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

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

< s:checkboxlist  list ="roles"  listKey ="id"  listValue ="name"  name ="selectRoleId" ></ s:checkboxlist >

出现了如下异常：

org.apache.jasper.JasperException: tag  ' checkboxlist ' , field  ' list ' , name  ' selectRoleId ' : 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
    }
}