关于struts2的checkboxlist、select等标签发生could not be resolved as a collection/array/ma

今天研究struts2的标签功能，在用checkboxlist的时候，
代码如下

<s:select name="cardsortId" list="cardsortList" listKey="csId" listValue="cardname" theme="simple"></s:select>

 出现了如下异常：

org.apache.jasper.JasperException: tag 'select', field 'cardsortList', name 'cardsortId': The requested list key 'roles' could not be resolved as a collection/array/map/enumeration/iterator type. Example: people or people.{name} - [unknown location] org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:541) org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:435) org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:320) org.apache.jasper.servlet.JspServlet.service(JspServlet.java:266) javax.servlet.http.HttpServlet.service(HttpServlet.java:803) org.apache.struts2.dispatcher.ServletDispatcherResult.doExecute(ServletDispatcherResult.java:154) org.apache.struts2.dispatcher.StrutsResultSupport.execute(StrutsResultSupport.java:186) com.opensymphony.xwork2.DefaultActionInvocation.executeResult(DefaultActionInvocation.java:361) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:265) com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:249) org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68) com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236) com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:122) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236) com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:195) com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236) com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:195) com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236) com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:148) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236) org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:93) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236) org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:235) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236) com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:89) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236) com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:128) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236) org.apache.struts2.interceptor.ProfilingActivationInterceptor.intercept(ProfilingActivationInterceptor.java:104) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236) org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:267) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236) com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:126) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236) com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:138) com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236) com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:148) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236) org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236) com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:128) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236) com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:176) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236) org.apache.struts2.interceptor.TokenInterceptor.handleValidToken(TokenInterceptor.java:178) org.apache.struts2.interceptor.TokenInterceptor.doIntercept(TokenInterceptor.java:144) com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87) com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236) org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:52) org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:468) org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77) org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:76)

 出现这个错误的原因可能如下：

1 刚进入该界面的时候发生错误，原因是 list="roles"中的这个集合是空的，导致错误

解决办法很简单，不能让list为空

2 刚进入该界面的时候list是有数据的，当点击提交等按钮的时候，数据被提交到后台,如果配置了验证框架或者在action中写了validate方法，校验没有通过，未走action，直接返回了input，又指定回了当前界面。
此时的checkboxlist中的list丢失了，导致了如上错误（这个错误提示的不太友好，让人认为是类转换错误）

解决办法是把初始化list的工作放到prepare拦截器中，因为prepare是在validate拦截器之前执行，即实现prepareble接口：

public class RoleAction extends ActionSupport implements Preparable{ @Override public void prepare() throws Exception { //初始化list } }