SearchGoodsByIdServlet.java:45 下一句 空指针异常

SearchGoodsByIdServlet.java:45 下一句 空指针异常

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严重: 在路径为[/Gim]的上下文中，servlet[com.imooc.gim.SearchGoodsByIdServlet]的Servlet.service()引发异常
java.lang.NullPointerException
    at com.imooc.gim.SearchGoodsByIdServlet.doPost(SearchGoodsByIdServlet.java:45)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:681)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:764)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:196)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:542)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:135)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:698)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:364)
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:624)
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:831)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1650)
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
    at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191)
    at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Unknown Source)
     
     
package com.imooc.gim;
import java.io.IOException;
import java.util.List;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
 * Servlet implementation class SearchGoodsByIdServlet
 */
@WebServlet("/sg")
public class SearchGoodsByIdServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
        
    /**
     * @see HttpServlet#HttpServlet()
     */
    public SearchGoodsByIdServlet() {
        super();
        // TODO Auto-generated constructor stub
    }
/**
 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
 */
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
doPost(request, response);
}
/**
 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
 */
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
  String id = request.getParameter("id");
  Integer goodsId = Integer.parseInt(id);
  ServletContext context=request.getServletContext();
  context.getAttribute("goods");
  List goodsList=(List)context.getAttribute("goods");
  for(int i=0; i<goodsList.size(); i++) {
  if( ((Goods) goodsList.get(i)).getGoodsId() == goodsId ) {  //这一句有问题？
  System.out.println("商品id已查询到");
  break;
   }else {
   System.out.println("商品id不存在");
   break;
   }
    
  }
}
}
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<%@ page contentType="text/html; charset=UTF-8" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %> 
<%@ taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt"%>
  
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width,initial-scale=1">
    <title>商品列表</title>
    <link href="css/bootstrap.css" type="text/css" rel="stylesheet">
    </link>
    <script type="text/javascript" src="js/jquery-1.11.1.min.js"></script>
    <script type="text/javascript" src="js/bootstrap.js"></script>
    <style type="text/css">
    .pagination {
        margin: 0px
    }
    .pagination>li>a,
    .pagination>li>span {
        margin: 0 5px;
        border: 1px solid #dddddd;
    }
    .glyphicon {
        margin-right: 3px;
    }
    .form-control[readonly] {
        cursor: pointer;
        background-color: white;
    }
    #dlgPhoto .modal-body {
        text-align: center;
    }
    .preview {
        max-width: 500px;
    }
    </style>
    <script>
        $(function () {
             
            $("#btnAdd").click(function () {
                $('#dlgForm').modal()
            });
        })
    </script>
</head>
<body>
    <div class="container">
        <div class="row">
            <h1 style="text-align: center">IMOOC商品信息表</h1>
            <div class="panel panel-default">
                <div class="clearfix panel-heading ">
                    <div>
                        <label class="form-inline">
                            <div class="form-group" style="width: 850px;">
                                <button class="btn btn-primary" id="btnAdd">新增
                                </button>
                            </div>
                            <div class="form-group pull-right">
                            <form action="/Gim/sg" method="post">
                                <input type="text" class="form-control" id="searchById" name="id" placeholder="根据商品id进行查询">
                                <button type="submit" class="btn btn-primary">查询</button>
                                </form>
                            </div>
                        </label>
                    </div>
                    <table class="table table-bordered table-hover">
                        <thead>
                            <tr>
                                <th>序号</th>
                                <th>商品编号</th>
                                <th>商品名称</th>
                                <th>商品类别</th>
                                <th>价格</th>
                                <th>备注</th>
                                <th>&nbsp;</th>
                            </tr>
                            <c:forEach items="${applicationScope.goods}" var ="good" varStatus="idx">
                            <tr>
                                <th>${idx.index+1}</th>
                                <th>${good.goodsId}</th>
                                <th>${good.goodsName}</th>
                                <th>${good.goodsType}</th>
                                <th>￥<fmt:formatNumber value = "${good.price}" pattern="0,000.00"></fmt:formatNumber></th>
                                <th>${good.goodsRemark}</th>
                                <th>&nbsp;</th>
                            </tr>
                            </c:forEach>
                        </thead>
                    </table>
                </div>
            </div>
        </div>
        <!-- 表单 -->
        <div class="modal fade" tabindex="-1" role="dialog" id="dlgForm">
            <div class="modal-dialog" role="document">
                <div class="modal-content">
                    <div class="modal-header">
                        <button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">&times;</span>
                        </button>
                        <h4 class="modal-title">新增商品</h4>
                    </div>
                    <div class="modal-body">
                        <form action="/Gim/create" method="post">
                            <div class="form-group">
                                <label>商品编号</label>
                                <input type="text" name="goodsId" class="form-control" id="empno" placeholder="请输入商品编号">
                            </div>
                            <div class="form-group">
                                <label>商品名称</label>
                                <input type="text" name="goodsName" class="form-control" id="ename" placeholder="请输入商品名称">
                            </div>
                            <div class="form-group">
                                <label>商品类别</label>
                                <select id="gname" name="goodsType" class="form-control">
                                    <option selected="selected">服装</option>
                                    <option value="家用">家用电器</option>
                                    <option value="生活">生活用品</option>
                                </select>
                            </div>
                            <div class="form-group">
                                <label>价格</label>
                                <input type="text" name="price" class="form-control" id="sal" placeholder="请输入价格">
                            </div>
                            <div class="form-group">
                                <label>备注</label>
                                <input type="text" name="description" class="form-control" id="sal" placeholder="请输入备注">
                            </div>
                            <div class="form-group" style="text-align: center;">
                                <button type="submit" class="btn btn-primary">保存</button>
                            </div>
                        </form>
                    </div>
                </div><!-- /.modal-content -->
            </div><!-- /.modal-dialog -->
        </div><!-- /.modal -->
</body>
</html>
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package com.imooc.gim;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
 * Servlet implementation class GoodsAddServlet
 */
@WebServlet("/gl")
public class GoodsListServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
    /**
     * Default constructor. 
     */
    public GoodsListServlet() {
        // TODO Auto-generated constructor stgaub
    }
/**
 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
 */
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
       doPost(request, response);
}
/**
 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
 */
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    request.setCharacterEncoding("utf-8");
    ServletContext context=request.getServletContext();
    if(context.getAttribute("goods")==null) {   
         
    List<Goods> goodsList=new ArrayList<>();
    goodsList.add(new Goods(1001,"格力空调","家用电器",3000.00f,"铂金品质"));
    goodsList.add(new Goods(1002,"LV","服装",18888.00f,"生活一场旅行"));
    context.setAttribute("goods", goodsList);
     
    }    
     
     
    request.getRequestDispatcher("/goods.jsp").forward(request, response);
     
   }
     
}
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package com.imooc.gim;
import java.io.IOException;
import java.util.List;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
 * Servlet implementation class GoodsAddServlet
 */
@WebServlet("/create")
public class GoodsAddServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
        
    /**
     * @see HttpServlet#HttpServlet()
     */
    public GoodsAddServlet() {
        super();
        // TODO Auto-generated constructor stub
    }
/**
 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
 */
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
doPost(request, response);
}
/**
 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
 */
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
request.setCharacterEncoding("utf-8");
String goodsId=request.getParameter("goodsId");
    String goodsName=request.getParameter("goodsName");
    String goodsType=request.getParameter("goodsType");
    String price=request.getParameter("price");
    String goodsRemark=request.getParameter("description");
    System.out.println(goodsId);
    Goods good = new Goods(Integer.parseInt(goodsId),goodsName,goodsType,Float.parseFloat(price),goodsRemark);
    ServletContext context=request.getServletContext();
    context.getAttribute("goods");
    List goodsList=(List)context.getAttribute("goods");
goodsList.add(good);
    context.setAttribute("goods", goodsList);
    request.getRequestDispatcher("/gl").forward(request, response);
}
}
package com.imooc.gim;
public class Goods {
 private Integer goodsId;
 private String goodsName;
 private String goodsType;
 private float price;
 private String goodsRemark;
public Goods(Integer goodsId, String goodsName, String goodsType, float price, String goodsRemark) {
super();
this.goodsId = goodsId;
this.goodsName = goodsName;
this.goodsType = goodsType;
this.price = price;
this.goodsRemark = goodsRemark;
}
public Integer getGoodsId() {
return goodsId;
}
public void setGoodsId(Integer goodsId) {
this.goodsId = goodsId;
}
public String getGoodsName() {
return goodsName;
}
public void setGoodsName(String goodsName) {
this.goodsName = goodsName;
}
public String getgoodsType() {
return goodsType;
}
public void setgoodsType(String goodsType) {
this.goodsType = goodsType;
}
public float getPrice() {
return price;
}
public void setPrice(float price) {
this.price = price;
}
public String getGoodsRemark() {
return goodsRemark;
}
public void setGoodsRemark(String goodsRemark) {
this.goodsRemark = goodsRemark;
}
@Override
public String toString() {
return "Goods [goodsId=" + goodsId + ", goodsName=" + goodsName + ", goodsType=" + goodsType
+ ", price=" + price + ", goodsRemark=" + goodsRemark + "]";
}
  
}
晓舟 2021-12-05
源自：JSTL与EL表达式   3-4 项目作业
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1回答
好帮手慕小蓝 回答被采纳获得+3积分 2021-12-05 13:31:13

同学你好，测试过同学代码之后，并未复现空指针异常。但同学的代码中存在逻辑问题：

当判断用户输入的ID在集合中是否有对应商品的时候，应当是将集合完整遍历结束之后才能确定没有找到。而同学的代码，循环只会进行一次，无论是否找到都会被break结束循环。

修改后的代码如下图所示：

https://img1.sycdn.imooc.com//climg/61ac4e80097b263d10230726.jpg

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//定义标记变量，如果没有找到flag的值为true，找到则更改为false
boolean flag = true;
//遍历集合
for (int i = 0; i < goodsList.size(); i++) {
    //获取集合中i下标的Goods对象
    Goods goods = (Goods)goodsList.get(i);
    //获取对象的id值
    Integer gid = goods.getGoodsId();
    //如果id和用户输入的id相同，则找到
    //注意：Integer类型数据对比需要使用equals而不是==
    if(gid.equals(goodsId)) {
        System.out.println("商品id已查询到");
        //将标记变量修改为false
        flag = false;
    }
}
//循环结束后，查看标记变量，判断是否
if(flag) System.out.println("商品id不存在");

如果同学依然出现空指针异常，建议将操作步骤和错误截图一并提供。

祝学习愉快~


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  • 晓舟 提问者 #1
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    package com.imooc.gim;
    import java.io.IOException;
    import java.util.List;
    import javax.servlet.ServletContext;
    import javax.servlet.ServletException;
    import javax.servlet.annotation.WebServlet;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    /**
     * Servlet implementation class SearchGoodsByIdServlet
     */
    @WebServlet("/sg")
    public class SearchGoodsByIdServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
            
        /**
         * @see HttpServlet#HttpServlet()
         */
        public SearchGoodsByIdServlet() {
            super();
            // TODO Auto-generated constructor stub
        }
    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // TODO Auto-generated method stub
    doPost(request, response);
    }
    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
      String id = request.getParameter("id");
      Integer goodsId = Integer.parseInt(id);
      ServletContext context=request.getServletContext();
      context.getAttribute("goods");
      List goodsList=(List)context.getAttribute("goods");
      for(int i=0; i<goodsList.size(); i++) {
      Integer good = ((Goods) goodsList.get(i)).getGoodsId();
      if(good.equals(goodsId) ) {
      System.out.println("商品id已查询到");
      break;     
      }
       }
    }
    }
     
    //修改成这样后，如果输入的商品id正好合list中匹配控制台应该输出 商品已查询到，为什么还是报空指针异常呢？
    严重: 在路径为[/Gim]的上下文中，servlet[com.imooc.gim.SearchGoodsByIdServlet]的Servlet.service()引发异常
    java.lang.NullPointerException
        at com.imooc.gim.SearchGoodsByIdServlet.doPost(SearchGoodsByIdServlet.java:45)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:681)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:764)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
        at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
        at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
        at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
        at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:196)
        at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97)
        at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:542)
        at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:135)
        at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
        at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:698)
        at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78)
        at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:364)
        at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:624)
        at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
        at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:831)
        at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1650)
        at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
        at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191)
        at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659)
        at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
        at java.lang.Thread.run(Unknown Source)
    2021-12-05 13:53:22
  • 好帮手慕小蓝 回复 提问者 晓舟 #2

        同学你好，老师这里使用同学的代码进行测试，并没有出现空指针异常。

        老师的测试方式是：

    1. 通过url：gl访问并展示页面；

    2. 在页面中输入1002进行查询；

    3. 在SearchGoodsByIdServlet接收数据并进行查询。

        以上步骤并不会出现空指针异常。

        如果同学在测试的时候，只是刷新页面，而不是通过地址栏重新访问gl的话，是会出现空指针的，因为List集合的数据时通过访问Servlet存储的，如果直接刷新页面就没有存储的过程，会在获取List的时候出现空指针。

        如果以上都不是同学出现空指针的情况，建议同学描述一下具体的操作过程。

        祝学习愉快~

    2021-12-05 14:34:52
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