MVC

MVC 模式代表 Model-View-Controller(模型-视图-控制器) 模式。这种模式用于应用程序的分层开发。

Model(模型) - 模型代表一个存取数据的对象或 JAVA POJO。它也可以带有逻辑，在数据变化时更新控制器。

View(视图) - 视图代表模型包含的数据的可视化。

Controller(控制器) - 控制器作用于模型和视图上。它控制数据流向模型对象，并在数据变化时更新视图。它使视图与模型分离开。

案例

下面我们来写一个简单的登录的案例，来提现MVC的设计模式

首先，我们有一个数据库，里面存放用户信息如下：

OK，下一步我们建立一个简单的登录界面前端的页面如下：

登录界面

${requestScope.err}

用户名：

密码：

如果我们不使用MVC的设计的话，程序应该如下：

package Servelet;

import utils.JDBCutil;

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.sql.Connection;

import java.sql.PreparedStatement;

import java.sql.ResultSet;

import java.sql.SQLException;

@WebServlet(name = "LoginServlet", urlPatterns = "/LoginServlet")

public class LoginServlet extends HttpServlet {

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

}

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

request.setCharacterEncoding("UTF-8");

String username = request.getParameter("username");

String password = request.getParameter("password");

System.out.println(username);

System.out.println(password);

System.out.println("测试");

try {

//1.数据库的连接

String JDBCDRVIER="com.mysql.jdbc.Driver";

String URL="jdbc:mysql://localhost:3306/ProjectServlet";

String USER="root";

String PASSWORD="3692512";

Class.forName(JDBCDRVIER);

Connection c = DriverManager.getConnection(URL,USER,PASSWORD)

System.out.println("数据库连接成功，正在查询....");

//2.数据的查询

String sql = "select * from user where username=? and password=?";

PreparedStatement p = c.prepareStatement(sql);

p.setObject(1,username);

p.setObject(2,password);

ResultSet rs = p.executeQuery();

//3.进行业务逻辑判断

if(rs.next()) {

request.getSession().setAttribute("username",username);

request.getSession().setAttribute("password",password);

System.out.println("用户查询成功！跳转主页中.....");

//不能转发，要用重定向，防止用户多次回车访问

//request.getRequestDispatcher("index.jsp");

response.sendRedirect("index.jsp");

}else{

System.out.println("用户名或密码不正确,跳转登录页面中.....");

request.setAttribute("err","用户名或密码不正确");

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

}

} catch (SQLException e) {

e.printStackTrace();

}

}

}

下面我们使用MVC设计模式，来重写上述案例。

Model

其实MVC中的M实际上就是根据数据库封装的Bean。这部分没什么好说的，我们利用Bean对象来存储我们的数据。

package Servelet.models;

//用于对应数据库中的信息

public class UserBean {

String username;

String password;

String nickname;

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;

}

public String getNickname() {

return nickname;

}

public void setNickname(String nickname) {

this.nickname = nickname;

}

@Override

public String toString() {

return "UserBean{" +

"username='" + username + '\'' +

", password='" + password + '\'' +

", nickname='" + nickname + '\'' +

'}';

}

}

Controller

这部分是Servelet程序，页面访问首先触发controller，主要作用就是

package Servelet.controllers;

import Servelet.models.UserBean;

import Servelet.services.UserServices;

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;

@WebServlet(name = "UserControllerServlet" ,urlPatterns = "/UserLogin")

public class UserControllerServlet extends HttpServlet {

private UserServices services;

@Override

public void init() throws ServletException {

services = new UserServices();

}

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

}

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

request.setCharacterEncoding("UTF-8");

String username = request.getParameter("username");

String password = request.getParameter("password");

System.out.println(username);

System.out.println(password);

try {

UserBean bean = services.checkLogin(username, password);

if(bean!=null){

System.out.println("用户查询成功！跳转主页中.....");

request.getSession().setAttribute("username",bean);

// request.getRequestDispatcher("index.jsp");

response.sendRedirect("index.jsp");

}else {

request.setAttribute("err","用户名或密码不正确");

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

}

} catch (Exception e) {

e.printStackTrace();

}

}

}

这个部分可以看到我们仅仅做了逻辑判断，和数据库交互的工作和业务逻辑我们交给services对象。

Services

package com.kkb.services;

import com.kkb.dao.UserDao;

import com.kkb.models.UserBean;

import java.sql.*;

//用于处理用户相关的业务逻辑

public class UserService {

private UserDao dao;

public UserService() {

dao = new UserDao();

}

//处理的登录逻辑 没有使用DAO之前

public UserBean checkLogin(String username,String pwd) throws Exception{

//连接数据库

String JDBCDRVIER="com.mysql.jdbc.Driver";

String URL="jdbc:mysql://localhost:3306/ProjectServlet";

String USER="root";

String PASSWORD="3692512";

Class.forName(JDBCDRVIER);

Connection connection = DriverManager.getConnection(URL,USER,PASSWORD)

System.out.println("数据库连接成功，正在查询....");

//查询数据库

PreparedStatement statement = connection.prepareStatement("select *from user where username = ? and password = ?");

statement.setObject(1,username);

statement.setObject(2,password);

ResultSet resultSet = statement.executeQuery();

if (resultSet.next()){

//将数据打包成一个UserBean

UserBean bean = new UserBean();

//从结果集中取出数据

bean.setId(resultSet.getInt(1));

bean.setName(resultSet.getString(2));

bean.setPwd(resultSet.getString(3));

//返回bean 表示登录成功了

return bean;

}else{

//表示用户名和密码不正确

return null;

}

}

但是，这里我们发现service里面既有业务逻辑的处理，也有和数据库的交互，这样代码的耦合程度高，不方便日后维护，所以我们考虑在原MVC的基础上添加DAO层，专门用来和数据库交互，这样能够进一步降低代码的耦合度。

DAO

这里的DAO就是我们专门和数据库打交道的地方，引入DAO后的逻辑可用下图表示。

这里我们dao层有两个java文件，一个是DBTool，是封装的数据库工具类，一个是UserDao，输入username返回bean对象(注意，这里仅仅是输入username，不执行是否密码一致的逻辑业务判断，判断是上一层service的范畴)

package dao;

import models.UserBean;

import java.sql.SQLException;

import java.util.List;

import java.util.Map;

public class UserDAO {

private DBTool dbTool;

public UserDAO() {

try {

this.dbTool = new DBTool();

} catch (ClassNotFoundException e) {

e.printStackTrace();

}

}

// 根据用户名获取一条数据

public UserBean selectUserByName(String username){

//编写sql

String sql = "select *from user where username = ?";

//调用连接类来执行sql

try {

List> maps = dbTool.executeQuery(sql, username);

if (maps.size() > 0){

return mapToUserBaen(maps.get(0));

}

} catch (SQLException e) {

e.printStackTrace();

}

return null;

}

//将map 转换为对象

public UserBean mapToUserBaen(Map map){

//把map转为UserBean

UserBean bean = new UserBean();

bean.setUsername((String) map.get("username"));

bean.setPassword((String) map.get("password"));

bean.setNickname((String) map.get("nickname"));

return bean;

}

public void insertUser(String name, String pwd) throws SQLException {

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

dbTool.executeUpdate(sql,name,pwd);

}

}

package dao;

import java.sql.*;

import java.util.ArrayList;

import java.util.HashMap;

import java.util.List;

import java.util.Map;

public class DBTool {

public String ip = "127.0.0.1";

public int port = 3306;

public String

user="root",

password="3692512",

charset ="utf8",

dbName="ProjectServlet";

private static boolean DriverLoaded=false;

//使用默认参数链接数据库

public DBTool() throws ClassNotFoundException {

if(DriverLoaded)return;

try {

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

System.out.println("DBTools message:数据库驱动加载成功!");

} catch (ClassNotFoundException e) {

System.out.println("DBTools Error:驱动程序加载失败!");

throw e;

}

DriverLoaded=true;

}

//自定义参数初始化

public DBTool(String ip, int port, String user, String password, String dbName) throws ClassNotFoundException {

this();

this.ip = ip;

this.port = port;

this.user = user;

this.password = password;

this.dbName = dbName;

}

//自定义参数初始化

public DBTool(String user, String password, String dbName) throws ClassNotFoundException {

this();

this.user = user;

this.password = password;

this.dbName = dbName;

}

//获取一个链接

public Connection getConnection() throws SQLException {

String url = String.format("jdbc:mysql://%s:%s/%s?characterEncoding=%s&user=%s&password=%s&useSSL=false",ip,port,dbName,charset,user,password);

try {

return DriverManager.getConnection(url);

} catch (SQLException e) {

System.out.println("DBTools Error 数据库连接失败!");

throw e;

}

}

//执行查询语句

public List> executeQuery(String sql, Object...args) throws SQLException {

ArrayList> res = new ArrayList<>();

ResultSet resultSet = null;

PreparedStatement preparedStatement = null;

Connection connection = null;

try {

connection = getConnection();

preparedStatement = getPreparedStatement(connection, sql, args);

resultSet = preparedStatement.executeQuery();

while (resultSet.next()) {

resultSet.getMetaData().getColumnCount();

HashMap map = new HashMap<>();

for (int i = 1; i <= resultSet.getMetaData().getColumnCount() ; i++) {

map.put(resultSet.getMetaData().getColumnName(i),resultSet.getObject(i));

}

res.add(map);

}

} catch (SQLException e) {

e.printStackTrace();

throw e;

} finally {

if(resultSet != null)

resultSet.close();

if(preparedStatement != null)

preparedStatement.close();

if(connection != null)

connection.close();

}

return res;

}

//sql参数预处理

private PreparedStatement getPreparedStatement(Connection connection, String sql, Object[] args) throws SQLException {

PreparedStatement preparedStatement = connection.prepareStatement(sql);

int count = sql.length() - sql.replace("?", "").length();

if(count != args.length){

throw new SQLException("DBTool Error: 参数个数不匹配");

}

for (int i = 0; i < args.length; i++) {

preparedStatement.setObject(i+1,args[i]);

}

return preparedStatement;

}

//执行更新语句 包括delete update insert

public boolean executeUpdate(String sql,Object...args) throws SQLException {

try {

Connection connection = getConnection();

PreparedStatement preparedStatement = getPreparedStatement(connection, sql, args);

int i = preparedStatement.executeUpdate();

if (i>0){return true;}

} catch (SQLException e) {

e.printStackTrace();

throw e;

}

return false;

}

}

最后当我们写好dao层后，我们修改service如下：

package services;

import dao.DBTool;

import dao.UserDAO;

import models.UserBean;

import java.sql.*;

public class UserServices {

private UserDAO dao;

public UserServices() {

dao = new UserDAO();

}

public UserBean checkLogin(String username, String password) throws Exception{

UserBean userBean = dao.selectUserByName(username);

if(userBean != null){

if(userBean.getPassword().equals(password)){

return userBean;

}

}

return null;

// //连接数据库

// String JDBCDRVIER="com.mysql.jdbc.Driver";

// String URL="jdbc:mysql://localhost:3306/ProjectServlet";

// String USER="root";

// String PASSWORD="3692512";

// Class.forName(JDBCDRVIER);

// Connection connection = DriverManager.getConnection(URL,USER,PASSWORD);

// System.out.println("数据库连接成功，正在查询....");

//

// //查询数据库

// PreparedStatement statement = connection.prepareStatement("select *from user where username = ? and password = ?");

// statement.setObject(1,username);

// statement.setObject(2,password);

// ResultSet resultSet = statement.executeQuery();

// if (resultSet.next()){

// //将数据打包成一个UserBean

// UserBean bean = new UserBean();

// //从结果集中取出数据

// bean.setUsername(resultSet.getString(1));

// bean.setPassword(resultSet.getString(2));

// bean.setNickname(resultSet.getString(3));

// //返回bean 表示登录成功了

// return bean;

// }else{

// //表示用户名和密码不正确

// return null;

// }

}

}

可以看到，我们通过MVC+Dao的形式，实现了程序的解耦，这样修改需求的时候，我们可以方便后序的维护。