JavaWeb中的MVC 下

2019-12-17 16:04:51来源：博客园阅读 ()

JavaWeb中的MVC 下

代码较多,请先略过代码,看懂逻辑在研究代码

引入

回顾上一节中的项目,最终的层次结构:

在MVC上中,我们分析了MVC设计模式具备的优点,以及不足,并在其基础上增了Service层用于处理业务逻辑,但是这还没完,对于大型项目来说,程序结构依然是不够清晰的,Service层不仅要处理业务逻辑,还要处理数据库操作;依然存在以下问题:

耦合度高
代码复用性差
Service层的功能职责不单一

DAO

为了解决这些问题,需要将Service层进一步的解耦,如何做到呢,通过增加数据访问层DAO即可实现;

定义:

DAO,(Data Access Object),数据访问对象,指提供了对数据的CRUD功能的对象;,负责与数据库打交道,处于业务逻辑层和数据库之间;

简单的说:

就是把原本在Service层中的数据库操作封装到DAO层中去;使得Service层可以专注于业务逻辑的处理,降低各功能间的耦合度;

增加DAO后的层次结构图:

DAO层的组成

DAO是针对数据访问层的设计模式,其完整组成包括如下部分:

看起来非常复杂,没错,DAO是一个用于解决数据库操作的完整解决方案,如果按照上述结构来实现的话,对于大型商业项目而言非常的规范,但是对小型的需要,快速开发的项目而言,让人望而生畏

老司机建议: 过度设计会让人看不清本质,学习设计模式时一定要牢记你要解决的关键问题,带着问题去看各部分的作用和重要性

实例

为了能够更清晰的认识到DAO的本质,这里采用简化后的DAO设计

DAO中有三个对象是必须的:

数据访问对象(Bean)
数据库连接类
DAO实现类

最终Service层需要的是DAO实现对象,数据库连接对象是为了将重复代码进行抽取而存在的,Bean也是在原来系统中已经存在的

现在需要两个新的类一个DAO,一个数据库连接类,创建它们像下面这样

DBTool:

数据库连接类,负责连接数据库执行查询,最后关闭资源

package com.kkb.test;

import java.sql.*;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;


public class DBTool {

    //默认参数
    public   String ip = "127.0.0.1";
    public   int port = 3306;
    public   String
            user="root",
            password="admin",
            charset ="utf8",
            dbName="db1";
    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<Map<String,Object>> executeQuery(String sql, Object...args) throws SQLException {
        ArrayList<Map<String, Object>> 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<String, Object> 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
    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;
    }
}

UserDao:

负责为Service层提供需要的CURD方法,本质就是封装了SQL的执行,和结果的解析

package com.kkb.models;

import com.kkb.test.DBTool;

import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

public class UserDao {
    private DBTool tools;

    //构造函数
    public UserDao() throws ClassNotFoundException {
        tools = new DBTool();
    }

    //新增用户
    public boolean insertUser(UserBean user) throws SQLException {
        String sql = "insert into user values(null,?,?)";
        return tools.executeUpdate(sql, user.getName(), user.getPwd());

    }

    //删除用户
    public boolean deleteUser(UserBean user) throws SQLException {
        String sql = "delete from user where id = ?";
        return tools.executeUpdate(sql, user.getId());

    }

    //更新用户
    public boolean updateUser(UserBean user) throws SQLException {
        String sql = "update user set name = ? , pwd = ?  where id = ?";
        return tools.executeUpdate(sql, user.getName(), user.getPwd(), user.getId());
    }

    //查询所有用户
    public List<UserBean> queryAllUser() throws SQLException {
        ArrayList<UserBean> beans = new ArrayList<>();

        List<Map<String, Object>> maps = tools.executeQuery("select *from user");
        //转List
        for (Map<String, Object> temp : maps) {
            UserBean bean = getUserBean(temp);
            beans.add(bean);
        }
        return beans;
    }

    //map 转 bean 方法
    private UserBean getUserBean(Map<String, Object> temp) {
        UserBean bean = new UserBean();
        bean.setId((Integer) temp.get("id"));
        bean.setName((String) temp.get("name"));
        bean.setPwd((String) temp.get("pwd"));
        return bean;
    }

    //通过ID查询
    public UserBean queryUserByID(Integer id) throws SQLException {
        List<Map<String, Object>> maps = tools.executeQuery("select *from user where id = ?", id);
        //转List
        for (Map<String, Object> temp : maps) {
            UserBean bean = getUserBean(temp);
            return bean;
        }
        return null;
    }

    //登录认证
    public UserBean checkLogin(UserBean login) throws SQLException {
        List<Map<String, Object>> maps = tools.executeQuery("select *from user where name = ? and pwd = ?", login.getName(), login.getPwd());
        for (Map<String, Object> temp : maps) {
            UserBean bean = getUserBean(temp);
            return bean;
        }
        return null;
    }

    //通过名字查询
    public UserBean queryUserByName(String name) throws SQLException {
        String sql = "select *from user where name = ?";
        List<Map<String, Object>> maps = tools.executeQuery(sql, name);
        if (!maps.isEmpty()){
            return getUserBean(maps.get(0));
        }
        return null;
    }
}

UserService:

替换原来使用JDBC的地方,改为使用UserDao来完成数据库操作

package com.kkb.srvices;

import com.kkb.exceptions.LoginException;
import com.kkb.models.UserBean;
import com.kkb.models.UserDao;

import java.sql.*;

//用来处理与用户相关的业务逻辑
public class UserService {

    private UserDao dao;

    public UserService() throws ClassNotFoundException {
        this.dao = new UserDao();
    }

    //用于检查登录的方法
    public UserBean checkLogin(UserBean reqBean) throws LoginException {
        //判断参数是否有效
        if(reqBean.getName() == null || reqBean.getPwd() == null ||
            reqBean.getName().equals("") || reqBean.getPwd().equals("")
        ){
            throw new LoginException("用户名或密码不能为空!");
        }else {
            try {
                UserBean bean = dao.checkLogin(reqBean);
                if(bean != null)
                    return bean;
                else
                    throw new LoginException("用户名或密码错误!");
            } catch (SQLException e) {
                e.printStackTrace();
                throw new LoginException("数据库炸了!");
            }
        }
    }
}

如此,就利用DAO(数据访问对象),来对Service层进行了解耦合,代码的可维护性提高了,但是相应的程序的复杂度也提高了