MVC系列教材 （三）- 结合Servlet和JSP 实现分页功能

随着数据中记录的增多，网页上显示的数据会越来越多。 

当多到一定程度的时候，就会影响用户的体验。 

解决办法是通过分页技术，一次只显示数据库中的部分数据，如果要看其他数据，可以通过"下一页" "最后一页" 等翻页操作实现

步骤1:首先准备 DAO
步骤2:只显示5条数据
步骤3:下一页
步骤4:上一页
步骤5:第一页
步骤6:最后一页
步骤7:边界处理
步骤8:套上Bootstrap

步骤 1 : 首先准备 DAO

在DAO中提供方法 

public List<Hero> list(int start, int count) 

start表示开始的个数，count表示取多少条
比如 list(0, 5) ， 即表示第一页，每页有5条数据
比如 list(5, 5) ， 即表示第二页，每页有5条数据

package dao;

import java.sql.Connection;

import java.sql.DriverManager;

import java.sql.PreparedStatement;

import java.sql.ResultSet;

import java.sql.SQLException;

import java.sql.Statement;

import java.util.ArrayList;

import java.util.List;

import bean.Hero;

public class HeroDAO {

    public int getTotal() {

        int total = 0;

        try {

            Class.forName("com.mysql.jdbc.Driver");

            Connection c = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/how2java?characterEncoding=UTF-8",

                    "root", "admin");

            Statement s = c.createStatement();

            String sql = "select count(*) from hero";

            ResultSet rs = s.executeQuery(sql);

            while (rs.next()) {

                total = rs.getInt(1);

            }

            System.out.println("total:" + total);

            s.close();

            c.close();

        } catch (ClassNotFoundException e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        } catch (SQLException e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        }

        return total;

    }

    public void add(Hero hero) {

        try {

            Class.forName("com.mysql.jdbc.Driver");

            Connection c = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/how2java?characterEncoding=UTF-8",

                    "root", "admin");

            String sql = "insert into hero values(null,?,?,?)";

            PreparedStatement ps = c.prepareStatement(sql);

            ps.setString(1, hero.name);

            ps.setFloat(2, hero.hp);

            ps.setInt(3, hero.damage);

            ps.execute();

            ResultSet rs = ps.getGeneratedKeys();

            if (rs.next()) {

                int id = rs.getInt(1);

                hero.id = id;

            }

            ps.close();

            c.close();

        } catch (ClassNotFoundException e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        } catch (SQLException e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        }

    }

    public void update(Hero hero) {

        try {

            Class.forName("com.mysql.jdbc.Driver");

            Connection c = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/how2java?characterEncoding=UTF-8",

                    "root", "admin");

            String sql = "update hero set name= ?, hp = ? , damage = ? where id = ?";

            PreparedStatement ps = c.prepareStatement(sql);

            ps.setString(1, hero.name);

            ps.setFloat(2, hero.hp);

            ps.setInt(3, hero.damage);

            ps.setInt(4, hero.id);

            ps.execute();

            ps.close();

            c.close();

        } catch (ClassNotFoundException e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        } catch (SQLException e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        }

    }

    public void delete(int id) {

        try {

            Class.forName("com.mysql.jdbc.Driver");

            Connection c = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/how2java?characterEncoding=UTF-8",

                    "root", "admin");

            Statement s = c.createStatement();

            String sql = "delete from hero where id = " + id;

            s.execute(sql);

            s.close();

            c.close();

        } catch (ClassNotFoundException e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        } catch (SQLException e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        }

    }

    public Hero get(int id) {

        Hero hero = null;

        try {

            Class.forName("com.mysql.jdbc.Driver");

            Connection c = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/how2java?characterEncoding=UTF-8",

                    "root", "admin");

            Statement s = c.createStatement();

            String sql = "select * from hero where id = " + id;

            ResultSet rs = s.executeQuery(sql);

            if (rs.next()) {

                hero = new Hero();

                String name = rs.getString(2);

                float hp = rs.getFloat("hp");

                int damage = rs.getInt(4);

                hero.name = name;

                hero.hp = hp;

                hero.damage = damage;

                hero.id = id;

            }

            s.close();

            c.close();

        } catch (ClassNotFoundException e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        } catch (SQLException e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        }

        return hero;

    }

    public List<Hero> list() {

        return list(0, Short.MAX_VALUE);

    }

    public List<Hero> list(int start, int count) {

        List<Hero> heros = new ArrayList<Hero>();

        try {

            Class.forName("com.mysql.jdbc.Driver");

            Connection c = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/how2java?characterEncoding=UTF-8",

                    "root", "admin");

            String sql = "select * from hero order by id desc limit ?,? ";

            PreparedStatement ps = c.prepareStatement(sql);

            ps.setInt(1, start);

            ps.setInt(2, count);

            ResultSet rs = ps.executeQuery();

            while (rs.next()) {

                Hero hero = new Hero();

                int id = rs.getInt(1);

                String name = rs.getString(2);

                float hp = rs.getFloat("hp");

                int damage = rs.getInt(4);

                hero.id = id;

                hero.name = name;

                hero.hp = hp;

                hero.damage = damage;

                heros.add(hero);

            }

            ps.close();

            c.close();

        } catch (ClassNotFoundException e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        } catch (SQLException e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        }

        return heros;

    }

}

步骤 2 : 只显示5条数据

修改HeroListServlet

int start = 0;

int count = 5;

List<Hero> heros = new HeroDAO().list(start, count);

即表示只获取5条数据

package servlet;

import java.io.IOException;

import java.util.List;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import bean.Hero;

import dao.HeroDAO;

public class HeroListServlet extends HttpServlet {

    protected void service(HttpServletRequest request, HttpServletResponse response)

            throws ServletException, IOException {

        response.setContentType("text/html; charset=UTF-8");

        int start = 0;

        int count = 5;

        List<Hero> heros = new HeroDAO().list(start, count);

        request.setAttribute("heros", heros);

        request.getRequestDispatcher("listHero.jsp").forward(request, response);

    }

}

步骤 3 : 下一页

HeroListServlet : 
通过参数获取start，如果浏览器没有传递参数，就设置为0。 
根据start，计算next. next的值就是start+count.
然后把next传递给listHero.jsp

listHero.jsp
在最后面增加一个超链

<a href="?start=${next}">[下一页]</a>

start=${next} 从服务器传递过来的next值

• HeroListServlet.java
• listHero.jsp

package servlet;

import java.io.IOException;

import java.util.List;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import bean.Hero;

import dao.HeroDAO;

public class HeroListServlet extends HttpServlet {

    protected void service(HttpServletRequest request, HttpServletResponse response)

            throws ServletException, IOException {

        response.setContentType("text/html; charset=UTF-8");

        int start = 0;

        int count = 5;

        try {

            start = Integer.parseInt(request.getParameter("start"));

        } catch (NumberFormatException e) {

            // 当浏览器没有传参数start时

        }

        int next = start + count;

        List<Hero> heros = new HeroDAO().list(start, count);

        request.setAttribute("next", next);

        request.setAttribute("heros", heros);

        request.getRequestDispatcher("listHero.jsp").forward(request, response);

    }

}

<%@ page language="java" contentType="text/html; charset=UTF-8"

    pageEncoding="UTF-8" import="java.util.*"%>

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

<table align='center' border='1' cellspacing='0'>

    <tr>

        <td>id</td>

        <td>name</td>

        <td>hp</td>

        <td>damage</td>

        <td>edit</td>

        <td>delete</td>

    </tr>

    <c:forEach items="${heros}" var="hero" varStatus="st">

        <tr>

            <td>${hero.id}</td>

            <td>${hero.name}</td>

            <td>${hero.hp}</td>

            <td>${hero.damage}</td>

            <td><a href="editHero?id=${hero.id}">edit</a></td>

            <td><a href="deleteHero?id=${hero.id}">delete</a></td>

        </tr>

    </c:forEach>

    <tr>

        <td colspan="6" align="center">

            <a href="?start=${next}">[下一页]</a>

        </td>

    </tr>

</table>

步骤 4 : 上一页

HeroListServlet:
根据start，计算pre. pre的值就是start-count.
然后把pre传递给listHero.jsp

listHero.jsp
在下一页前增加一个超链

<a href="?start=${pre}">[上一页]</a>

start=${pre} 从服务器传递过来的pre值

• HeroListServlet.java
• listHero.jsp

package servlet;

import java.io.IOException;

import java.util.List;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import bean.Hero;

import dao.HeroDAO;

public class HeroListServlet extends HttpServlet {

    protected void service(HttpServletRequest request, HttpServletResponse response)

            throws ServletException, IOException {

        response.setContentType("text/html; charset=UTF-8");

        int start = 0;

        int count = 5;

        try {

            start = Integer.parseInt(request.getParameter("start"));

        } catch (NumberFormatException e) {

            // 当浏览器没有传参数start时

        }

        int next = start + count;

        int pre = start - count;

        request.setAttribute("next", next);

        request.setAttribute("pre", pre);

        List<Hero> heros = new HeroDAO().list(start, count);

        request.setAttribute("heros", heros);

        request.getRequestDispatcher("listHero.jsp").forward(request, response);

    }

}

<%@ page language="java" contentType="text/html; charset=UTF-8"

    pageEncoding="UTF-8" import="java.util.*"%>

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

<table align='center' border='1' cellspacing='0'>

    <tr>

        <td>id</td>

        <td>name</td>

        <td>hp</td>

        <td>damage</td>

        <td>edit</td>

        <td>delete</td>

    </tr>

    <c:forEach items="${heros}" var="hero" varStatus="st">

        <tr>

            <td>${hero.id}</td>

            <td>${hero.name}</td>

            <td>${hero.hp}</td>

            <td>${hero.damage}</td>

            <td><a href="editHero?id=${hero.id}">edit</a></td>

            <td><a href="deleteHero?id=${hero.id}">delete</a></td>

        </tr>

    </c:forEach>

    <tr>

        <td colspan="6" align="center">

            <a href="?start=${pre}">[上一页]</a>

            <a href="?start=${next}">[下一页]</a>

        </td>

    </tr>

</table>

步骤 5 : 第一页

只需要修改listHero.jsp即可

增加 

<a href="?start=0">[首  页]</a>

因为首页的start永远都是0

<%@ page language="java" contentType="text/html; charset=UTF-8"

    pageEncoding="UTF-8" import="java.util.*"%>

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

<table align='center' border='1' cellspacing='0'>

    <tr>

        <td>id</td>

        <td>name</td>

        <td>hp</td>

        <td>damage</td>

        <td>edit</td>

        <td>delete</td>

    </tr>

    <c:forEach items="${heros}" var="hero" varStatus="st">

        <tr>

            <td>${hero.id}</td>

            <td>${hero.name}</td>

            <td>${hero.hp}</td>

            <td>${hero.damage}</td>

            <td><a href="editHero?id=${hero.id}">edit</a></td>

            <td><a href="deleteHero?id=${hero.id}">delete</a></td>

        </tr>

    </c:forEach>

    <tr>

        <td colspan="6" align="center">

            <a href="?start=0">[首  页]</a>

            <a href="?start=${pre}">[上一页]</a>

            <a href="?start=${next}">[下一页]</a>

        </td>

    </tr>

</table>

步骤 6 : 最后一页

HeroListServlet:
在HeroListServlet中计算last
last需要根据总数total和每页有多少条数据count来计算得出。

同时，还要看total是否能够整除count
假设总数是50，是能够被5整除的，那么最后一页的开始就是45

if (0 == total % count)

  last = total - count;

假设总数是51，不能够被5整除的，那么最后一页的开始就是50

last = total - total % count;

listHero.jsp
在下一页后增加一个超链

<a href="?start=${last}">[末 页]</a>

start=${last} 从服务器传递过来的last值

• HeroListServlet.java
• listHero.jsp

package servlet;

import java.io.IOException;

import java.util.List;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import bean.Hero;

import dao.HeroDAO;

public class HeroListServlet extends HttpServlet {

    protected void service(HttpServletRequest request, HttpServletResponse response)

            throws ServletException, IOException {

        response.setContentType("text/html; charset=UTF-8");

        int start = 0;

        int count = 5;

        try {

            start = Integer.parseInt(request.getParameter("start"));

        } catch (NumberFormatException e) {

            // 当浏览器没有传参数start时

        }

        int next = start + count;

        int pre = start - count;

        int total = new HeroDAO().getTotal();

        int last;

        // 假设总数是50，是能够被5整除的，那么最后一页的开始就是45

        if (0 == total % count)

            last = total - count;

        // 假设总数是51，不能够被5整除的，那么最后一页的开始就是50

        else

            last = total - total % count;

        request.setAttribute("next", next);

        request.setAttribute("pre", pre);

        request.setAttribute("last", last);

        List<Hero> heros = new HeroDAO().list(start, count);

        request.setAttribute("heros", heros);

        request.getRequestDispatcher("listHero.jsp").forward(request, response);

    }

}

<%@ page language="java" contentType="text/html; charset=UTF-8"

    pageEncoding="UTF-8" import="java.util.*"%>

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

<table align='center' border='1' cellspacing='0'>

    <tr>

        <td>id</td>

        <td>name</td>

        <td>hp</td>

        <td>damage</td>

        <td>edit</td>

        <td>delete</td>

    </tr>

    <c:forEach items="${heros}" var="hero" varStatus="st">

        <tr>

            <td>${hero.id}</td>

            <td>${hero.name}</td>

            <td>${hero.hp}</td>

            <td>${hero.damage}</td>

            <td><a href="editHero?id=${hero.id}">edit</a></td>

            <td><a href="deleteHero?id=${hero.id}">delete</a></td>

        </tr>

    </c:forEach>

    <tr>

        <td colspan="6" align="center">

            <a href="?start=0">[首  页]</a>

            <a href="?start=${pre}">[上一页]</a>

            <a href="?start=${next}">[下一页]</a>

            <a href="?start=${last}">[末  页]</a>

        </td>

    </tr>

</table>

步骤 7 : 边界处理

上一页，下一页有一个问题，
如果在第一页点击上一页，就会看不到数据了，因为在第一页的时候，pre=-5,也就导致传递到serlvet的start=-5;
同样的在最后一页的时候，点击下一页，也有类似的问题。

解决办法是进行边界处理：

pre = pre < 0 ? 0 : pre;

如果pre是负数了，就使用0 

next = next > last ? last : next;

如果next大于last，就使用last

package servlet;

import java.io.IOException;

import java.util.List;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import bean.Hero;

import dao.HeroDAO;

public class HeroListServlet extends HttpServlet {

    protected void service(HttpServletRequest request, HttpServletResponse response)

            throws ServletException, IOException {

        response.setContentType("text/html; charset=UTF-8");

        int start = 0;

        int count = 5;

        try {

            start = Integer.parseInt(request.getParameter("start"));

        } catch (NumberFormatException e) {

            // 当浏览器没有传参数start时

        }

        int next = start + count;

        int pre = start - count;

        int total = new HeroDAO().getTotal();

        int last;

        if (0 == total % count)

            last = total - count;

        else

            last = total - total % count;

        pre = pre < 0 ? 0 : pre;

        next = next > last ? last : next;

        request.setAttribute("next", next);

        request.setAttribute("pre", pre);

        request.setAttribute("last", last);

        List<Hero> heros = new HeroDAO().list(start, count);

        request.setAttribute("heros", heros);

        request.getRequestDispatcher("listHero.jsp").forward(request, response);

    }

}

步骤 8 : 套上Bootstrap

<%@ page language="java" contentType="text/html; charset=UTF-8"

    pageEncoding="UTF-8" import="java.util.*"%>

<!DOCTYPE html>

<script src="https://how2j.cn/study/js/jquery/2.0.0/jquery.min.js"></script>

<link href="https://how2j.cn/study/css/bootstrap/3.3.6/bootstrap.min.css" rel="stylesheet">

<script src="https://how2j.cn/study/js/bootstrap/3.3.6/bootstrap.min.js"></script>

     

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>

<script>

$(function(){

    $("a").addClass("btn btn-default btn-xs");

     

});

</script> 

<table style="500px; margin:44px auto" class="table table-striped table-bordered table-hover  table-condensed" align='center' border='1' cellspacing='0'>

    <tr>

        <td>id</td>

        <td>name</td>

        <td>hp</td>

        <td>damage</td>

        <td>edit</td>

        <td>delete</td>

    </tr>

    <c:forEach items="${heros}" var="hero" varStatus="st">

        <tr>

            <td>${hero.id}</td>

            <td>${hero.name}</td>

            <td>${hero.hp}</td>

            <td>${hero.damage}</td>

            <td><a href="editHero?id=${hero.id}">编辑</a></td>

            <td><a href="deleteHero?id=${hero.id}">删除</a></td>

        </tr>

    </c:forEach>

</table>

        <nav>

          <ul class="pager">

            <li><a href="?start=0">首  页</a></li>

            <li><a href="?start=${pre}">上一页</a></li>

            <li><a href="?start=${next}">下一页</a></li>

            <li><a href="?start=${last}">末  页</a></li>

          </ul>            

        </nav>

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