用ssm框架做联表查询，报出PersistenceException异常，求助如何解决
这是一个对Student和Classes两张表的联表查询，查所有学生信息，包括他们的班级名称。
表的关系很简单，c表的主键id与s表classid列的外键关联。Student类中有Classes cla属性。
以下是mapperxml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="cn.bdqn.dao.StuDao">
<resultMap type="Student" id="stuResultMap">
<id column="id" javaType="java.lang.Integer" property="sid"/>
<result column="name" property="sname"/>
<result column="telephone" property="telephone"/>
<result column="email" property="email"/>
<result column="age" property="age"/>
<association property="cla" javaType="Classes">
<result column="id" property="cid"/>
<result column="name" property="cname"/>
</association>
</resultMap>
<select id="selectAllStu" resultType="Student">
select s.id sid, s.name sname, s.telephone telephone,
s.email email,s.age age,c.name cname ,c.id cid
from student s left join classes c on s.classid=c.id
</select>
</mapper>
一下是执行action时候的报错信息。页面报错，后台不报错。
exception
org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.exceptions.PersistenceException:
### Error querying database. Cause: java.lang.NullPointerException
### The error may exist in mappers/StuDaoMapper.xml
### The error may involve cn.bdqn.dao.StuDao.selectAllStu
### The error occurred while executing a query
### Cause: java.lang.NullPointerException
org.mybatis.spring.MyBatisExceptionTranslator.translateExceptionIfPossible(MyBatisExceptionTranslator.java:73)
org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:364)
com.sun.proxy.$Proxy9.selectList(Unknown Source)
org.mybatis.spring.SqlSessionTemplate.selectList(SqlSessionTemplate.java:194)
org.apache.ibatis.binding.MapperMethod.executeForMany(MapperMethod.java:119)
org.apache.ibatis.binding.MapperMethod.execute(MapperMethod.java:63)
org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:52)
com.sun.proxy.$Proxy10.selectAllStu(Unknown Source)
cn.bdqn.service.impl.StuServiceImpl.selectAllStu(StuServiceImpl.java:21)
cn.bdqn.action.StuAction.toStuList(StuAction.java:19)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
java.lang.reflect.Method.invoke(Unknown Source)
com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:452)
com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:291)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:254)
com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:176)
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:263)
org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68)
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:133)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:207)
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:207)
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:190)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
org.apache.struts2.interceptor.MultiselectInterceptor.intercept(MultiselectInterceptor.java:75)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:94)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:243)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:100)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:141)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:267)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:142)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:166)
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:176)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:190)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:187)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:52)
org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:485)
org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77)
org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:91)
以前做过极为类似的联表查询，没出现过这个异常，鼓捣一会儿还不行，求助各位！
