今天非常高兴，解决了一个关于java类转json时有关联对象而且困扰我很久的BUG

之前在做毕设的时候，在java类转json且有关联对象的时候总是会遇到如下的错误，真的是困扰我很久

 freemarker.template.TemplateModelException: Method public java.lang.String org.apache.commons.lang.exception.NestableRuntimeException.getMessage(int) threw an exception when invoked on net.sf.json.JSONException: java.lang.reflect.InvocationTargetException        at freemarker.ext.beans.SimpleMethodModel.exec(SimpleMethodModel.java:130)        at freemarker.ext.beans.SimpleMethodModel.get(SimpleMethodModel.java:138)        at freemarker.core.DynamicKeyName.dealWithNumericalKey(DynamicKeyName.java:111)        at freemarker.core.DynamicKeyName._getAsTemplateModel(DynamicKeyName.java:90)        at freemarker.core.Expression.getAsTemplateModel(Expression.java:89)        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.IfBlock.accept(IfBlock.java:82)        at freemarker.core.Environment.visit(Environment.java:221)        at freemarker.core.IteratorBlock$Context.runLoop(IteratorBlock.java:179)        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.IfBlock.accept(IfBlock.java:82)        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 org.apache.struts2.dispatcher.Dispatcher.sendError(Dispatcher.java:845)        at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:567)        at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77)        at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:91)        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)        at org.springframework.orm.hibernate3.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java:233)        at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)        at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)        at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243)        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:472)        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:99)        at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:953)        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)        at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1008)        at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:589)        at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:312)        at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)        at java.lang.Thread.run(Unknown Source)Caused by: java.lang.NullPointerException        at freemarker.ext.beans.SimpleMemberModel.unwrapArguments(SimpleMemberModel.java:86)        at freemarker.ext.beans.SimpleMethodModel.exec(SimpleMethodModel.java:106)        ... 51 more

之前每次都是明奇妙的解决掉了，连自己也不清楚到底什么原因，网上有说因为延迟加载问题，我也试了但是没解决，终于在昨天再次遇到了不过这次我却怎么也绕不过去，尝试千方百计，主要有以下几种

1. 延迟加载设为lazy，
2. 排除管对象
3. 类要是public

不过这几种方法都没能解决我的问题，今天终于发现问题最终出现在哪儿了，之前有过要获取某个关联对象中的某个属性，但是是关联对象又不能加入转换json因为会陷入死循环，所以我就加了一些get方法用于获取这些属性，问题就在这里！！！看下面这段代码，被涂色的就是我写的两个get方法用于获得某个关联对象的某个属性的，黄色是修改之前的写法，这种写法在关联对象确实存在的时候是不会出错的，但是一旦不存在就蒙蔽了，所以加了个判断如红色部分。

public class Teacher implements java.io.Serializable {// Fieldsprivate String tid;private Specialty specialty;//专业private Academy academy;//学院private String password;private String tname;private String tsex;//性别private Integer age;//年龄private String educationrecoder;//学历private String positiontitle;//职称private String tposition;//职位private String telephone;private Set<Graduationsubject> graduationsubjects = new HashSet<Graduationsubject>(0);//毕设题目private Set<MyRole> myRoles = new HashSet<MyRole>(0);private Set<Student> students = new HashSet<Student>(0);//指导学生private Set<Replygroup> replygroups = new HashSet<Replygroup>(0);//答辩小组public String getSpecialtyname(){return this.specialty.getSpname();}public String getAcademyname(){if(this.academy != null)return this.academy.getAcname();elsereturn "";}