写网上商城作业java.lang.NullPointerException

1. 报错的：HTTP Status 500 -

type Exception report

message

description The server encountered an internal error that prevented it from fulfilling this request.

exception

java.lang.NullPointerException
com.eshore.action.GoodServlet.doPost(GoodServlet.java:56)
com.eshore.action.GoodServlet.doGet(GoodServlet.java:26)
javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
com.eshore.filters.EncodeFilter.doFilter(EncodeFilter.java:45)

note The full stack trace of the root cause is available in the Apache Tomcat/7.0.30 logs.

Apache Tomcat/7.0.30

2.goodservlet代码
package com.eshore.action;

import java.io.IOException;
import java.util.Vector;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.eshore.factory.DAOFactory;
import com.eshore.pojo.Goods;

/**

• 商品的Servlet类
  */
  @WebServlet(
  urlPatterns = { "/goods" },
  name = "goodsServlet"
  )
  public class GoodServlet extends HttpServlet {
  private static final long serialVersionUID = 1L;
  public void doGet(HttpServletRequest request, HttpServletResponse response)
  throws ServletException, IOException {
  this.doPost(request, response);
  }
  public void doPost(HttpServletRequest request, HttpServletResponse response)
  throws ServletException, IOException {
  //判断action类型
  String action=request.getParameter("action");
  String path=null;
  Vector gVector=new Vector();

  try{
      if(action.equals("index-select")){//查询商品列表
          String keyWord=request.getParameter("keyWord");//获取查询的输入值
          String keyClass=request.getParameter("keyClass");//获取查询类别
          gVector=DAOFactory.getGoodDAOInstance().//获得所有商品
              queryAll(keyWord, keyClass);
          request.setAttribute("goods", gVector);
          path="goods/goodslist.jsp";
      }else if(action.equals("goodslist-select")){//指定商品列表
          Goods good=new Goods();
          String sid=request.getParameter("sid");//获得商品的id
          gVector=DAOFactory.getGoodDAOInstance().//获得指定的商品对象
              queryGoodBySid(Integer.valueOf(sid));
          if(gVector.size()>0&&gVector!=null)
              good =(Goods)gVector.get(0); 
          request.setAttribute("good", good);
          path="goods/good.jsp";
      }
  }catch(Exception e){
      e.printStackTrace();
  }
  request.getRequestDispatcher(path).forward(request, response);
  

  }

}

调试下，从错误信息看，是有变量为null，比如request.getParameter("xxx")这里有没有获得参数

你看下表里面有字段为非空，前面是空的，用debug看看，那个值为null了，或者开日志文件看下运行信息，就知道了

空指针异常，看页面与servlet之间传值的 名称 是否一致！

最底层的异常信息就是GoodServlet.java:56)这个类的56行，看看对象.操作有什么对象可能为空。
说时候，Java的空指针异常是最好解决的的了。自己调试下吧，少年。