression.SimpleExpression.executeSimple(SimpleExpression.java:66) 66 at org.thymeleaf.standard.expression.Expression.execute(Expression.java:109) 67 at org.thymeleaf.standard.expression.Expression.execute(Expression.java:138) 68 at org.thymeleaf.standard.processor.AbstractStandardExpressionAttributeTagProcessor.doProcess(AbstractStandardExpressionAttributeTagProcessor.java:144) 69 at org.thymeleaf.processor.element.AbstractAttributeTagProcessor.doProcess(AbstractAttributeTagProcessor.java:74) 70 at org.thymeleaf.processor.element.AbstractElementTagProcessor.process(AbstractElementTagProcessor.java:95) 71 at org.thymeleaf.util.ProcessorConfigurationUtils$ElementTagProcessorWrapper.process(ProcessorConfigurationUtils.java:633) 72 at org.thymeleaf.engine.ProcessorTemplateHandler.handleStandaloneElement(ProcessorTemplateHandler.java:918) 73 at org.thymeleaf.engine.StandaloneElementTag.beHandled(StandaloneElementTag.java:228) 74 at org.thymeleaf.engine.Model.process(Model.java:282) 75 at org.thymeleaf.engine.Model.process(Model.java:290) 76 at org.thymeleaf.engine.IteratedGatheringModelProcessable.processIterationModel(IteratedGatheringModelProcessable.java:367) 77 at org.thymeleaf.engine.IteratedGatheringModelProcessable.process(IteratedGatheringModelProcessable.java:293) 78 at org.thymeleaf.engine.ProcessorTemplateHandler.handleCloseElement(ProcessorTemplateHandler.java:1640) 79 at org.thymeleaf.engine.TemplateHandlerAdapterMarkupHandler.handleCloseElementEnd(TemplateHandlerAdapterMarkupHandler.java:388) 80 at org.thymeleaf.templateparser.markup.InlinedOutputExpressionMarkupHandler$InlineMarkupAdapterPreProcessorHandler.handleCloseElementEnd(InlinedOutputExpressionMarkupHandler.java:322) 81 at org.thymeleaf.standard.inline.OutputExpressionInlinePreProcessorHandler.handleCloseElementEnd(OutputExpressionInlinePreProcessorHandler.java:220) 82 at org.thymeleaf.templateparser.markup.InlinedOutputExpressionMarkupHandler.handleCloseElementEnd(InlinedOutputExpressionMarkupHandler.java:164) 83 at org.attoparser.HtmlElement.handleCloseElementEnd(HtmlElement.java:169) 84 at org.attoparser.HtmlMarkupHandler.handleCloseElementEnd(HtmlMarkupHandler.java:412) 85 at org.attoparser.MarkupEventProcessorHandler.handleCloseElementEnd(MarkupEventProcessorHandler.java:473) 86 at org.attoparser.ParsingElementMarkupUtil.parseCloseElement(ParsingElementMarkupUtil.java:201) 87 at org.attoparser.MarkupParser.parseBuffer(MarkupParser.java:725) 88 at org.attoparser.MarkupParser.parseDocument(MarkupParser.java:301) 89 ... 50 more 90 Caused by: org.springframework.expression.spel.SpelEvaluationException: EL1007E: Property or field 'headerUrl' cannot be found on null 91 at org.springframework.expression.spel.ast.PropertyOrFieldReference.readProperty(PropertyOrFieldReference.java:213) 92 at org.springframework.expression.spel.ast.PropertyOrFieldReference.getValueInternal(PropertyOrFieldReference.java:104) 93 at org.springframework.expression.spel.ast.PropertyOrFieldReference.access$000(PropertyOrFieldReference.java:51) 94 at org.springframework.expression.spel.ast.PropertyOrFieldReference$AccessorLValue.getValue(PropertyOrFieldReference.java:406) 95 at org.springframework.expression.spel.ast.CompoundExpression.getValueInternal(CompoundExpression.java:92) 96 at org.springframework.expression.spel.ast.SpelNodeImpl.getValue(SpelNodeImpl.java:112) 97 at org.springframework.expression.spel.standard.SpelExpression.getValue(SpelExpression.java:337) 98 at org.thymeleaf.spring5.expression.SPELVariableExpressionEvaluator.evaluate(SPELVariableExpressionEvaluator.java:263) 99 ... 75 more
其实这句：
An error happened during template parsing (template: "class path resource [templates/index.html]")
并没有什么用，只是告诉我们thymeleaf生成页面时出现了错误，具体错误还是在下面
Caused by: org.attoparser.ParseException: Exception evaluating SpringEL expression: "map.user.headerUrl" (template: "index" - line 123, col 13) ………… Caused by: org.thymeleaf.exceptions.TemplateProcessingException: Exception evaluating SpringEL expression: "map.user.headerUrl" (template: "index" - line 123, col 13) ………… Caused by: org.springframework.expression.spel.SpelEvaluationException: EL1007E: Property or field 'headerUrl' cannot be found on null …………
说明 map.user.headerUrl 这个参数有问题，
网上说将 ${map.user.headerUrl} 改为 #{map.user.headerUrl}
我改了后确实生成页面时不报错了，但是现实不出东西，如下图
查阅$和#这两个的区别后（https://www.cnblogs.com/hjwublog/p/5051632.html）
发现这两个东西作用完全不同，这样改根本没意义，只是不报错而已，治标不治本
然后我把代码版本倒退一天，发现还是这个错误，而且只有index页面有这个错误，其他用户登录注册等页面都没有问题。
最后我在数据库中发现最近发的几个帖子的user_id字段都是0
原来是我发帖时插入数据到数据库的时候没有设置user_id字段
