三层架构-----实践篇

上篇论述了三层的基本概念，作用，关系，如今展示下系统中用户登陆过程中简单应用三层结构设计思想。

vb.NET的操作例如以下

首先建立下面windows应用程序以及类库



首先建立实体类

Public Class userInfo
Private _username As String
Public Property UserName As String
Get
Return _username

End Get
Set(ByVal value As String)
_username = value

End Set
End Property
Private _pwd As String
Public Property Pwd As String
Get
Return _pwd

End Get
Set(value As String)
_pwd = value
End Set
End Property
End Class

UI层，前提是引用实体类（model）和BLL

Public Class Form1

Private Sub btnCancel_Click(sender As Object, e As EventArgs) Handles btnCancel.Click
Me.Close()
End Sub

Private Sub btnOK_Click(sender As Object, e As EventArgs) Handles btnOK.Click
Try
Dim euser2 As New Model.userInfo
Dim euser3 As New Model.userInfo

euser2.UserName = txtUserName.Text.Trim '将U层数据传递给实体
euser2.Pwd = txtPassword.Text.Trim

Dim mgr As New BLL.LoginMannager '将数据传递给B层
euser3 = mgr.UserLogin(euser2)
Catch ex As Exception
MessageBox.Show(ex.Message.ToString())

End Try
End Sub
End Class

DAL层：前提引用实体类

Imports System.Data
Imports System.Data.SqlClient
Imports Model

Public Class user
'创建数据库链接
Public conn As New SqlConnection("server=LXY;database=login;user Id=sa;password=123456")
Public Function SelectUser(ByVal User As Model.userInfo) As Model.userInfo
'传递实体uesrinfo而不是參数id.username 等。这样方便对实体中參数进行调用
Dim reader As SqlDataReader
Dim eUser As New Model.userInfo
Dim sql As String = "select UserName,PassWord FROM User_info WHERE UserName=@username and PassWord=@password"
Dim cmd As New SqlCommand(sql, conn)

cmd.CommandText = sql
cmd.CommandType = CommandType.Text
cmd.Parameters.Add(New SqlParameter("@username", User.UserName))
cmd.Parameters.Add(New SqlParameter("@password", User.Pwd))
conn.Open()
reader = cmd.ExecuteReader
While reader.Read()
eUser.UserName = reader.GetString(0)
eUser.Pwd = reader.GetString(1)
End While
Return eUser
conn.Close()

End Function
End Class

接下来是BLL层：前提引用DAL和实体类

Public Class LoginMannager
Public Function UserLogin(ByVal User As Model.userInfo) As Model.userInfo
Dim uDao As New DAL.user
Dim eUser1 As New Model.userInfo

eUser1 = uDao.SelectUser(User) '将数据传给D层

If IsNothing(eUser1.UserName) Then
Throw New Exception("登录失败。请验证请验证登陆名和password！")
Else
MsgBox("登陆成功。")

Return eUser1
End If

End Function
End Class

引用：我们看到三层架构的主要特点：UI调用BLL中类的方法,即UI知道BLL的方法,但不知道方法实现的细节，而BLL层并不知道UI的存在.相同BLL层调用DAL，而DAL也不知道BLL的存在。

三者中不论什么一者发生变化都不会影响其它俩者的正常工作。仅仅对变化者进行又一次调整就可以。

三层把Entity实体类的实例作为參数来传递.

总之，表示层仅仅提供软件系统与用户交互的接口。业务逻辑层是界面层和数据訪问层之间的桥梁，负责数据处理和传递；数据訪问层仅仅负责数据的存取工作。