FreeMarker template error
代码提测时，在Linux中的日志中出现如下错误：
2019-07-05 19:10:59 ERROR [freemarker.runtime:60] - Error executing FreeMarker template
FreeMarker template error:
The following has evaluated to null or missing:
==> error [in template "satisfiedScore/videoWitness.ftl" at line 47, column 24]
----
Tip: If the failing expression is known to be legally refer to something that's sometimes null or missing, either specify a default value like myOptionalVar!myDefault, or use <#if myOptionalVar??>when-present<#else>when-missing. (These only cover the last step of the expression; to cover the whole expression, use parenthesis: (myOptionalVar.foo)!myDefault, (myOptionalVar.foo)??
----
----
FTL stack trace ("~" means nesting-related):
- Failed at: ${error} [in template "satisfiedScore/videoWitness.ftl" at line 47, column 22]
----
Java stack trace (for programmers):
----
freemarker.core.InvalidReferenceException: [... Exception message was already printed; see it above ...]
at freemarker.core.InvalidReferenceException.getInstance(InvalidReferenceException.java:131)
at freemarker.core.EvalUtil.coerceModelToString(EvalUtil.java:355)
at freemarker.core.Expression.evalAndCoerceToString(Expression.java:82)
at freemarker.core.DollarVariable.accept(DollarVariable.java:41)
at freemarker.core.Environment.visit(Environment.java:324)
at freemarker.core.MixedContent.accept(MixedContent.java:54)
at freemarker.core.Environment.visit(Environment.java:324)
at freemarker.core.Environment.process(Environment.java:302)
at freemarker.template.Template.process(Template.java:325)
at org.springframework.web.servlet.view.freemarker.FreeMarkerView.processTemplate(FreeMarkerView.java:367)
at org.springframework.web.servlet.view.freemarker.FreeMarkerView.doRender(FreeMarkerView.java:284)
at org.springframework.web.servlet.view.freemarker.FreeMarkerView.renderMergedTemplateModel(FreeMarkerView.java:234)
at org.springframework.web.servlet.view.AbstractTemplateView.renderMergedOutputModel(AbstractTemplateView.java:167)
at org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:303)
at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1286)
at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1041)
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:984)
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:901)
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)
at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.shiro.web.servlet.ProxiedFilterChain.doFilter(ProxiedFilterChain.java:61)
at org.apache.shiro.web.servlet.AdviceFilter.executeChain(AdviceFilter.java:108)
at org.apache.shiro.web.servlet.AdviceFilter.doFilterInternal(AdviceFilter.java:137)
at org.apache.shiro.web.servlet.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:125)
at org.apache.shiro.web.servlet.ProxiedFilterChain.doFilter(ProxiedFilterChain.java:66)
at org.apache.shiro.web.servlet.AbstractShiroFilter.executeChain(AbstractShiroFilter.java:449)
at org.apache.shiro.web.servlet.AbstractShiroFilter$1.call(AbstractShiroFilter.java:365)
at org.apache.shiro.subject.support.SubjectCallable.doCall(SubjectCallable.java:90)
at org.apache.shiro.subject.support.SubjectCallable.call(SubjectCallable.java:83)
at org.apache.shiro.subject.support.DelegatingSubject.execute(DelegatingSubject.java:383)
at org.apache.shiro.web.servlet.AbstractShiroFilter.doFilterInternal(AbstractShiroFilter.java:362)
at org.apache.shiro.web.servlet.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:125)
at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:346)
at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:262)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:242)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at com.tebon.trans.filter.EnviromentSetFilter.doFilter(EnviromentSetFilter.java:57)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:242)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at com.tebon.trans.filter.HttpToHttpsFilter.doFilter(HttpToHttpsFilter.java:87)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:242)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:197)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:242)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:243)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:201)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:163)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:108)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:556)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:401)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:242)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:267)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:245)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:260)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)简单的说，就是前端模板变量写的有问题，例如变量可能未定义或为null，
例如在前端模板的js内定义变量：
var test = ${test}解决方法：
在未声明的变量后面增加叹号
var test = ${test!}或者设置默认值,在叹号后面增加默认值
var test = ${test!'TEST'}扩展介绍：
FreeMarker 是一款 模板引擎： 即一种基于模板和要改变的数据， 并用来生成输出文本(HTML网页，电子邮件，配置文件，源代码等)的通用工具。 它不是面向最终用户的，而是一个Java类库，是一款程序员可以嵌入他们所开发产品的组件。
模板编写为FreeMarker Template Language (FTL)。它是简单的，专用的语言， 不是 像PHP那样成熟的编程语言。 那就意味着要准备数据在真实编程语言中来显示，比如数据库查询和业务运算， 之后模板显示已经准备好的数据。在模板中，你可以专注于如何展现数据， 而在模板之外可以专注于要展示什么数据。
这种方式通常被称为 MVC (模型 视图 控制器) 模式，对于动态网页来说，是一种特别流行的模式。 它帮助从开发人员(Java 程序员)中分离出网页设计师(HTML设计师)。设计师无需面对模板中的复杂逻辑， 在没有程序员来修改或重新编译代码时，也可以修改页面的样式。
以上引自free marker手册：http://freemarker.foofun.cn/
参考链接：
http://freemarker.foofun.cn/
https://hw1287789687.iteye.com/blog/2263466