javaweb的分页实现

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

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
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() {
        String sql = "select * from user ";
        return DBUtils.executeQuery(sql);
    }
}
dao层
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
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
<%--
  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="${user}" varStatus="i">
          <tr>
            <td>${i.count}</td>
            <td>${p.username}</td>
            <td>${p.password}</td>
          </tr>
        </c:forEach>
      </table>
  </body>
</html>
页面

运行测试，可以看到表中数据以表格形式展示

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：

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：

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
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
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
<%--
  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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原文地址：https://www.cnblogs.com/Zs-book1/p/11125640.html