空指针异常问题

这是jsp页面的代码
<%@ page language="java" import="java.util.*" pageEncoding="utf-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <title>My JSP 'jspShopCar.jsp' starting page</title>
  </head>
  <body>
    <h3>选择所需的商品</h3>
    <form name = "form1" action = "jspShopCar.jsp" method = "post">
     <select name = "item">
     <option>ASP.NET</option>
     <option>C++</option>
     <option>C语言</option>
     <option>THINGING IN JAVA </option>
     <option>ORCALE</option>
     <option>SQL/PLUS</option>
     </select>
     <input type = "submit" name = "submit" value = "Add">
     <input type = "submit" name = "submit" value = "Remove">
    </form>
  </body>
</html>
<jsp:useBean id = "beanCar" scope = "session" class = "chap7.ex.ShoppingCar"/>
<jsp:setProperty name = "beanCar" property = "*"/>
<%
String tmpbtn = request.getParameter("submit");
String tmpname = request.getParameter("item");
if(tmpbtn!=null){
out.print("<hr size = '1' noshade = 'noshade' />");
out.print("<h4>购物车中的商品:</h4>");
}
if(tmpbtn.equals("Add")){
beanCar.AddItem(tmpname);
}
if(tmpbtn.equals("Remove")){
beanCar.RemoveItem(tmpname);
}
out.print("<ol>");
String[] item = beanCar.getItems();
for(int i = 0;i<item.length;i++){
out.print("<li>" +item[i] +"</li>");
}
out.print("</ol>");
beanCar.reset();
%>
==============================================
这个是bean的代码
package chap7.ex;
import java.util.*;
public class ShoppingCar {
Vector v = new Vector();
String submit = null;
String item = null;
public void AddItem(String name){
v.addElement(name);
}
public void RemoveItem(String name){
v.removeElement(name);
}
public void setItem(String name){
item = name;
}
public void setSubmit(String s){
submit= s;
}
public String[] getItems(){
String[] s = new String[v.size()];
return s;
}
public void reset(){
item = null;
submit = null;
}
}
我一执行jsp就会出现空指针异常，调了好几天了，大家帮帮忙吧

解决方案 »

  1.   

    Error 500--Internal Server Error 
    java.lang.NullPointerException
    at jsp_servlet.__jspshopcar._jspService(__jspshopcar.java:112)
    at weblogic.servlet.jsp.JspBase.service(JspBase.java:34)
    at weblogic.servlet.internal.StubSecurityHelper$ServletServiceAction.run(StubSecurityHelper.java:226)
    at weblogic.servlet.internal.StubSecurityHelper.invokeServlet(StubSecurityHelper.java:124)
    at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:283)
    at weblogic.servlet.internal.ServletStubImpl.onAddToMapException(ServletStubImpl.java:391)
    at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:309)
    at weblogic.servlet.internal.ServletStubImpl.execute(ServletStubImpl.java:175)
    at weblogic.servlet.internal.WebAppServletContext$ServletInvocationAction.run(WebAppServletContext.java:3370)
    at weblogic.security.acl.internal.AuthenticatedSubject.doAs(AuthenticatedSubject.java:321)
    at weblogic.security.service.SecurityManager.runAs(Unknown Source)
    at weblogic.servlet.internal.WebAppServletContext.securedExecute(WebAppServletContext.java:2117)
    at weblogic.servlet.internal.WebAppServletContext.execute(WebAppServletContext.java:2023)
    at weblogic.servlet.internal.ServletRequestImpl.run(ServletRequestImpl.java:1359)
    at weblogic.work.ExecuteThread.execute(ExecuteThread.java:200)
    at weblogic.work.ExecuteThread.run(ExecuteThread.java:172)
     
      

  2.   

    if(tmpbtn.equals("Add")){
    beanCar.AddItem(tmpname);
    }
    if(tmpbtn.equals("Remove")){
    beanCar.RemoveItem(tmpname);
    }
    改if("Add".equals(tmpbtn)){
    beanCar.AddItem(tmpname);
    }
    if("Remove".equals(tmpbtn)){
    beanCar.RemoveItem(tmpname);
    }如果tmpbtn获取是null值，那是用equals方法的时候当然会出现npe
      

  3.   

    但是这样改了之后我所获取的tmpname怎么都是null呀？
      

  4.   

    at jsp_servlet.__jspshopcar._jspService(__jspshopcar.java:112)