exception 



javax.servlet.ServletException: BeanUtils.populate

	org.apache.struts.util.RequestUtils.populate(RequestUtils.java:495)

	org.apache.struts.action.RequestProcessor.processPopulate(RequestProcessor.java:805)

	org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:203)

	org.apache.struts.action.ActionServlet.process(ActionServlet.java:1194)

	org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)

	javax.servlet.http.HttpServlet.service(HttpServlet.java:710)

	javax.servlet.http.HttpServlet.service(HttpServlet.java:803)





root cause 



java.lang.NullPointerException

	org.apache.commons.beanutils.PropertyUtilsBean.setIndexedProperty(PropertyUtilsBean.java:1414)

	org.apache.commons.beanutils.BeanUtilsBean.setProperty(BeanUtilsBean.java:1016)

	org.apache.commons.beanutils.BeanUtilsBean.populate(BeanUtilsBean.java:811)

	org.apache.commons.beanutils.BeanUtils.populate(BeanUtils.java:298)

	org.apache.struts.util.RequestUtils.populate(RequestUtils.java:493)

	org.apache.struts.action.RequestProcessor.processPopulate(RequestProcessor.java:805)

	org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:203)

	org.apache.struts.action.ActionServlet.process(ActionServlet.java:1194)

	org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)

	javax.servlet.http.HttpServlet.service(HttpServlet.java:710)

	javax.servlet.http.HttpServlet.service(HttpServlet.java:803)

actionForm类 

/*

 * Generated by MyEclipse Struts

 * Template path: templates/java/JavaClass.vtl

 */

package com.xsm.struts.form;



import javax.servlet.http.HttpServletRequest;

import org.apache.struts.action.ActionErrors;

import org.apache.struts.action.ActionForm;

import org.apache.struts.action.ActionMapping;



/** 

 * MyEclipse Struts

 * Creation date: 01-04-2011

 * 

 * XDoclet definition:

 * @struts.form name="subject_lib_showForm"

 */

public class Subject_lib_showForm extends ActionForm {





	/** id property */

	private int subject_id;

	private String subject_name;

	private String answer_a;

	private String answer_b;

	private String answer_c;

	private String answer_d;

	private  int[] idArrS;//记录单选题的试题ID属性

	private String[] answerArrS;//在显示试题和提交试卷时 记录单选题答案的属性



	



	/** 

	 * Method validate

	 * @param mapping

	 * @param request

	 * @return ActionErrors

	 */

	public ActionErrors validate(ActionMapping mapping,

			HttpServletRequest request) {

		// TODO Auto-generated method stub

		return null;

	}



	/** 

	 * Method reset

	 * @param mapping

	 * @param request

	 */

	public void reset(ActionMapping&n