为什么在同一个thymeleaf模板html文件中，同一个对象不是null，而在其他地方是null

Question

请帮忙解决一个问题，谢谢。

下面是一个控制器：

@GetMapping(path="/getAllAccessoriesInPark")
 public String getAllAccessoriesInPark (
        @RequestParam(value = "parkId", required = true) int parkId,
        Model model,
        HttpServletRequest sRequest ) {
    if(!SessionHelper.authSession(sRequest, "show_accessory")) {
        return "redirect:/panel/login"; }
    HttpSession session = SessionHelper.getCurrentUserSession(sRequest);
    Iterable<Accessory> accessories = accessoryRepository.getByParkId(parkId);
    model.addAttribute("getAllAccessoriesInPark", accessories);
    return "panel/order/make"; }

这里是来自html文件的cobe：

<ul>
     <a th:name="parkId"
        th:href="@{/getAllAccessoriesInPark(parkId=${parkData.getPid()})}"
     ></a>

     <li  th:each="accessory: ${getAllAccessoriesInPark}">

         <div class="c-checkbox-wrapper quantityS">
              <input class="o-input o-input-checkbox quantitySocksMCheck" type="checkbox" id="${accessory.getPid()}" promo-id="3" name="orderType" value="standard">
              
              <label class="o-label-checkbox" for="${accessory.getPid()}"
                     th:text="${accessory.getName() + ' (' + accessory.getAccessoryPrice() + ' zł)'}">
                     Akcesoria
              </label>
              <div class="u-flex quantitySocksM">
                   <button class="o-button" option="minus"><span>-</span></button>
                   <input class="o-input" type="text" value="1" pattern="[0-9.]+">
                   <button class="o-button" option="plus"><span >+</span></button>
              </div>
         </div>
            
     </li>
</ul>

以下是一个错误(SpelEvaluationException:el1011e:方法调用：试图对空上下文对象调用方法getPid()):

021-09-24 11:48:24.951 ERROR 205027 --- [nio-8080-exec-1] org.thymeleaf.TemplateEngine             : [THYMELEAF][http-nio-8080-exec-1] Exception processing template "panel/order/make": An error happened during template parsing (template: "class path resource [templates/panel/order/make.html]")

org.thymeleaf.exceptions.TemplateInputException: An error happened during template parsing (template: "class path resource [templates/panel/order/make.html]")
at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parse(AbstractMarkupTemplateParser.java:241) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parseStandalone(AbstractMarkupTemplateParser.java:100) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.engine.TemplateManager.parseAndProcess(TemplateManager.java:666) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1098) [thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1072) [thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.spring5.view.ThymeleafView.renderFragment(ThymeleafView.java:354) [thymeleaf-spring5-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.spring5.view.ThymeleafView.render(ThymeleafView.java:187) [thymeleaf-spring5-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1325) [spring-webmvc-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1069) [spring-webmvc-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1008) [spring-webmvc-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:925) [spring-webmvc-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:978) [spring-webmvc-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:870) [spring-webmvc-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:635) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:855) [spring-webmvc-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:742) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) [tomcat-embed-websocket-8.5.28.jar:8.5.28]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.28.jar:8.5.28]
at fds.CorsFilter.doFilterInternal(CorsFilter.java:24) [classes/:na]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) [spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:109) [spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:81) [spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200) [spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) [spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:496) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:803) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:790) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1459) [tomcat-embed-core-8.5.28.jar:8.5.28]
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-8.5.28.jar:8.5.28]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_302]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_302]
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.5.28.jar:8.5.28]
at java.lang.Thread.run(Thread.java:748) [na:1.8.0_302]
Caused by: org.attoparser.ParseException: Exception evaluating SpringEL expression: "parkData.getPid()" (template: "panel/order/make" - line 296, col 44)
at org.attoparser.MarkupParser.parseDocument(MarkupParser.java:393) ~[attoparser-2.0.4.RELEASE.jar:2.0.4.RELEASE]
at org.attoparser.MarkupParser.parse(MarkupParser.java:257) ~[attoparser-2.0.4.RELEASE.jar:2.0.4.RELEASE]
at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parse(AbstractMarkupTemplateParser.java:230) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
... 56 common frames omitted
Caused by: org.thymeleaf.exceptions.TemplateProcessingException: Exception evaluating SpringEL expression: "parkData.getPid()" (template: "panel/order/make" - line 296, col 44)
at org.thymeleaf.spring5.expression.SPELVariableExpressionEvaluator.evaluate(SPELVariableExpressionEvaluator.java:290) ~[thymeleaf-spring5-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.standard.expression.VariableExpression.executeVariableExpression(VariableExpression.java:165) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.standard.expression.SimpleExpression.executeSimple(SimpleExpression.java:66) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.standard.expression.Expression.execute(Expression.java:109) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.standard.expression.Expression.execute(Expression.java:138) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.standard.expression.LinkExpression.resolveParameters(LinkExpression.java:337) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.standard.expression.LinkExpression.executeLinkExpression(LinkExpression.java:283) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.standard.expression.SimpleExpression.executeSimple(SimpleExpression.java:85) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.standard.expression.Expression.execute(Expression.java:109) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.standard.expression.Expression.execute(Expression.java:138) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.standard.processor.AbstractStandardExpressionAttributeTagProcessor.doProcess(AbstractStandardExpressionAttributeTagProcessor.java:126) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.processor.element.AbstractAttributeTagProcessor.doProcess(AbstractAttributeTagProcessor.java:74) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.processor.element.AbstractElementTagProcessor.process(AbstractElementTagProcessor.java:95) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.util.ProcessorConfigurationUtils$ElementTagProcessorWrapper.process(ProcessorConfigurationUtils.java:633) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.engine.ProcessorTemplateHandler.handleOpenElement(ProcessorTemplateHandler.java:1314) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.engine.TemplateHandlerAdapterMarkupHandler.handleOpenElementEnd(TemplateHandlerAdapterMarkupHandler.java:304) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.templateparser.markup.InlinedOutputExpressionMarkupHandler$InlineMarkupAdapterPreProcessorHandler.handleOpenElementEnd(InlinedOutputExpressionMarkupHandler.java:278) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.standard.inline.OutputExpressionInlinePreProcessorHandler.handleOpenElementEnd(OutputExpressionInlinePreProcessorHandler.java:186) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.thymeleaf.templateparser.markup.InlinedOutputExpressionMarkupHandler.handleOpenElementEnd(InlinedOutputExpressionMarkupHandler.java:124) ~[thymeleaf-3.0.9.RELEASE.jar:3.0.9.RELEASE]
at org.attoparser.HtmlElement.handleOpenElementEnd(HtmlElement.java:109) ~[attoparser-2.0.4.RELEASE.jar:2.0.4.RELEASE]
at org.attoparser.HtmlMarkupHandler.handleOpenElementEnd(HtmlMarkupHandler.java:297) ~[attoparser-2.0.4.RELEASE.jar:2.0.4.RELEASE]
at org.attoparser.MarkupEventProcessorHandler.handleOpenElementEnd(MarkupEventProcessorHandler.java:402) ~[attoparser-2.0.4.RELEASE.jar:2.0.4.RELEASE]
at org.attoparser.ParsingElementMarkupUtil.parseOpenElement(ParsingElementMarkupUtil.java:159) ~[attoparser-2.0.4.RELEASE.jar:2.0.4.RELEASE]
at org.attoparser.MarkupParser.parseBuffer(MarkupParser.java:710) ~[attoparser-2.0.4.RELEASE.jar:2.0.4.RELEASE]
at org.attoparser.MarkupParser.parseDocument(MarkupParser.java:301) ~[attoparser-2.0.4.RELEASE.jar:2.0.4.RELEASE]
... 58 common frames omitted
Caused by: org.springframework.expression.spel.SpelEvaluationException: EL1011E: Method call: Attempted to call method getPid() on null context object
at org.springframework.expression.spel.ast.MethodReference.throwIfNotNullSafe(MethodReference.java:149) ~[spring-expression-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.springframework.expression.spel.ast.MethodReference.getValueRef(MethodReference.java:78) ~[spring-expression-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.springframework.expression.spel.ast.CompoundExpression.getValueRef(CompoundExpression.java:67) ~[spring-expression-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.springframework.expression.spel.ast.CompoundExpression.getValueInternal(CompoundExpression.java:88) ~[spring-expression-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.springframework.expression.spel.ast.SpelNodeImpl.getValue(SpelNodeImpl.java:111) ~[spring-expression-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.springframework.expression.spel.standard.SpelExpression.getValue(SpelExpression.java:334) ~[spring-expression-5.0.4.RELEASE.jar:5.0.4.RELEASE]
at org.thymeleaf.spring5.expression.SPELVariableExpressionEvaluator.evaluate(SPELVariableExpressionEvaluator.java:263) ~[thymeleaf-spring5-3.0.9.RELEASE.jar:3.0.9.RELEASE]
... 82 common frames omitted

这个问题转化为如何从thymeleaf模板htlm文件中的控制器调用方法

Answer 1

它无法生成模板，因为当parkData为null时，这段代码生成了一个null指针：

<a 
  th:if="${parkData != null && parkData.getPid() != null}"
  th:name="parkId"
  th:href="@{/getAllAccessoriesInPark(parkId=${parkData.getPid()})}"
>
</a>

您必须将th:if拉出到父元素中，比如div。此外，还可以使用elivs运算符来验证公园数据-${parkData？:parkData.getpid（）}

有关更多信息，您可以查看Baeldung关于spring-thymeleaf-conditionals的文档