三层架构之抽象工厂加反射----实现数据库转换

注意：文中代码有误，为保留历史痕迹在本文不做修改，正确代码详见《纠错》

在做系统的时候有意识的用到了抽象工厂这个设计模式，主要解决的是数据库更换的问题。

下面就以简单的登录来逐步的分析一下这个模式。

经典的三层架构

数据库如下

1.      一般的数据库连接方式

界面层

 

Public Class Login
    Private Sub btnLogin_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnLogin.Click
        Dim LUser As New Entity.User
        Dim BCheck As New BLL.B_Login
        LUser.User_ID = txtName.Text
        LUser.User_Pwd = txtPwd.Text
        If BCheck.Check(LUser) = True Then
            MsgBox("登录成功！")
        Else
            MsgBox(“"登录失败！")
        End If
    End Sub

    Private Sub btnCancle_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnCancle.Click
        End
    End Sub
End Class

业务逻辑层

Public Class B_Login
    Function Check(ByVal User As Entity.User) As Boolean
        Dim DaUser As New DAL.D_UserInfo
        Dim BlUser As New Entity.User
        Bluser.User_ID=User.ID
        BlUser = DaUser.Check(BlUser)
          If BlUser.User_Pwd = User.User_Pwd Then
             Return True
         Else
              Return False
         End If
      End Function
End Class

数据持久层

Imports System.Data.SqlClient
Public Class D_UserInfo
Dim ConnStr As String = "Data Source=******;Initial Catalog=Student;User ID=sa;Password=******"
    Dim conn As SqlConnection = New SqlConnection(ConnStr)
    Function Check(ByVal User As Entity.User) As Entity.User
        Dim sql As String = "select * from UserInfo where UserInfo=" & User.User_ID
        Dim cmd As SqlCommand = New SqlCommand(sql, conn)
        Dim read As SqlDataReader
        Try
            conn.Open()
            read = cmd.ExecuteReader
            User.User_ID = read.Item(0)
            User.User_Pwd = read.Item(1)
Return User
        Catch ex As Exception
            User.User_Pwd = ""
            Return User
        End Try
    End Function
End Class

2.      简单工厂

添加一个工厂类和一个接口

接口类

Public Interface IUserInfo
    Function Check(ByVal IUser As Entity.User) As Entity.User
End Interface

工厂类

Imports [Interface]
Public Class DFactory
    'Dim DataBase As String = "Access"
    Dim DataBase As String = "Sql"
    Function CreateUserInfo() As IUserInfo
        Dim DB As IUserInfo
        Select Case DataBase
            Case "Sql"
                DB = New D_UserInfoSql
                'Case "Access"
                '    DB = New D_UserInfoAccess
        End Select
        Return DB
    End Function
End Class

当然D_UserInfoSql实现接口，代码基本不变

当有新的数据库使用时候（例如Access数据库）可以将工厂中的注释部分添上，然后重新写Dal层就可以直接使用，但是这样的不足是还是需要再次编译工厂，利用反射可以解决这个问题。

3.抽象工厂加反射

工厂类

Imports [Interface]
Imports System.Reflection
Public Class DFactory
    '抽象工厂加反射
    Dim DBString As String = System.Configuration.ConfigurationSettings.AppSettings("DBString")
    Function CreateUserInfo() As IUserInfo
        Return CType(Assembly.Load("DAL").CreateInstance("DAL.D_UserInfo" & DBString), IUserInfo)
    End Function
End Class

数据持久层

Imports System.Data.SqlClient
Public Class D_UserInfoSql : Implements [Interface].IUserInfo
    'Dim ConnStr As String = "Data Source=******;Initial Catalog=Student;User ID=sa;Password=******"
    Dim ConnStr As String = System.Configuration.ConfigurationSettings.AppSettings("ConnStr")
    Dim conn As SqlConnection = New SqlConnection(ConnStr)
    Public Function Check(ByVal IUser As Entity.User) As Entity.User Implements [Interface].IUserInfo.Check
        Dim sql As String = "select * from UserInfo where UserInfo=" & IUser.User_ID
        Dim cmd As SqlCommand = New SqlCommand(sql, conn)
        Dim read As SqlDataReader
        Try
            conn.Open()
            read = cmd.ExecuteReader
            IUser.User_ID = read.Item(0)
            IUser.User_Pwd = read.Item(1)
            Return IUser
        Catch ex As Exception
            IUser.User_Pwd = ""
            Return IUser
        End Try
    End Function
End Class

配置文件

  <appSettings>
    <add key="ConnStr" value ="Data Source=******;Initial Catalog=Student;User ID=sa;Password=******"></add>
    <add key="DBString" value ="Sql"></add>
  </appSettings>

添加两个Key，一个是连接数据库的字符串，一个是通过反射来产生不同数据库的Dal层的

这样一来就可以实现设计模式中的“开闭原则”，如果更换数据库只需要增加类（DAL），而不需要更改，更不需要重新编译。

PS：配置文件必须在界面层

1、         反射的写法:

objType=Assembly.Load(AssemblyPath).CreateInstance(className);其中：AssemblyPath指程序集名。className指命名空间.类名称。

2、         反射的一个原则:一切皆以UI层的bin文件夹中的dll名称为中心。(原因很简单:.net类加载的机制就是默认从本程序集的bin文件中找,所以bin文件夹中一定要有要加载的程序集的dll)。UI层中bin文件夹中dll叫什么名字AssemblyPath就使用什么名字,bin内部类的全名叫什么，className就写成什么全名。.net中的引用:加入对某个程序集的引用就能在程序集有变化时自动拷贝dll。