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

}

}