在Spring Roo生成的项目中调用update方法时，我正在遇到NullPointerException。
尝试更新JPA实体（该对象具有带有日历字段的嵌套对象）时，将引发异常。仅当嵌套实体不为null时才发生此异常，以便应用延迟加载并且嵌套实体的日历字段为null。
这是我的roo命令：
project setup --topLevelPackage de.dk.test
jpa setup --provider HIBERNATE --database HYPERSONIC_PERSISTENT
entity jpa --class ~.model.Student --serializable
field string --fieldName username --notNull --unique
entity jpa --class ~.model.Course --serializable --entityFormatExpression "#{description} "
field string --fieldName description --notNull
field date --fieldName courseBeginn --type java.util.Calendar
field set --fieldName students --type ~.model.Student --cardinality ONE_TO_MANY --aggregation --joinTable my_courses_students --joinColumns my_courses --referencedColumns id --inverseJoinColumns my_students --inverseReferencedColumns id
repository jpa --all
service --all
// Spring MVC setup
web mvc setup
// JSON Rest controllers
web mvc controller --all --responseType JSON --pathPrefix /api
// THYMELEAF views
web mvc view setup --type THYMELEAF
web mvc controller --all --responseType THYMELEAF
/// Include templates inside generated project to allow developers to customize them
web mvc templates setup --type THYMELEAF
push-in --all
push-in --all --force
尝试致电 http://127.0.0.1:8080/students/1/edit-form 时，我看到了例外情况：
2019-03-11 10:20:46.934 ERROR 58 --- [nio-8080-exec-6] o.a.c.c.C.[.[.[/].[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.thymeleaf.exceptions.TemplateInputException: An error happened during template parsing (template: "class path resource [templates/students/edit.html]")] with root cause
java.lang.NullPointerException: null
at de.dk.test.model.Course.toString(Course.java:235) ~[classes/:na]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_181]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_181]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_181]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_181]
at org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer.invoke(JavassistLazyInitializer.java:84) ~[hibernate-core-5.0.11.Final.jar:5.0.11.Final]
at de.dk.test.model.Course_$$_jvst670_0.toString(Course_$$_jvst670_0.java) ~[classes/:na]
at org.springframework.util.ObjectUtils.nullSafeToString(ObjectUtils.java:660) ~[spring-core-4.3.3.RELEASE.jar:4.3.3.RELEASE]
at org.springframework.util.ObjectUtils.getDisplayString(ObjectUtils.java:606) ~[spring-core-4.3.3.RELEASE.jar:4.3.3.RELEASE]
at org.springframework.web.servlet.tags.form.SelectedValueComparator.exhaustiveCompare(SelectedValueComparator.java:162) ~[spring-webmvc-4.3.3.RELEASE.jar:3.0.0.RELEASE]
at org.springframework.web.servlet.tags.form.SelectedValueComparator.isSelected(SelectedValueComparator.java:98) ~[spring-webmvc-4.3.3.RELEASE.jar:3.0.0.RELEASE]
at org.springframework.web.servlet.tags.form.SelectedValueComparatorWrapper.isSelected(SelectedValueComparatorWrapper.java:34) ~[thymeleaf-spring4-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.spring4.processor.SpringOptionFieldTagProcessor.doProcess(SpringOptionFieldTagProcessor.java:68) ~[thymeleaf-spring4-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.spring4.processor.AbstractSpringFieldTagProcessor.doProcess(AbstractSpringFieldTagProcessor.java:179) ~[thymeleaf-spring4-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.processor.element.AbstractAttributeTagProcessor.doProcess(AbstractAttributeTagProcessor.java:74) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.processor.element.AbstractElementTagProcessor.process(AbstractElementTagProcessor.java:95) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.util.ProcessorConfigurationUtils$ElementTagProcessorWrapper.process(ProcessorConfigurationUtils.java:633) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.engine.ProcessorTemplateHandler.handleOpenElement(ProcessorTemplateHandler.java:1304) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.engine.OpenElementTag.beHandled(OpenElementTag.java:205) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.engine.Model.process(Model.java:282) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.engine.ProcessorTemplateHandler.handleOpenElement(ProcessorTemplateHandler.java:1567) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.engine.OpenElementTag.beHandled(OpenElementTag.java:205) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.engine.Model.process(Model.java:282) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.engine.Model.process(Model.java:290) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.engine.GatheringModelProcessable.process(GatheringModelProcessable.java:78) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.engine.ProcessorTemplateHandler.handleCloseElement(ProcessorTemplateHandler.java:1620) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.engine.CloseElementTag.beHandled(CloseElementTag.java:139) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.engine.Model.process(Model.java:282) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.engine.ProcessorTemplateHandler.handleOpenElement(ProcessorTemplateHandler.java:1567) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.engine.OpenElementTag.beHandled(OpenElementTag.java:205) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.engine.Model.process(Model.java:282) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.engine.Model.process(Model.java:290) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.engine.GatheringModelProcessable.process(GatheringModelProcessable.java:78) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.engine.ProcessorTemplateHandler.handleCloseElement(ProcessorTemplateHandler.java:1620) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.engine.TemplateHandlerAdapterMarkupHandler.handleCloseElementEnd(TemplateHandlerAdapterMarkupHandler.java:388) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.templateparser.markup.InlinedOutputExpressionMarkupHandler$InlineMarkupAdapterPreProcessorHandler.handleCloseElementEnd(InlinedOutputExpressionMarkupHandler.java:322) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.standard.inline.OutputExpressionInlinePreProcessorHandler.handleCloseElementEnd(OutputExpressionInlinePreProcessorHandler.java:220) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.templateparser.markup.InlinedOutputExpressionMarkupHandler.handleCloseElementEnd(InlinedOutputExpressionMarkupHandler.java:164) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.attoparser.HtmlElement.handleCloseElementEnd(HtmlElement.java:169) ~[attoparser-2.0.0.RELEASE.jar:2.0.0.RELEASE]
at org.attoparser.HtmlMarkupHandler.handleCloseElementEnd(HtmlMarkupHandler.java:412) ~[attoparser-2.0.0.RELEASE.jar:2.0.0.RELEASE]
at org.attoparser.MarkupEventProcessorHandler.handleCloseElementEnd(MarkupEventProcessorHandler.java:473) ~[attoparser-2.0.0.RELEASE.jar:2.0.0.RELEASE]
at org.attoparser.ParsingElementMarkupUtil.parseCloseElement(ParsingElementMarkupUtil.java:201) ~[attoparser-2.0.0.RELEASE.jar:2.0.0.RELEASE]
at org.attoparser.MarkupParser.parseBuffer(MarkupParser.java:725) ~[attoparser-2.0.0.RELEASE.jar:2.0.0.RELEASE]
at org.attoparser.MarkupParser.parseDocument(MarkupParser.java:301) ~[attoparser-2.0.0.RELEASE.jar:2.0.0.RELEASE]
at org.attoparser.MarkupParser.parse(MarkupParser.java:257) ~[attoparser-2.0.0.RELEASE.jar:2.0.0.RELEASE]
at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parse(AbstractMarkupTemplateParser.java:230) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parseStandalone(AbstractMarkupTemplateParser.java:100) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.engine.TemplateManager.parseAndProcess(TemplateManager.java:667) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1087) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1061) ~[thymeleaf-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.spring4.view.ThymeleafView.renderFragment(ThymeleafView.java:335) ~[thymeleaf-spring4-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.thymeleaf.spring4.view.ThymeleafView.render(ThymeleafView.java:189) ~[thymeleaf-spring4-3.0.0.RELEASE.jar:3.0.0.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1257) ~[spring-webmvc-4.3.3.RELEASE.jar:4.3.3.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1037) ~[spring-webmvc-4.3.3.RELEASE.jar:4.3.3.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:980) ~[spring-webmvc-4.3.3.RELEASE.jar:4.3.3.RELEASE]
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:897) ~[spring-webmvc-4.3.3.RELEASE.jar:4.3.3.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970) ~[spring-webmvc-4.3.3.RELEASE.jar:4.3.3.RELEASE]
at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861) ~[spring-webmvc-4.3.3.RELEASE.jar:4.3.3.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:622) ~[tomcat-embed-core-8.5.5.jar:8.5.5]
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846) ~[spring-webmvc-4.3.3.RELEASE.jar:4.3.3.RELEASE]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729) ~[tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:230) ~[tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165) ~[tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) ~[tomcat-embed-websocket-8.5.5.jar:8.5.5]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:192) ~[tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165) ~[tomcat-embed-core-8.5.5.jar:8.5.5]
at org.springframework.web.servlet.resource.ResourceUrlEncodingFilter.doFilterInternal(ResourceUrlEncodingFilter.java:53) ~[spring-webmvc-4.3.3.RELEASE.jar:4.3.3.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.3.3.RELEASE.jar:4.3.3.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:192) ~[tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165) ~[tomcat-embed-core-8.5.5.jar:8.5.5]
at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) ~[spring-web-4.3.3.RELEASE.jar:4.3.3.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.3.3.RELEASE.jar:4.3.3.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:192) ~[tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165) ~[tomcat-embed-core-8.5.5.jar:8.5.5]
at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:89) ~[spring-web-4.3.3.RELEASE.jar:4.3.3.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.3.3.RELEASE.jar:4.3.3.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:192) ~[tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165) ~[tomcat-embed-core-8.5.5.jar:8.5.5]
at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:77) ~[spring-web-4.3.3.RELEASE.jar:4.3.3.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.3.3.RELEASE.jar:4.3.3.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:192) ~[tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165) ~[tomcat-embed-core-8.5.5.jar:8.5.5]
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:197) ~[spring-web-4.3.3.RELEASE.jar:4.3.3.RELEASE]
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-4.3.3.RELEASE.jar:4.3.3.RELEASE]
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:192) ~[tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165) ~[tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198) ~[tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:108) [tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472) [tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) [tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79) [tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:349) [tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:784) [tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:802) [tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1410) [tomcat-embed-core-8.5.5.jar:8.5.5]
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-8.5.5.jar:8.5.5]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_181]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_181]
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.5.5.jar:8.5.5]
at java.lang.Thread.run(Thread.java:748) [na:1.8.0_181]
这是相关代码（由Roo完全生成）：
public String toString() {
return "Course {" + "id='" + id + '\'' + ", version='" + version + '\'' + ", description='" + description + '\'' + ", courseBeginn='" + courseBeginn == null ? null : java.text.DateFormat.getDateTimeInstance().format(courseBeginn.getTime()) + '\'' + "}" + super.toString();
}
据我了解，该问题已经in this thread进行了讨论，但尚未解决。
更新
这将引发java.lang.NullPointerException：
courseBeginn.getTime()
你能解释为什么空检查不起作用吗？
courseBeginn == null ? null : java.text.DateFormat.getDateTimeInstance().format(courseBeginn.getTime())
有工作要解决吗？请指教。预先感谢！