用jsf <t:DataTable>是的错误是什么原因?--up有分

Question

错误异常
javax.servlet.ServletException: Cannot get value for expression '#{dataPage.data}'
   javax.faces.webapp.FacesServlet.service(FacesServlet.java:121)
   org.apache.myfaces.component.html.util.ExtensionsFilter.doFilter(ExtensionsFilter.java:122)我用ShowTable(bb)里调用另一个BB里的方法 用<t:DataTable>加在那个getData 这样出现了错误.
ShowTable.java  import moudel.compoent.DataPage;
import java.util.ArrayList;
import java.util.List;import javax.faces.event.ActionEvent;import data.managerSession.SessionUtil;
import data.mapping.Suser;public class ShowTable {
  List userList;
  SessionUtil su;
  String userName="";
  String passWord="";
  DataPage dataPage;
  public ShowTable(){
      su=new SessionUtil();
        try{
            
           userList=su.findAllValues("Suser");
          dataPage=new DataPage(9,6,userList);
        }catch(Exception exp){
            System.out.println(exp.toString());
        }finally{
            su.closeSession();
        }
  }
  public void ss(ActionEvent event){
    
  }public String getPassWord() {
   System.out.println("getpass");
   return passWord;
}
public void setPassWord(String passWord) {
   System.out.println("setpass");
   this.passWord = passWord;
}
public String getUserName() {
   
   return userName;
}
public void setUserName(String userName) {
   
   this.userName = userName;
}
--------------------------------------------------------------------------
DataPage.javapackage moudel.compoent;import java.util.ArrayList;
import java.util.List;import data.managerSession.SessionUtil;public class DataPage {
   private int datasetSize;
  private int startRow;
  private List data=new ArrayList();
  SessionUtil su;
  /** *//**
  * Create an object representing a sublist of a dataset.
  * 
  * @param datasetSize
  *         is the total number of matching rows available.
  * 
  * @param startRow
  *         is the index within the complete dataset of the first element
  *         in the data list.
  * 
  * @param data
  *         is a list of consecutive objects from the dataset.
  */
  public DataPage(int datasetSize, int startRow, List list)
  {
    this.datasetSize = 1;
    this.startRow = 2;
    this.setData(list);
    
    
  }  /** *//**
  * Return the number of items in the full dataset.
  */
  public void setData(List data){
     this.data=data;
  }
  public int getDatasetSize()
  {
      System.out.println("getDatasetSize");
    return datasetSize;
  }  /** *//**
  * Return the offset within the full dataset of the first element in the
  * list held by this object.
  */
  public int getStartRow()
  {
     System.out.println("getDatasetStartRow");
    return startRow;
  }  /** *//**
  * Return the list of objects held by this object, which is a continuous
  * subset of the full dataset.
  */
  public List getData()
  {
     
    return data;
  }}
face-config.xml  <faces-config >
   <managed-bean>
       <managed-bean-name>login</managed-bean-name>
       <managed-bean-class>web.login</managed-bean-class>
       <managed-bean-scope>session</managed-bean-scope>
   </managed-bean>
   <managed-bean>
       <managed-bean-name>user</managed-bean-name>
       <managed-bean-class>data.mapping.Suser</managed-bean-class>
       <managed-bean-scope>request</managed-bean-scope>
   </managed-bean>
   <managed-bean>
       <managed-bean-name>    showTable</managed-bean-name>
       <managed-bean-class>moudel.ShowTable</managed-bean-class>
       <managed-bean-scope>session</managed-bean-scope>
   </managed-bean>
   <managed-bean>
       <managed-bean-name>dataPage</managed-bean-name>
       <managed-bean-class>moudel.compoent.DataPage</managed-bean-class>
       <managed-bean-scope>request</managed-bean-scope>
   </managed-bean>
</faces-config>
----------------------------------------------
show.jsp
<t:dataTable value="#{dataPage.data}" var="user" id="data" rows="5">
        <t:column>
        <f:facet name="header">
          <h:outputText value="username"></h:outputText>
        </f:facet>
        <h:outputText value="#{user.username}"></h:outputText>
        </t:column>
          <t:column>
          <f:facet name="header">
          <h:outputText value="password"></h:outputText>
        </f:facet>
        <h:outputText value="#{user.password}"></h:outputText>
        </t:column>
       </t:dataTable>