struts1 nullpointerexception解决思路

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 mapping, HttpServletRequest request) {
		// TODO Auto-generated method stub
	}

	/** 
	 * Returns the id.
	 * @return String
	 */

	public String getAnswer_a() {
		return answer_a;
	}

	public void setAnswer_a(String answer_a) {
		this.answer_a = answer_a;
	}

	public String getAnswer_b() {
		return answer_b;
	}

	public void setAnswer_b(String answer_b) {
		this.answer_b = answer_b;
	}

	public String getAnswer_c() {
		return answer_c;
	}

	public void setAnswer_c(String answer_c) {
		this.answer_c = answer_c;
	}

	public String getAnswer_d() {
		return answer_d;
	}

	public void setAnswer_d(String answer_d) {
		this.answer_d = answer_d;
	}

	public int getSubject_id() {
		return subject_id;
	}

	public void setSubject_id(int subject_id) {
		this.subject_id = subject_id;
	}

	public String getSubject_name() {
		return subject_name;
	}

	public void setSubject_name(String subject_name) {
		this.subject_name = subject_name;
	}

	public String[] getAnswerArrS() {
		return answerArrS;
	}

	public void setAnswerArrS(String[] answerArrS) {
		this.answerArrS = answerArrS;
	}

	public int[] getIdArrS() {
		return idArrS;
	}

	public void setIdArrS(int[] idArrS) {
		this.idArrS = idArrS;
	}
	public void setSize(int size){
		answerArrS=new String[size];
		idArrS=new int[size];
		System.out.println("单选题的数组大小："+answerArrS.length);
	}
	
}

public ActionForward execute(ActionMapping mapping, ActionForm form,