javaweb的分页实现

爱被打了一巴掌 2021-11-10 12:14 369阅读 0赞

其实分页这一篇本来我是我不想写的，但是下来打算将泛型，为了泛型能够更加的容易理解，所以分页拖到了现在才写。

分页的实现有两种方式：

1.后台获取所有数据，然后通过前端框架来分页（不推荐，效率低，浪费资源）；

2.通过后台数据库sql语句分页获取数据（在mysql和oracle中，分页的实现语句是不一样的，mysql通过limit关键字来实现分页，而oracle通过条件语句来实现；

在这里我们使用更加方便的limit来实现分页。下面我们使用一个案例来实现分页：

创建一个web项目，遍历user数据表的所有信息，以表格的形式展示，具体操作如下，工具类就不在这里写了。同时为了方便接口这里也不在创建了，直接写dao，service的实现

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package com.zs.entity;
    
    public class User {
        private int uid;
        private String username;
        private String password;
    
        public int getUid() {
            return uid;
        }
    
        public void setUid(int uid) {
            this.uid = uid;
        }
    
        public String getUsername() {
            return username;
        }
    
        public void setUsername(String username) {
            this.username = username;
        }
    
        public String getPassword() {
            return password;
        }
    
        public void setPassword(String password) {
            this.password = password;
        }
    }

实体类

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dao层

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package com.zs.service;
    
    import com.zs.dao.UserDao;
    import com.zs.entity.User;
    
    import java.util.ArrayList;
    import java.util.List;
    import java.util.Map;
    
    public class UserService {
        public List<User> allUser() {
            UserDao userDao = new UserDao();
            List<Map<String, Object>> maps = userDao.allUser();
            List<User> list = fengzhuang(maps);
            return list;
        }
    
        private List<User> fengzhuang(List<Map<String,Object>> maps) {
            List<User> list= new ArrayList<>();
            for (Map<String, Object> map : maps) {
                User user = new User();
                user.setUid((Integer) map.get("uid"));
                user.setUsername(map.get("username").toString());
                user.setPassword(map.get("password").toString());
                list.add(user);
            }
            return list;
        }
    }

service

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package com.zs.servlet;
    
    import com.zs.entity.User;
    import com.zs.service.UserService;
    
    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 java.io.IOException;
    import java.util.List;
    
    @WebServlet("/home")
    public class UserServlet extends HttpServlet {
        @Override
        protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
            UserService userService = new UserService();
            List<User> users = userService.allUser();
            req.setAttribute("user", users);
            req.getRequestDispatcher("index.jsp").forward(req, resp);
        }
    }

servlet

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页面

运行测试，可以看到表中数据以表格形式展示

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但是这中方式，是一次性将所有的信息都读出来，如果信息量大的话，效率低而且展示数据表格会非常长，这很不方便阅读，因此就想，如何实现分页，将所有的数据分成一页一页 的，mysql使用limit关键字来实现分页，如下：

/**
         * startNum 开始下标，
         * num  每页的记录数
         */
        select * from user limit startNum,num;

通过limit实现分页，当点击下一页时，就根据发送过来的页数信息查询下一页的数据，然后在响应给前端页面，为了实现这种方式，我们创建一个分页的工具类，通过工具类来计算页数以及传递数据，工具类如下：

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package com.zs.util;
    
    import com.zs.entity.User;
    
    import java.util.List;
    
    public class PageUtil {
        private int firstPage;
        private int proPage;
        private int currentPage;
        private int nextPage;
        private int lastPage;
        private List<User> list;
    
        public PageUtil() {
    
        }
    
        /**
         * 创建构造方法，通过传入当前页的值以及数据总数，计算下一页，上一页，首页，尾页以及当前页的数据等信息
         * @param currentPage   当前页数
         * @param countPage     总页数
         * @param list          数据集合
         */
        public PageUtil(int currentPage,int countPage, List<User> list) {
            this.firstPage = 1;
            this.currentPage = currentPage;
            this.lastPage = countPage;
            this.proPage = this.currentPage == 1 ? 1 : (this.currentPage - 1);
            this.nextPage = this.currentPage == this.lastPage ? this.currentPage : this.currentPage + 1;
            this.list = list;
        }
    
        public int getFirstPage() {
            return firstPage;
        }
    
        public void setFirstPage(int firstPage) {
            this.firstPage = firstPage;
        }
    
        public int getProPage() {
            return proPage;
        }
    
        public void setProPage(int proPage) {
            this.proPage = proPage;
        }
    
        public int getCurrentPage() {
            return currentPage;
        }
    
        public void setCurrentPage(int currentPage) {
            this.currentPage = currentPage;
        }
    
        public int getNextPage() {
            return nextPage;
        }
    
        public void setNextPage(int nextPage) {
            this.nextPage = nextPage;
        }
    
        public int getLastPage() {
            return lastPage;
        }
    
        public void setLastPage(int lastPage) {
            this.lastPage = lastPage;
        }
    
        public List<User> getList() {
            return list;
        }
    
        public void setList(List<User> list) {
            this.list = list;
        }
    }

分页工具类

然后修改dao层，service层，servlet以及页面如下：

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package com.zs.dao;
    
    import com.zs.util.DBUtils;
    
    import java.util.List;
    import java.util.Map;
    
    public class UserDao {
        /**
         * 获取所有的用户信息
         * @return
         */
        public List<Map<String,Object>> allUser(int startNum) {
            String sql = "select * from user limit ?,3 ";
            return DBUtils.executeQuery(sql,startNum);
        }
    
        /**
         * 获取数据的记录数，方便计算总页数
         * @return
         */
        public Map<String,Object> countUser() {
            String sql = "select count(uid) as num from user";
            return DBUtils.executeQuery(sql).get(0);
        }
    }

dao

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package com.zs.service;
    
    import com.zs.dao.UserDao;
    import com.zs.entity.User;
    import com.zs.util.PageUtil;
    
    import java.util.ArrayList;
    import java.util.List;
    import java.util.Map;
    
    public class UserService {
        public PageUtil allUser(int currentPage) {
    
            UserDao userDao = new UserDao();
            Map<String, Object> map = userDao.countUser();
            /*获取总记录数*/
            int num =Integer.parseInt(map.get("num").toString()) ;
            /*获取当前页数据, (currentPage-1)*10计算开始下标*/
            List<Map<String, Object>> maps = userDao.allUser((currentPage-1)*3);
            List<User> list = fengzhuang(maps);
            PageUtil page = new PageUtil(currentPage, (num+2)/3, list);
            return page;
        }
    
        private List<User> fengzhuang(List<Map<String,Object>> maps) {
            List<User> list= new ArrayList<>();
            for (Map<String, Object> map : maps) {
                User user = new User();
                user.setUid((Integer) map.get("uid"));
                user.setUsername(map.get("username").toString());
                user.setPassword(map.get("password").toString());
                list.add(user);
            }
            return list;
        }
    }

service

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package com.zs.servlet;
    
    import com.zs.entity.User;
    import com.zs.service.UserService;
    import com.zs.util.PageUtil;
    
    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 java.io.IOException;
    import java.util.List;
    
    @WebServlet("/home")
    public class UserServlet extends HttpServlet {
        @Override
        protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
            Integer currentPage = 1;
            try {
                currentPage = Integer.parseInt(req.getParameter("page"));
            } catch (Exception e) {
                System.out.println("第一次访问");
            }
            UserService userService = new UserService();
    
            PageUtil page = userService.allUser(currentPage);
            req.setAttribute("page", page);
            req.getRequestDispatcher("index.jsp").forward(req, resp);
        }
    }

servlet

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<%--
      Created by IntelliJ IDEA.
      User: Administrator
      Date: 2019/7/3
      Time: 14:18
      To change this template use File | Settings | File Templates.
    --%>
    <%@ page contentType="text/html;charset=UTF-8" language="java" %>
    <%@taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
    <html>
      <head>
        <title>$Title$</title>
      </head>
      <body>
          <table border="1" cellpadding="1">
            <tr>
              <th>序号</th>
              <th>用户名</th>
              <th>密码</th>
            </tr>
            <c:forEach var="p" items="${page.list}" varStatus="i">
              <tr>
                <td>${i.count}</td>
                <td>${p.username}</td>
                <td>${p.password}</td>
              </tr>
            </c:forEach>
          </table>
          <%--分页工具--%>
          <div>
            <a href="<c:url value='/home?page=${page.firstPage}'/>">首页</a>
            <a href="<c:url value='/home?page=${page.proPage}'/>">上一页</a>
            第${page.currentPage}/${page.lastPage}页
            <a href="<c:url value='/home?page=${page.nextPage}'/>">下一页</a>
            <a href="<c:url value='/home?page=${page.lastPage}'/>">尾页</a>
          </div>
      </body>
    </html>

前端页面

然后运行测试：

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转载于:https://www.cnblogs.com/Zs-book1/p/11125640.html

[ContractedBlock.gif]: https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif
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[1602537-20190703145518819-1166743710.png]: /images/20211109/7a6a212b74b746b1bfb28fde3b54cd36.png
[1602537-20190703154442241-247182620.png]: /images/20211109/72df8e53b45a4a60a1f8e8d99f59ab60.png