谈谈MVC

一、个人理解

​ 如果我们将数据库增删改的操作代码、业务代码、界面的html脚本代码都写在Servlet文件中，这样Servlet代码将会变得臃肿，可读性非常差，维护的时会相当麻烦。为此我们需要需找一个架构方法让代码的结构更清晰，更容易维护，MVC就是解决该问题的架构模式之一。

​ MVC是三个单词首字母的首拼，分别是Model（模型）、View（视图）和Contorller（控制）。该结构可以简单的看成三层

1）最上面的一层，是直接面向最终用户的"视图层"（View）。它是提供给用户的操作界面，是程序的外壳。

2）中间的一层，就是"控制层"（Controller），它负责根据用户从"视图层"输入的指令，选取"数据层"中的数据，然后对其进行相应的操作，产生最终结果。

3）最底下的一层，是核心的"数据层"（Model），也就是程序需要操作的数据或信息。

二、使用MVC思想实现查询功能

1、实体类Hero

public class Hero{
    public int id;
    public String name;
    public float hp;
    public int damage;
    public int getId(){return id;}
    public String getName(){return name;}
    public float getHp(){return hp;}
    public int getDamage(){return damage;}
    public void setId(int id){this.id=id;}
    public void setName(String name){this.name=name;}
    public void setHp(float hp){this.hp=hp;}
    public void setDamage(int damage){this.damage=damage;}
}

2、HeroDao

public class HeroDAO {
   
    public HeroDAO() {
        try {
            Class.forName("com.mysql.jdbc.Driver");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
   
    public Connection getConnection() throws SQLException {
        return DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/how2java?characterEncoding=UTF-8", "root",
                "admin");
    }
   
    public int getTotal() {
        int total = 0;
        try (Connection c = getConnection(); 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);
   
        } catch (SQLException e) {
   
            e.printStackTrace();
        }
        return total;
    }
   
    public void add(Hero hero) {
   
        String sql = "insert into hero values(null,?,?,?)";
        try (Connection c = getConnection(); 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;
            }
        } catch (SQLException e) {
   
            e.printStackTrace();
        }
    }
   
    public void update(Hero hero) {
   
        String sql = "update hero set name= ?, hp = ? , damage = ? where id = ?";
        try (Connection c = getConnection(); 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();
   
        } catch (SQLException e) {
   
            e.printStackTrace();
        }
   
    }
   
    public void delete(int id) {
   
        try (Connection c = getConnection(); Statement s = c.createStatement();) {
   
            String sql = "delete from hero where id = " + id;
   
            s.execute(sql);
   
        } catch (SQLException e) {
   
            e.printStackTrace();
        }
    }
   
    public Hero get(int id) {
        Hero hero = null;
   
        try (Connection c = getConnection(); 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;
            }
   
        } catch (SQLException e) {
   
            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>();
   
        String sql = "select * from hero order by id desc limit ?,? ";
   
        try (Connection c = getConnection(); 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);
            }
        } catch (SQLException e) {
   
            e.printStackTrace();
        }
        return heros;
    }
   
}

3、HeroServlet

public class HeroListServlet extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        try {
            List<Hero> heros=new HeroDao().list();
            req.setAttribute("heros",heros);
            req.getRequestDispatcher("listHero.jsp").forward(req, resp);
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
//配置web.xml

4、listHero.jsp

<%@ page language="java" contentType="text/html;charset=UTF-8" pageEncoding="UTF-8" import="java.util.*"%>
<%@page isELIgnored="false" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<html>
<head>
    <title>Title</title>
</head>
<body>
<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></tr>
</table>
</body>
</html>

效果：

三、使用mvc思想实现分页功能

1、修改HeroServlet

protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        try {
            int total=new HeroDao().total();
            int start=0;
            int count=5;
            try {
                start = Integer.parseInt(req.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 last;
            if (0 == total % count)
  			last = total - count;
            else 
                last=total-tatal%count;
            //第一页和最后一页的边界处理
             pre = pre < 0 ? 0 : pre;
        next = next > last ? last : next;
            List<Hero> heros=new HeroDao().list(start,count);
            req.setAttribute("next", next); 
            req.setAttribute("pre", pre);
            req.setAttribute("last", last); 
            req.setAttribute("heros",heros);
            req.getRequestDispatcher("listHero.jsp").forward(req, resp);
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

2、修改listHero.jsp

添加了首页、上一页，下一页，末页的超链接标签。

<%--
  Created by IntelliJ IDEA.
  User: QY
  Date: 2021/2/23
  Time: 11:11
  To change this template use File | Settings | File Templates.
--%>

<%@ page language="java" contentType="text/html;charset=UTF-8" pageEncoding="UTF-8" import="java.util.*"%>
<%@page isELIgnored="false" %>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<html>
<head>
    <title>Title</title>
</head>
<body>
<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>
</body>
</html>

效果：