急急急，关于新建图书分类类在jsp页面不显示问题

-categoryList.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"<br> pageEncoding="UTF-8"%><br><%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%><br><!DOCTYPE html><br><html><br><head><br><meta charset="UTF-8"><br><title>图书后台管理</title><br><link rel="stylesheet" href="css/index.css"><br><link rel="stylesheet" href="css/bootstrap.min.css"><br></head><br><br><body><br> <header><br> <div class="container"><br> <nav><br> <a href="bookList.html"

>图书信息管理</a><br>          </nav><br>            <nav><br>             <a href=

"categoryList.html">分类管理</a><br> </nav><br><br> </div><br> </header><br> <section class="banner"><br> <div class="container"

><br>           <div><br>             <h1>图书管理系统</h1><br>             <p>图书分类管理</p><br>           </div><br>        </div><br>    </section><br>    <section

class="main"><br> <div class="container"><br> <table class="table table-striped"

><br>               <thead><br>                   <tr><br>                      <th>序号</th><br>                     <th>分类编号</th><br>                       <th>分类名称</th><br>                       <th>操作</th><br>                 </tr><br>             </thead><br>              <tbody><br>                   <c:forEach items=

"${applicationScope.categoryDb }" var="ctg" varStatus="idx"

><br>                     <tr><br>                          <td>${idx.index+

1

}</td><br>                            <td>${ctg.categoryId }</td><br>                         <td>${ctg.categoryName }</td><br>                           <td><a href=

"/deleteCategory?categoryId=ca0001"

>删除</a></td><br>                         <!--在循环显示数据时，此处的ca0001可以用EL表达式进行替换--><br><br>                     </tr><br>                 </c:forEach><br>              </tbody><br>          </table><br>      </div><br>    </section><br>    <section

class="page"><br> <div class="container"><br> <div id="fatie"><br> <a href="addCategory.jsp"

><button>新建</button></a><br>           </div><br>        </div><br>    </section><br>    <footer> copy@慕课网 </footer><br></body><br></html><br>

-addCategory.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"<br> pageEncoding="UTF-8"%><br><!DOCTYPE html><br><html><br><head><br><meta charset="UTF-8"><br><title>新建图书分类</title><br><link rel="stylesheet" href="css/bootstrap.min.css"><br><link rel="stylesheet" href="css/add.css"><br></head><br><body><br> <nav class="navbar navbar-default"><br> <div class="container"><br> <div class="navbar-header"><br> <a class="navbar-brand" href=""> 图书分类管理 </a><br> </div><br> </div><br> </nav><br> <div class="container"><br> <div class="jumbotron"

><br>           <h1>Hello, XXX!</h1><br>            <p>请小心地新增图书分类，要是建了一个错误的就不好了。。。</p><br>      </div><br>        <div

class="page-header"

><br>         <h3><br>              <small>新建</small><br>           </h3><br>     </div><br>        <form

class="form-horizontal"<br> action="${pageContext.request.contextPath }/AddCategoryServlet"<br> method="post"><br> <div class="form-group"><br> <label for="categoryId" class="col-sm-2 control-label">分类ID<br> ：</label><br> <div class="col-sm-8"><br> <input name="categoryId" class="form-control" id="categoryId"><br> </div><br> </div><br> <div class="form-group"><br> <label for="categoryName" class="col-sm-2 control-label">分类名称<br> ：</label><br> <div class="col-sm-8"><br> <input name="categoryName" class="form-control" id="categoryName"><br> </div><br> </div><br><br> <div class="form-group"><br> <div class="col-sm-offset-2 col-sm-10"><br> <button type="submit" class="btn btn-primary"

>保存</button><br>                   <p>${ msg}</p><br>                  &nbsp;&nbsp;&nbsp;<br>              </div><br>            </div><br>        </form><br>   </div><br>    <footer

class="text-center"> copy@imooc </footer><br></body><br></html><br>

-InitServletContextListener

package com.imooc.web.listener;<br><br>import java.util.ArrayList;<br>import java.util.List;<br><br>import javax.servlet.ServletContext;<br>import javax.servlet.ServletContextEvent;<br>import javax.servlet.ServletContextListener;<br><br>import com.imooc.domain.Category;<br>import com.imooc.domain.User;<br><br>public class InitServletContextListener implements ServletContextListener {<br><br> public InitServletContextListener() {<br> }<br><br>

// 销毁<br>   public void contextDestroyed(ServletContextEvent sce) {<br> }<br><br>   // 初始化用户和图书分类类<br>  public void contextInitialized(ServletContextEvent sce) {<br>       ServletContext context = sce.getServletContext();<br>       // 创建一个用于保存用户信息的List集合<br>      List<User> userList = new ArrayList<User>();<br>        // 将List集合存入到ServletContext域中<br>       userList.add(new User("aaa", "123456"));<br>        userList.add(new User("bbb", "234567"));<br>        userList.add(new User("ccc", "345678"));<br>        context.setAttribute("userList", userList);<br>     if (context.getAttribute("cateogryDb") == null) {<br>           List<Category> ctgList = new ArrayList<Category>();<br>         ctgList.add(new Category("ca0001", "计算机类"));<br>            context.setAttribute("categoryDb", ctgList);<br>        }<br>   }<br>}<br>

-接口

package com.imooc.service;<br><br>import java.util.ArrayList;<br><br>import java.util.List;<br><br>import com.imooc.domain.Category;<br>

//负责简洁清晰的展示接口中必要的方法和属性<br>public interface CategoryService {<br>    public static final List<Category> categoryDb =new ArrayList<Category>();<br><br>   public void addCategory(String categoryId, String categoryName);<br><br>    public void deleteCatgory(String categoryId);<br><br>}<br>

-实现类

package com.imooc.service.impl;<br><br>import java.util.ArrayList;<br>import java.util.List;<br><br>import com.imooc.domain.Category;<br>import com.imooc.service.CategoryService;<br>

//用来实现接口中属性和方法的实现类<br>public class CategoryServiceImpl implements CategoryService {<br><br>   private static final List<Category> categoryDb = new ArrayList<Category>();<br><br> public static List<Category> getCategoryDb() {<br>        return categoryDb;<br>  }<br><br>   //纯添加，判断在servlet里进行<br> @Override<br>   public void addCategory(String categoryId, String categoryName) {<br>       Category category = new Category(categoryId, categoryName);<br>     categoryDb.add(category);<br>   }<br><br>   @Override<br>   public void deleteCatgory(String categoryId) {<br>      Category categoryExist = null;<br>      for (Category c : categoryDb) {<br>         if (categoryId.equals(c.getCategoryId())) {<br>             categoryExist = c;<br>          }<br>       }<br>   }<br>}<br>

-AddCategoryServlet

package com.imooc.web.servlet;<br><br>import java.io.IOException;<br>import java.util.List;<br><br>import javax.servlet.ServletException;<br>import javax.servlet.annotation.WebServlet;<br>import javax.servlet.http.HttpServlet;<br>import javax.servlet.http.HttpServletRequest;<br>import javax.servlet.http.HttpServletResponse;<br><br>import com.imooc.domain.Category;<br>import com.imooc.service.CategoryService;<br>import com.imooc.service.impl.CategoryServiceImpl;<br><br>@WebServlet("/AddCategoryServlet")<br>public class AddCategoryServlet extends HttpServlet {<br> private static final long serialVersionUID = 1L;<br><br> public AddCategoryServlet() {<br> super();<br> }<br><br> protected void doGet(HttpServletRequest request, HttpServletResponse response)<br> throws ServletException, IOException {<br>

// 接受数据<br>     String categoryId = request.getParameter("categoryId");<br>     String categoryName = request.getParameter("categoryName");<br>     // 在这里判断输入的是否为空（根据id）<br>       if (categoryId.isEmpty() && categoryName.isEmpty()) {<br>           request.setAttribute("msg", "id及名称为空！");<br>            request.getRequestDispatcher("/addCategory.jsp").forward(request, response);<br>        } else {<br>            // 封装数据<br>         Category ctg = new Category(categoryId, categoryName);<br>          CategoryService ctgService = new CategoryServiceImpl();// 接口引用指向具体的实现类，多态的体现<br>            // 处理数据<br>         // 在这里判断一下列表里是否已经有id重名的情况再存进去<br>           List<Category> list = CategoryServiceImpl.getCategoryDb();<br>            for (Category cExist : list) {<br>              if (cExist.getCategoryId().equals(ctg.getCategoryId())<br>                      && cExist.getCategoryName().equals(ctg.getCategoryName())) {<br>                    request.setAttribute("msg", "该分类已存在！");<br>                 request.getRequestDispatcher("/addCategory.jsp").forward(request, response);<br>                    return;<br>             } else {<br>                    ctgService.addCategory(categoryId, categoryName);<br>                   request.getServletContext().setAttribute("categoryDb", CategoryServiceImpl.getCategoryDb());<br>                }<br>           }<br>       }<br>       // 交给jsp展示数据<br>        request.getRequestDispatcher("/categoryList.jsp").forward(request, response);<br>   }<br><br>   protected void doPost(HttpServletRequest request, HttpServletResponse response)<br>         throws ServletException, IOException {<br>      doGet(request, response);<br>   }<br><br>}<br>

老师请问

1、为什么我新建图书分类在jsp页面不显示，点击前后如图。自己已经找到了一些问题了，比如因为监听器初始化不显示设置在了全局域中，应该从ServletContext域中获取

1	`request.getServletContext().setAttribute("categoryDb", CategoryServiceImpl.getCategoryDb());<br>`

2、初始化放在监听器中可以吗，有没有更好的方法

3、怎么可以把判断放在实现类中，我看了老师其他答案在void add方法中返回boolean变量，但在AddServlet中怎么获取这个变量啊

4、关于msg，我判断为空的提示正常输出显示，但重名已存在的msg怎么不显示啊

孫瑪戈 2020-11-20

源自：项目：登录注册功能 6-2 项目作业

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7回答

好帮手慕小班回答被采纳获得+3积分 2020-11-23 16:11:23

同学你好，老师是将同学的Servlet拼接到自己的项目中，让代码能跑起来，现在测试同学的代码有前面提出的三个问题，同学使用如下修改后的代码再来试试。

package com.imooc.web.servlet;
 
import java.io.IOException;
import java.util.List;
 
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.imooc.domain.Category;
import com.imooc.service.CategoryService;
import com.imooc.service.impl.CategoryServiceImpl;
 
@WebServlet("/AddCategoryServlet")
public class AddCategoryServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
 
    public AddCategoryServlet() {
        super();
    }
 
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // 接受数据
        String categoryId = request.getParameter("categoryId");
        String categoryName = request.getParameter("categoryName");
        // 在这里判断输入的是否为空（根据id）
        if (categoryId.isEmpty() && categoryName.isEmpty()) {
            request.setAttribute("msg", "id及名称为空！");
            request.getRequestDispatcher("/addCategory.jsp").forward(request, response);
        } else {
            // 封装数据
            Category ctg = new Category(categoryId, categoryName);
            CategoryService ctgService = new CategoryServiceImpl();// 接口引用指向具体的实现类，多态的体现
            // 处理数据
            // 在这里判断一下列表里是否已经有id重名的情况再存进去
            List<Category> list = CategoryServiceImpl.getCategoryDb();
            boolean flag = false;
            for (Category cExist : list) {
                if (cExist.getCategoryId().equals(ctg.getCategoryId())
                        && cExist.getCategoryName().equals(ctg.getCategoryName())) {
                    flag = true;
                    System.out.println("存在相同的");
                    return;
                } else {
                    flag = false;
                    System.out.println("没有相同的");
                    //return;  //这里else不需要再次return
                }
            }
            //if (flag = false) {  //flag判断要使用==
            if (flag == false) {
                ctgService.addCategory(categoryId, categoryName);
                request.getServletContext().setAttribute("categoryDb", CategoryServiceImpl.getCategoryDb());
                System.out.println("添加成功");
                request.getRequestDispatcher("/categoryList.jsp").forward(request, response);
            } else {
                System.out.println("存在相同的没有添加");
                request.setAttribute("msg", "该分类已存在！");
                request.getRequestDispatcher("/addCategory.jsp").forward(request, response);
            }
        }
        // 交给jsp展示数据 //if-else条件语句执行完后不需要再次执行跳转语句
        //request.getRequestDispatcher("/categoryList.jsp").forward(request, response);
    }
 
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doGet(request, response);
    }
 
}

如上所示就是修改后的代码，同学再来测试运行一下。

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好帮手慕小班 2020-11-23 10:38:12

同学你好，测试拼接同学贴出的代码有如下问题：

1、提交响应后无法转发。

因为同学在if--else的条件语句中已经存在转发，在if的条件外面又进行了转发，导致出现这个问题，建议同学直接在if条件中书写对应的跳转，比如：

2、在判断列表里是否已经有id重名的情况时，不要在else中写return语句，return表示返回不再继续执行。

这样书写条件语句有问题，比如：当传入的分类属性与集合中的第一个对象不一样，它直接判断完成后直接跳出，最后得到的结果就是列表里没有重名的id，但是它只比较了集合中的第一个对象。

修改建议如下：

3、注意判断是"=="，赋值是"="，在最后的判断语句中，注意是==，

因为是拼接同学的代码，可能某些方法名或属性与同学的不一样，同学理解意思就好。如上所示的问题修改后再来试试。

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提问者孫瑪戈 #1

不是，是我少贴了哪一段代码老师要告诉我啊，怎么问了这么久还要拼接代码呢？？

2020-11-23 15:21:07

提问者孫瑪戈 2020-11-23 06:52:27

// 接受数据<br>     String categoryId = request.getParameter("categoryId");<br>     String categoryName = request.getParameter("categoryName");<br>     // 在这里判断输入的是否为空（根据id）<br>       if (categoryId.isEmpty() && categoryName.isEmpty()) {<br>           request.setAttribute("msg", "id及名称为空！");<br>            request.getRequestDispatcher("/addCategory.jsp").forward(request, response);<br>        } else {<br>            // 封装数据<br>         Category ctg = new Category(categoryId, categoryName);<br>          CategoryService ctgService = new CategoryServiceImpl();// 接口引用指向具体的实现类，多态的体现<br>            // 处理数据<br>         // 在这里判断一下列表里是否已经有id重名的情况再存进去<br>           List<Category> list = CategoryServiceImpl.getCategoryDb();<br>            boolean flag = false;<br>           for (Category cExist : list) {<br>              if (cExist.getCategoryId().equals(ctg.getCategoryId())<br>                      && cExist.getCategoryName().equals(ctg.getCategoryName())) {<br>                    flag = true;<br>                    System.out.println("存在相同的");<br>                    return;<br>             } else {<br>                    flag = false;<br>                   System.out.println("没有相同的");<br>                    return;<br>             }<br>           }<br>           if (flag = false) {<br>             ctgService.addCategory(categoryId, categoryName);<br>               request.getServletContext().setAttribute("categoryDb", CategoryServiceImpl.getCategoryDb());<br>                System.out.println("添加成功");<br>         } else {<br>                System.out.println("存在相同的没有添加");<br>                request.setAttribute("msg", "该分类已存在！");<br>             request.getRequestDispatcher("/addCategory.jsp").forward(request, response);<br>            }<br>       }<br>       // 交给jsp展示数据<br>        request.getRequestDispatcher("/categoryList.jsp").forward(request, response);<br>   }<br><br>   protected void doPost(HttpServletRequest request, HttpServletResponse response)<br>         throws ServletException, IOException {<br>      doGet(request, response);<br>   }<br><br>}<br>