案例一

1.1、错误描述

 五月 28, 2014 9:56:48 下午 freemarker.log.JDK14LoggerFactory$JDK14Logger error
 严重: Template processing error: "Expression student.course is undefined on line 15, column 24 in course.ftl."

 Expression student.course is undefined on line 15, column 24 in course.ftl.
 The problematic instruction:
 ----------
 ==> ${student.course} [on line 15, column 22 in course.ftl]
 ----------

 Java backtrace for programmers:
 ----------
 freemarker.core.InvalidReferenceException: Expression student.course is undefined on line 15, column 24 in course.ftl.
     at freemarker.core.TemplateObject.assertNonNull(TemplateObject.java:125)
     at freemarker.core.Expression.getStringValue(Expression.java:118)
     at freemarker.core.Expression.getStringValue(Expression.java:93)
     at freemarker.core.DollarVariable.accept(DollarVariable.java:76)
     at freemarker.core.Environment.visit(Environment.java:221)
     at freemarker.core.MixedContent.accept(MixedContent.java:92)
     at freemarker.core.Environment.visit(Environment.java:221)
     at freemarker.core.Environment.process(Environment.java:199)
     at freemarker.template.Template.process(Template.java:259)
     at com.you.freemarker.FreemarkerTemplate.printFtl(FreemarkerTemplate.java:77)
     at com.you.test.freemarker.FreemarkerTest.studentPrint(FreemarkerTest.java:141)
     at com.you.test.freemarker.FreemarkerTest.testCourse(FreemarkerTest.java:128)
     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
     at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
     at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
     at java.lang.reflect.Method.invoke(Unknown Source)
     at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:45)
     at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
     at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42)
     at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
     at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
     at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:263)
     at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:68)
     at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:47)
     at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231)
     at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60)
     at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229)
     at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50)
     at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222)
     at org.junit.runners.ParentRunner.run(ParentRunner.java:300)
     at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
     at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

 Expression student.course is undefined on line 15, column 24 in course.ftl.
 The problematic instruction:
 ----------
 ==> ${student.course} [on line 15, column 22 in course.ftl]
 ----------

 Java backtrace for programmers:
 ----------
 freemarker.core.InvalidReferenceException: Expression student.course is undefined on line 15, column 24 in course.ftl.
     at freemarker.core.TemplateObject.assertNonNull(TemplateObject.java:125)
     at freemarker.core.Expression.getStringValue(Expression.java:118)
     at freemarker.core.Expression.getStringValue(Expression.java:93)
     at freemarker.core.DollarVariable.accept(DollarVariable.java:76)
     at freemarker.core.Environment.visit(Environment.java:221)
     at freemarker.core.MixedContent.accept(MixedContent.java:92)
     at freemarker.core.Environment.visit(Environment.java:221)
     at freemarker.core.Environment.process(Environment.java:199)
     at freemarker.template.Template.process(Template.java:259)
     at com.you.freemarker.FreemarkerTemplate.printFtl(FreemarkerTemplate.java:77)
     at com.you.test.freemarker.FreemarkerTest.studentPrint(FreemarkerTest.java:141)
     at com.you.test.freemarker.FreemarkerTest.testCourse(FreemarkerTest.java:128)
     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
     at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
     at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
     at java.lang.reflect.Method.invoke(Unknown Source)
     at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:45)
     at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
     at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42)
     at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
     at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
     at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:263)
     at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:68)
     at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:47)
     at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231)
     at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60)
     at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229)
     at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50)
     at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222)
     at org.junit.runners.ParentRunner.run(ParentRunner.java:300)
     at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
     at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)
 <html>
   <head>
     <meta http-equiv="content-type" content="text/html; charset=UTF-8">
     <title>学生信息</title>

   </head>

   <body>
         姓名：张三丰
          性别：男
          年龄：16
          生日：1970-01-01
         地址：湖北省武汉市武昌洪山区
           QQ：78,451,214
                   课程：
 Expression student.course is undefined on line 15, column 24 in course.ftl.
 The problematic instruction:
 ----------
 ==> ${student.course} [on line 15, column 22 in course.ftl]
 ----------

 Java backtrace for programmers:
 ----------
 freemarker.core.InvalidReferenceException: Expression student.course is undefined on line 15, column 24 in course.ftl.
     at freemarker.core.TemplateObject.assertNonNull(TemplateObject.java:125)
     at freemarker.core.Expression.getStringValue(Expression.java:118)
     at freemarker.core.Expression.getStringValue(Expression.java:93)
     at freemarker.core.DollarVariable.accept(DollarVariable.java:76)
     at freemarker.core.Environment.visit(Environment.java:221)
     at freemarker.core.MixedContent.accept(MixedContent.java:92)
     at freemarker.core.Environment.visit(Environment.java:221)
     at freemarker.core.Environment.process(Environment.java:199)
     at freemarker.template.Template.process(Template.java:259)
     at com.you.freemarker.FreemarkerTemplate.printFtl(FreemarkerTemplate.java:77)
     at com.you.test.freemarker.FreemarkerTest.studentPrint(FreemarkerTest.java:141)
     at com.you.test.freemarker.FreemarkerTest.testCourse(FreemarkerTest.java:128)
     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
     at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
     at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
     at java.lang.reflect.Method.invoke(Unknown Source)
     at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:45)
     at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
     at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42)
     at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
     at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
     at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:263)
     at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:68)
     at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:47)
     at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231)
     at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60)
     at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229)
     at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50)
     at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222)
     at org.junit.runners.ParentRunner.run(ParentRunner.java:300)
     at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
     at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

1.2、错误原因

Student类中的course属性没有赋值，而freemarker对为空的属性就会报错

  课程：${student.course}

1.3、解决办法

在属性值后添加“!”

二，案例二

2.1，分析

上面的案例的前提条件是student不能为空，假如现在student为空了怎么办。

2.2，解决方案

freemarker.core.InvalidReferenceException: Expression ctt.contentType is undefined on line 46, column 91 in WEB-INF/ftl_lib/cms/style_content/style1-1_core.ftl.
百度，google半天只说ftl文件中不能带人空值，但不知如何在ftl文件中判断为空，然后容掉这个错误。后来终于找到方法：
[#if ctt.contentType??][${ctt.contentType.name} - ${ctt.ctgName}] [#else][${ctt.ctgName}][/#if]
“??”判断是否为空。
记下方便以后备用，同样希望给遇到同样问题的人带来帮助。

这种解决方案就是加一个if判断就好了。

三，案例三

1 <#list books as book>
2         <tr>
3         <td>书名:</td>
4         <td>${book}</td>
5         </tr>
6         </#list>

报错

 1 freemarker.core.InvalidReferenceException: Expression books is undefined on line 9, column 24 in WEB-INF/ftl/showBook.ftl.
 2     at freemarker.core.TemplateObject.assertNonNull(TemplateObject.java:125)
 3     at freemarker.core.IteratorBlock.accept(IteratorBlock.java:100)
 4     at freemarker.core.Environment.visit(Environment.java:221)
 5     at freemarker.core.MixedContent.accept(MixedContent.java:92)
 6     at freemarker.core.Environment.visit(Environment.java:221)
 7     at freemarker.core.Environment.process(Environment.java:199)
 8     at freemarker.template.Template.process(Template.java:259)
 9     at org.apache.struts2.views.freemarker.FreemarkerResult.doExecute(FreemarkerResult.java:201)
10     at org.apache.struts2.dispatcher.StrutsResultSupport.execute(StrutsResultSupport.java:186)
11     at com.opensymphony.xwork2.DefaultActionInvocation.executeResult(DefaultActionInvocation.java:374)
12     at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:278)
13     at org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:256)
14     at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
15     at com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:176)
16     at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
17     at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
18     at com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:265)
19     at org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68)
20     at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
21     at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
22     at com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:138)
23     at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
24     at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:211)
25     at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
26     at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
27     at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:211)
28     at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
29     at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
30     at com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:190)
31     at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
32     at org.apache.struts2.interceptor.MultiselectInterceptor.intercept(MultiselectInterceptor.java:75)
33     at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
34     at org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:90)
35     at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
36     at org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:243)
37     at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
38     at com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:100)
39     at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
40     at com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:141)
41     at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
42     at com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:145)
43     at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
44     at com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:171)
45     at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
46     at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
47     at com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:176)
48     at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
49     at org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164)
50     at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
51     at com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:192)
52     at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
53     at com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:187)
54     at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
55     at org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:54)
56     at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:511)
57     at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77)
58     at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:91)
59     at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
60     at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
61     at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
62     at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
63     at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
64     at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
65     at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
66     at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
67     at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:857)
68     at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
69     at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
70     at java.lang.Thread.run(Thread.java:619)
71 2012-11-26 14:11:33 org.apache.catalina.core.StandardWrapperValve invoke

解决办法是：因为ftl是不能传入空值的。如果有空值就会报这个的错误：

只要做一下判断就行了

<#if books ??>

</#if>

代码如下：

1 <#if books ??>
2         <#list books as book>
3         <tr>
4         <td>书名:</td>
5         <td>${book}</td>
6         </tr>
7         </#list>
8         </#if>

四，案例四

4，1、错误描述

 六月 04, 2014 11:04:03 下午 freemarker.log.JDK14LoggerFactory$JDK14Logger error
 严重: Template processing error: "Expression maps.key is undefined on line 15, column 25 in map.ftl."

 Expression maps.key is undefined on line 15, column 25 in map.ftl.
 The problematic instruction:
 ----------
 ==> ${maps.key} [on line 15, column 23 in map.ftl]
 ----------

 Java backtrace for programmers:
 ----------
 freemarker.core.InvalidReferenceException: Expression maps.key is undefined on line 15, column 25 in map.ftl.
     at freemarker.core.TemplateObject.assertNonNull(TemplateObject.java:125)
     at freemarker.core.Expression.getStringValue(Expression.java:118)
     at freemarker.core.Expression.getStringValue(Expression.java:93)
     at freemarker.core.DollarVariable.accept(DollarVariable.java:76)
     at freemarker.core.Environment.visit(Environment.java:221)
     at freemarker.core.MixedContent.accept(MixedContent.java:92)
     at freemarker.core.Environment.visit(Environment.java:221)
     at freemarker.core.IteratorBlock$Context.runLoop(IteratorBlock.java:167)
     at freemarker.core.Environment.visit(Environment.java:428)
     at freemarker.core.IteratorBlock.accept(IteratorBlock.java:102)
     at freemarker.core.Environment.visit(Environment.java:221)
     at freemarker.core.MixedContent.accept(MixedContent.java:92)
     at freemarker.core.Environment.visit(Environment.java:221)
     at freemarker.core.Environment.process(Environment.java:199)
     at freemarker.template.Template.process(Template.java:259)
     at com.you.freemarker.FreemarkerTemplate.printFtl(FreemarkerTemplate.java:77)
     at com.you.test.freemarker.FreemarkerTest.studentPrint(FreemarkerTest.java:210)
     at com.you.test.freemarker.FreemarkerTest.testMap(FreemarkerTest.java:197)
     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
     at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
     at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
     at java.lang.reflect.Method.invoke(Unknown Source)
     at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:45)
     at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
     at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42)
     at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
     at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
     at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:263)
     at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:68)
     at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:47)
     at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231)
     at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60)
     at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229)
     at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50)
     at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222)
     at org.junit.runners.ParentRunner.run(ParentRunner.java:300)
     at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
     at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

 Expression maps.key is undefined on line 15, column 25 in map.ftl.
 The problematic instruction:
 ----------
 ==> ${maps.key} [on line 15, column 23 in map.ftl]
 ----------

 Java backtrace for programmers:
 ----------
 freemarker.core.InvalidReferenceException: Expression maps.key is undefined on line 15, column 25 in map.ftl.
     at freemarker.core.TemplateObject.assertNonNull(TemplateObject.java:125)
     at freemarker.core.Expression.getStringValue(Expression.java:118)
     at freemarker.core.Expression.getStringValue(Expression.java:93)
     at freemarker.core.DollarVariable.accept(DollarVariable.java:76)
     at freemarker.core.Environment.visit(Environment.java:221)
     at freemarker.core.MixedContent.accept(MixedContent.java:92)
     at freemarker.core.Environment.visit(Environment.java:221)
     at freemarker.core.IteratorBlock$Context.runLoop(IteratorBlock.java:167)
     at freemarker.core.Environment.visit(Environment.java:428)
     at freemarker.core.IteratorBlock.accept(IteratorBlock.java:102)
     at freemarker.core.Environment.visit(Environment.java:221)
     at freemarker.core.MixedContent.accept(MixedContent.java:92)
     at freemarker.core.Environment.visit(Environment.java:221)
     at freemarker.core.Environment.process(Environment.java:199)
     at freemarker.template.Template.process(Template.java:259)
     at com.you.freemarker.FreemarkerTemplate.printFtl(FreemarkerTemplate.java:77)
     at com.you.test.freemarker.FreemarkerTest.studentPrint(FreemarkerTest.java:210)
     at com.you.test.freemarker.FreemarkerTest.testMap(FreemarkerTest.java:197)
     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
     at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
     at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
     at java.lang.reflect.Method.invoke(Unknown Source)
     at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:45)
     at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
     at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42)
     at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
     at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
     at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:263)
     at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:68)
     at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:47)
     at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231)
     at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60)
     at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229)
     at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50)
     at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222)
     at org.junit.runners.ParentRunner.run(ParentRunner.java:300)
     at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
     at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)
 <html>
   <head>
     <meta http-equiv="content-type" content="text/html; charset=UTF-8">
     <title>freemarker处理哈希表的内建函数</title>

   </head>

   <body>
           张三强

         1----
 Expression maps.key is undefined on line 15, column 25 in map.ftl.
 The problematic instruction:
 ----------
 ==> ${maps.key} [on line 15, column 23 in map.ftl]
 ----------

 Java backtrace for programmers:
 ----------
 freemarker.core.InvalidReferenceException: Expression maps.key is undefined on line 15, column 25 in map.ftl.
     at freemarker.core.TemplateObject.assertNonNull(TemplateObject.java:125)
     at freemarker.core.Expression.getStringValue(Expression.java:118)
     at freemarker.core.Expression.getStringValue(Expression.java:93)
     at freemarker.core.DollarVariable.accept(DollarVariable.java:76)
     at freemarker.core.Environment.visit(Environment.java:221)
     at freemarker.core.MixedContent.accept(MixedContent.java:92)
     at freemarker.core.Environment.visit(Environment.java:221)
     at freemarker.core.IteratorBlock$Context.runLoop(IteratorBlock.java:167)
     at freemarker.core.Environment.visit(Environment.java:428)
     at freemarker.core.IteratorBlock.accept(IteratorBlock.java:102)
     at freemarker.core.Environment.visit(Environment.java:221)
     at freemarker.core.MixedContent.accept(MixedContent.java:92)
     at freemarker.core.Environment.visit(Environment.java:221)
     at freemarker.core.Environment.process(Environment.java:199)
     at freemarker.template.Template.process(Template.java:259)
     at com.you.freemarker.FreemarkerTemplate.printFtl(FreemarkerTemplate.java:77)
     at com.you.test.freemarker.FreemarkerTest.studentPrint(FreemarkerTest.java:210)
     at com.you.test.freemarker.FreemarkerTest.testMap(FreemarkerTest.java:197)
     at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
     at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
     at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
     at java.lang.reflect.Method.invoke(Unknown Source)
     at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:45)
     at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
     at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42)
     at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
     at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
     at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:263)
     at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:68)
     at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:47)
     at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231)
     at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60)
     at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229)
     at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50)
     at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222)
     at org.junit.runners.ParentRunner.run(ParentRunner.java:300)
     at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
     at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
     at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

4.2、错误原因

 <#assign maps={"1":"张三丰","2":"李思思","3":"张三强","4":"王五"}>
      ${maps["3"]}

      <#assign keys=maps?keys>
      <#list keys as key>
         ${key}----${maps.key}
      </#list>

其中，${maps.key}未定义，获取hashmap中的描述不能这样取值

4.3、解决办法

 <#--freemarker HashMap取值-->
      <#assign maps={"1":"张三丰","2":"李思思","3":"张三强","4":"王五"}>
      ${maps["3"]}

      <#assign keys=maps?keys>
      <#list keys as key>
         ${key}----${maps[key]}
      </#list>

正确取值：${key}----${maps[key]}

maps.key改成

maps[key]

运行结果：

 <html>
   <head>
     <meta http-equiv="content-type" content="text/html; charset=UTF-8">
     <title>freemarker处理哈希表的内建函数</title>

   </head>

   <body>
           张三强

         1----张三丰
         2----李思思
         3----张三强
         4----王五
   </body>
 </html>

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异常-----freemarker.core.InvalidReferenceException问题解决

案例一