简单工厂模式就是根据提供给它的数据,返回几个可能类中的一个类的实例.通常它返回的类都有一个共同的父类和共同的方法，但是每个方法执行的任务不同，而且根据不同的数据进行了优化。比如X是一个基类，xy和xz是从它派生出来的，XFactory类根据给出的参数决定返回那一个子类，返回那一个类的实例与程序员无关，因为这些类有同样的方法，只是实现不同，返回那个类的实例取决于工厂，工厂功能可能很复杂，但通常都是相当简间的.
接下来我们用一个例子来说明这个简单的工厂模式.假如我们要输入一个人的姓名，
有两个方式，一种是“firstname lastname” 和“fristname,lastname”形式.我们的任务就是要确定lastname和firstname是否是逗号来确定姓名的先后.
第一步，设计一个父类:
public class CName
{
protected string frName ,lName;
public string getFristName() { return frName;}
public string getLastName() { return lName;}
}
第二步,设计两个子类,它是由类CName的派生的。它们现实了接口，并在构造function中将名字分成了两部分.在CFirstFirst类中，做了一个简单的假设：最后一个空格前面的所有部分都属与firstname.
public class CFirstFirst : CName
{
public CFirstFirst (string sName)
{
int i = sName.Trim().Indexof(” “);
if ( i >0 )
{
frName = name.Substring(0,i).Trim();
lName = name.Substring(i+1).Trim();
}
else
{
lName = sName;
frName = “”;
}
}
}
在LastFist类中，用逗号给lastname划分界限.当空格或逗号不存在时，两个类都提供了错误校正处理.
public class LastFrist :CName
{
public LastFrist(string sName)
{
int i = sName.Indexof(”,”);
if ( i>0 )
{
if ( i >0 )
{
lName = name.Substring(0,i).Trim();
frName = name.Substring(i+1).Trim();
}
else
{
lName = sName;
frName = “”;
}
}
}
}
两种情况下，都把分开的名字保存在基类的CName中保护变量中.所以在派生类中根本不需要任何getFirstName和getLastName方法，因为基类中已给出.
第三步: 构造简单工厂.
现在很容易给出简单工厂类.只检查逗号来决定返回那个类的实例
public class NameFactory ()
{
public CName getName(string sname)
{
int i = sname.Indexof(”,”);
if ( i >0 )
{
return new LastFrist (sname);
}
else
{
return new CFirstFirst (sname);
}
}
}
第四步。使用工厂类:
string sName = “cedy hao”;
NameFactory nf = new NameFactory ();
CName name = nf.getName(sName);
string sFristName = name.getFristName();
string sLastName = name.getLastName();
这种方法把和数据相关的问题与类的其它的方法分隔开来。采用抽象工厂模式设计出的系统类图如下：
数据库层抽象工厂解决方案归档：
(1)AbstractDbFactory.cs
using System;
using System.Data;
namespace DbService
{
/// <summary>
/// 数据库抽象工厂接口
/// </summary>
public interface AbstractDbFactory
{
/// <summary>
/// 建立默认连接
/// </summary>
/// <returns>数据库连接</returns>
IDbConnection CreateConnection();
/// <summary>
/// 根据连接字符串建立Connection对象
/// </summary>
/// <param name="strConn">连接字符串</param>
/// <returns>Connection对象</returns>
IDbConnection CreateConnection(string strConn);
/// <summary>
/// 建立Command对象
/// </summary>
/// <returns>Command对象</returns>
IDbCommand CreateCommand();
/// <summary>
/// 建立DataAdapter对象
/// </summary>
/// <returns>DataAdapter对象</returns>
IDbDataAdapter CreateDataAdapter();
/// <summary>
/// 根据Connection建立Transaction
/// </summary>
/// <param name="myDbConnection">Connection对象</param>
/// <returns>Transaction对象</returns>
IDbTransaction CreateTransaction(IDbConnection myDbConnection);
/// <summary>
/// 根据Command建立DataReader
/// </summary>
/// <param name="myDbCommand">Command对象</param>
/// <returns>DataReader对象</returns>
IDataReader CreateDataReader(IDbCommand myDbCommand);
/// <summary>
/// 获得连接字符串
/// </summary>
/// <returns>连接字符串</returns>
string GetConnectionString();
}
}
(2)Factory.cs
using System;
using System.Configuration;
namespace DbService
{
/// <summary>
/// Factory类
/// </summary>
public sealed class Factory
{
private static volatile Factory singleFactory = null;
private static object syncObj = new object();
/// <summary>
/// Factory类构造函数
/// </summary>
private Factory()
{
}
/// <summary>
/// 获得Factory类的实例
/// </summary>
/// <returns>Factory类实例</returns>
public static Factory GetInstance()
{
if(singleFactory == null)
{
lock(syncObj)
{
if(singleFactory == null)
{
singleFactory = new Factory();
}
}
}
return singleFactory;
}
/// <summary>
/// 建立Factory类实例
/// </summary>
/// <returns>Factory类实例</returns>
public AbstractDbFactory CreateInstance()
{
AbstractDbFactory abstractDbFactory = null;
switch(ConfigurationSettings.AppSettings["DatabaseType"].ToLower())
{
case "sqlserver":
{
abstractDbFactory = new SqlFactory();
break;
}
case "oledb":
{
abstractDbFactory = new OleDbFactory();
break;
}
case "odbc":
{
abstractDbFactory = new OdbcFactory();
break;
}
}
return abstractDbFactory;
}
}
}
以下3个类分别是Factory针对SqlServer专用连接、OleDb连接和Odbc连接时的具体实现：
(3)SqlFactory.cs
using System;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
namespace DbService
{
/// <summary>
/// 针对SqlServer专用连接的工厂
/// </summary>
public class SqlFactory : AbstractDbFactory
{
/// <summary>
/// 构造函数
/// </summary>
public SqlFactory()
{
}
/// <summary>
/// 建立默认Connection对象
/// </summary>
/// <returns>Connection对象</returns>
public IDbConnection CreateConnection()
{
return new SqlConnection();
}
/// <summary>
/// 根据连接字符串建立Connection对象
/// </summary>
/// <param name="strConn">连接字符串</param>
/// <returns>Connection对象</returns>
public IDbConnection CreateConnection(string strConn)
{
return new SqlConnection(strConn);
}
/// <summary>
/// 建立Command对象
/// </summary>
/// <returns>Command对象</returns>
public IDbCommand CreateCommand()
{
return new SqlCommand();
}
/// <summary>
/// 建立DataAdapter对象
/// </summary>
/// <returns>DataAdapter对象</returns>
public IDbDataAdapter CreateDataAdapter()
{
return new SqlDataAdapter();
}
/// <summary>
/// 根据Connection建立Transaction
/// </summary>
/// <param name="myDbConnection">Connection对象</param>
/// <returns>Transaction对象</returns>
public IDbTransaction CreateTransaction(IDbConnection myDbConnection)
{
return myDbConnection.BeginTransaction();
}
/// <summary>
/// 根据Command建立DataReader
/// </summary>
/// <param name="myDbCommand">Command对象</param>
/// <returns>DataReader对象</returns>
public IDataReader CreateDataReader(IDbCommand myDbCommand)
{
return myDbCommand.ExecuteReader();
}
/// <summary>
/// 获得连接字符串
/// </summary>
/// <returns>连接字符串</returns>
public string GetConnectionString()
{
string strServer = ConfigurationSettings.AppSettings["SqlServerServer"];
string strDatabase = ConfigurationSettings.AppSettings["SqlServerDatabase"];
string strUid = ConfigurationSettings.AppSettings["SqlServerUid"];
string strPwd = ConfigurationSettings.AppSettings["SqlServerPwd"];
string strConnectionString = "Server = " + strServer + "; Database = " + strDatabase + "; Uid = " + strUid + "; Pwd = " + strPwd + ";";
return strConnectionString;
}
}
}
(4)OleDbFactory.cs
using System;
using System.Data;
using System.Data.OleDb;
using System.Configuration;
namespace DbService
{
/// <summary>
/// 针对OleDb连接的工厂
/// </summary>
public class OleDbFactory : AbstractDbFactory
{
/// <summary>
/// 构造函数
/// </summary>
public OleDbFactory()
{
}
/// <summary>
/// 建立默认Connection对象
/// </summary>
/// <returns>Connection对象</returns>
public IDbConnection CreateConnection()
{
return new OleDbConnection();
}
/// <summary>
/// 根据连接字符串建立Connection对象
/// </summary>
/// <param name="strConn">连接字符串</param>
/// <returns>Connection对象</returns>
public IDbConnection CreateConnection(string strConn)
{
return new OleDbConnection(strConn);
}
/// <summary>
/// 建立Command对象
/// </summary>
/// <returns>Command对象</returns>
public IDbCommand CreateCommand()
{
return new OleDbCommand();
}
/// <summary>
/// 建立DataAdapter对象
/// </summary>
/// <returns>DataAdapter对象</returns>
public IDbDataAdapter CreateDataAdapter()
{
return new OleDbDataAdapter();
}
/// <summary>
/// 根据Connection建立Transaction
/// </summary>
/// <param name="myDbConnection">Connection对象</param>
/// <returns>Transaction对象</returns>
public IDbTransaction CreateTransaction(IDbConnection myDbConnection)
{
return myDbConnection.BeginTransaction();
}
/// <summary>
/// 根据Command建立DataReader
/// </summary>
/// <param name="myDbCommand">Command对象</param>
/// <returns>DataReader对象</returns>
public IDataReader CreateDataReader(IDbCommand myDbCommand)
{
return myDbCommand.ExecuteReader();
}
/// <summary>
/// 获得连接字符串
/// </summary>
/// <returns>连接字符串</returns>
public string GetConnectionString()
{
string strProvider = ConfigurationSettings.AppSettings["OleDbProvider"];
string strDataSource = ConfigurationSettings.AppSettings["OleDbDataSource"];
string strConnectionString = "Provider = " + strProvider + ";Data Source = " + strDataSource + ";";
return strConnectionString;
}
}
}
(5)OdbcFactory.cs
using System;
using System.Data;
using System.Data.Odbc;
using System.Configuration;
namespace DbService
{
/// <summary>
/// 针对Odbc连接的工厂
/// </summary>
public class OdbcFactory : AbstractDbFactory
{
/// <summary>
/// 构造函数
/// </summary>
public OdbcFactory()
{
}
/// <summary>
/// 建立默认Connection对象
/// </summary>
/// <returns>Connection对象</returns>
public IDbConnection CreateConnection()
{
return new OdbcConnection();
}
/// <summary>
/// 根据连接字符串建立Connection对象
/// </summary>
/// <param name="strConn">连接字符串</param>
/// <returns>Connection对象</returns>
public IDbConnection CreateConnection(string strConn)
{
return new OdbcConnection(strConn);
}
/// <summary>
/// 建立Command对象
/// </summary>
/// <returns>Command对象</returns>
public IDbCommand CreateCommand()
{
return new OdbcCommand();
}
/// <summary>
/// 建立DataAdapter对象
/// </summary>
/// <returns>DataAdapter对象</returns>
public IDbDataAdapter CreateDataAdapter()
{
return new OdbcDataAdapter();
}
/// <summary>
/// 根据Connection建立Transaction
/// </summary>
/// <param name="myDbConnection">Connection对象</param>
/// <returns>Transaction对象</returns>
public IDbTransaction CreateTransaction(IDbConnection myDbConnection)
{
return myDbConnection.BeginTransaction();
}
/// <summary>
/// 根据Command建立DataReader
/// </summary>
/// <param name="myDbCommand">Command对象</param>
/// <returns>DataReader对象</returns>
public IDataReader CreateDataReader(IDbCommand myDbCommand)
{
return myDbCommand.ExecuteReader();
}
/// <summary>
/// 获得连接字符串
/// </summary>
/// <returns></returns>
public string GetConnectionString()
{
string strDriver = ConfigurationSettings.AppSettings["OdbcDriver"];
string strDBQ = ConfigurationSettings.AppSettings["OdbcDBQ"];
string strConnectionString = "Driver={" + strDriver + "}; DBQ=" + strDBQ + ";";
return strConnectionString;
}
}
}
用C#实现的数据库抽象工厂(三)
以下是在应用时真正要调用到的类：
(6)DbAccess.cs
using System;
using System.Data;
namespace DbService
{
/// <summary>
/// DbAccess类,即进行数据库访问时需要调用的类
/// </summary>
public sealed class DbAccess
{
/// <summary>
/// DbAccess构造函数
/// </summary>
private DbAccess()
{
}
/// <summary>
/// 无条件查询操作,即查询表中所有记录
/// </summary>
/// <param name="strTableName">表名</param>
/// <param name="strColumn">列名组</param>
/// <returns>无条件查询结果</returns>
public static DataSet SelectAll(string strTableName, string[] strColumn)
{
DataSet ds = new DataSet();
Factory factory = Factory.GetInstance();
AbstractDbFactory abstractDbFactory = factory.CreateInstance();
IDbConnection concreteDbConn = abstractDbFactory.CreateConnection();
concreteDbConn.ConnectionString = abstractDbFactory.GetConnectionString();
concreteDbConn.Open();
IDbCommand concreteDbCommand = abstractDbFactory.CreateCommand();
IDbTransaction concreteDbTrans = abstractDbFactory.CreateTransaction(concreteDbConn);
concreteDbCommand.Connection = concreteDbConn;
concreteDbCommand.Transaction = concreteDbTrans;
IDbDataAdapter concreteDbAdapter = abstractDbFactory.CreateDataAdapter();
try
{
string strSql = "SELECT ";
for(int i = 0; i < strColumn.Length - 1; i++)
{
strSql += (strColumn[i] + ", ");
}
strSql += (strColumn[strColumn.Length - 1] + " FROM " + strTableName);
concreteDbCommand.CommandText = strSql;
concreteDbAdapter.SelectCommand = concreteDbCommand;
concreteDbAdapter.Fill(ds);
concreteDbTrans.Commit();
}
catch
{
concreteDbTrans.Rollback();
ds.Clear();
throw;
}
finally
{
concreteDbConn.Close();
}
return ds;
}
/// <summary>
/// 条件查询操作
/// </summary>
/// <param name="strTableName">表名</param>
/// <param name="strColumn">列名组</param>
/// <param name="strCondition">条件</param>
/// <returns>条件查询结果</returns>
public static DataSet Select(string strTableName, string[] strColumn, string strCondition)
{
DataSet ds = new DataSet();
Factory factory = Factory.GetInstance();
AbstractDbFactory abstractDbFactory = factory.CreateInstance();
IDbConnection concreteDbConn = abstractDbFactory.CreateConnection();
concreteDbConn.ConnectionString = abstractDbFactory.GetConnectionString();
concreteDbConn.Open();
IDbCommand concreteDbCommand = abstractDbFactory.CreateCommand();
IDbTransaction concreteDbTrans = abstractDbFactory.CreateTransaction(concreteDbConn);
concreteDbCommand.Connection = concreteDbConn;
concreteDbCommand.Transaction = concreteDbTrans;
IDbDataAdapter concreteDbAdapter = abstractDbFactory.CreateDataAdapter();
try
{
string strSql = "SELECT ";
for(int i = 0; i < strColumn.Length - 1; i++)
{
strSql += (strColumn[i] + ", ");
}
strSql += (strColumn[strColumn.Length - 1] + " FROM " + strTableName + " WHERE " + strCondition);
concreteDbCommand.CommandText = strSql;
concreteDbAdapter.SelectCommand = concreteDbCommand;
concreteDbAdapter.Fill(ds);
concreteDbTrans.Commit();
}
catch
{
concreteDbTrans.Rollback();
ds.Clear();
throw;
}
finally
{
concreteDbConn.Close();
}
return ds;
}
/// <summary>
/// 单条记录的插入操作
/// </summary>
/// <param name="strTableName">表名</param>
/// <param name="strColumn">列名组</param>
/// <param name="strValue">值组</param>
public static void Insert(string strTableName, string[] strColumn, object[] strValue)
{
Factory factory = Factory.GetInstance();
AbstractDbFactory abstractDbFactory = factory.CreateInstance();
IDbConnection concreteDbConn = abstractDbFactory.CreateConnection();
concreteDbConn.ConnectionString = abstractDbFactory.GetConnectionString();
concreteDbConn.Open();
IDbCommand concreteDbCommand = abstractDbFactory.CreateCommand();
IDbTransaction concreteDbTrans = abstractDbFactory.CreateTransaction(concreteDbConn);
concreteDbCommand.Connection = concreteDbConn;
concreteDbCommand.Transaction = concreteDbTrans;
try
{
string strSql = "INSERT INTO " + strTableName + " (";
for(int i = 0; i < strColumn.Length - 1; i++)
{
strSql += (strColumn[i] + ", ");
}
strSql += (strColumn[strColumn.Length - 1] + ") VALUES ('");
for(int i = 0; i < strValue.Length - 1; i++)
{
strSql += (strValue[i] + "', '");
}
strSql += (strValue[strValue.Length - 1] + "')");
concreteDbCommand.CommandText = strSql;
concreteDbCommand.ExecuteNonQuery();
concreteDbTrans.Commit();
}
catch
{
concreteDbTrans.Rollback();
throw;
}
finally
{
concreteDbConn.Close();
}
}
/// <summary>
/// 批量记录的插入操作,即可一次向多张表中插入不同的批量记录
/// </summary>
/// <param name="ds">批量记录组成的DataSet,DataSet中的各个DataTable名为表名,各DataTable中的DataColumn名为列名</param>
public static void InsertSet(ref DataSet ds)
{
Factory factory = Factory.GetInstance();
AbstractDbFactory abstractDbFactory = factory.CreateInstance();
IDbConnection concreteDbConn = abstractDbFactory.CreateConnection();
concreteDbConn.ConnectionString = abstractDbFactory.GetConnectionString();
concreteDbConn.Open();
IDbCommand concreteDbCommand = abstractDbFactory.CreateCommand();
IDbTransaction concreteDbTrans = abstractDbFactory.CreateTransaction(concreteDbConn);
concreteDbCommand.Connection = concreteDbConn;
concreteDbCommand.Transaction = concreteDbTrans;
try
{
foreach(DataTable dt in ds.Tables)
{
foreach(DataRow dr in dt.Rows)
{
string strSql = "INSERT INTO " + dt.TableName + " (";
for(int i = 0; i < dt.Columns.Count - 1; i++)
{
strSql += (dt.Columns[i].Caption + ", ");
}
strSql += (dt.Columns[dt.Columns.Count - 1].Caption + ") VALUES ('");
for(int i = 0; i < dt.Columns.Count - 1; i++)
{
strSql += (dr[i] + "', '");
}
strSql += (dr[dt.Columns.Count - 1] + "')");
concreteDbCommand.CommandText = strSql;
concreteDbCommand.ExecuteNonQuery();
}
}
concreteDbTrans.Commit();
}
catch
{
concreteDbTrans.Rollback();
throw;
}
finally
{
concreteDbConn.Close();
}
}
/// <summary>
/// 无条件删除操作,即删除表中所有记录
/// </summary>
/// <param name="strTableName">表名</param>
public static void DeleteAll(string strTableName)
{
Factory factory = Factory.GetInstance();
AbstractDbFactory abstractDbFactory = factory.CreateInstance();
IDbConnection concreteDbConn = abstractDbFactory.CreateConnection();
concreteDbConn.ConnectionString = abstractDbFactory.GetConnectionString();
concreteDbConn.Open();
IDbCommand concreteDbCommand = abstractDbFactory.CreateCommand();
IDbTransaction concreteDbTrans = abstractDbFactory.CreateTransaction(concreteDbConn);
concreteDbCommand.Connection = concreteDbConn;
concreteDbCommand.Transaction = concreteDbTrans;
try
{
string strSql = "DELETE FROM " + strTableName;
concreteDbCommand.CommandText = strSql;
concreteDbCommand.ExecuteNonQuery();
concreteDbTrans.Commit();
}
catch
{
concreteDbTrans.Rollback();
throw;
}
finally
{
concreteDbConn.Close();
}
}
/// <summary>
/// 条件删除操作
/// </summary>
/// <param name="strTableName">表名</param>
/// <param name="strCondition">条件</param>
public static void Delete(string strTableName, string strCondition)
{
Factory factory = Factory.GetInstance();
AbstractDbFactory abstractDbFactory = factory.CreateInstance();
IDbConnection concreteDbConn = abstractDbFactory.CreateConnection();
concreteDbConn.ConnectionString = abstractDbFactory.GetConnectionString();
concreteDbConn.Open();
IDbCommand concreteDbCommand = abstractDbFactory.CreateCommand();
IDbTransaction concreteDbTrans = abstractDbFactory.CreateTransaction(concreteDbConn);
concreteDbCommand.Connection = concreteDbConn;
concreteDbCommand.Transaction = concreteDbTrans;
try
{
string strSql = "DELETE FROM " + strTableName + " WHERE " + strCondition;
concreteDbCommand.CommandText = strSql;
concreteDbCommand.ExecuteNonQuery();
concreteDbTrans.Commit();
}
catch
{
concreteDbTrans.Rollback();
throw;
}
finally
{
concreteDbConn.Close();
}
}
/// <summary>
/// 无条件更新操作,即更新表中所有记录
/// </summary>
/// <param name="strTableName">表名</param>
/// <param name="strColumn">列名组</param>
/// <param name="strValue">值组</param>
public static void UpdateAll(string strTableName, string[] strColumn, object[] strValue)
{
Factory factory = Factory.GetInstance();
AbstractDbFactory abstractDbFactory = factory.CreateInstance();
IDbConnection concreteDbConn = abstractDbFactory.CreateConnection();
concreteDbConn.ConnectionString = abstractDbFactory.GetConnectionString();
concreteDbConn.Open();
IDbCommand concreteDbCommand = abstractDbFactory.CreateCommand();
IDbTransaction concreteDbTrans = abstractDbFactory.CreateTransaction(concreteDbConn);
concreteDbCommand.Connection = concreteDbConn;
concreteDbCommand.Transaction = concreteDbTrans;
try
{
string strSql = "UPDATE " + strTableName + " SET ";
for(int i = 0; i < strColumn.Length - 1; i++)
{
strSql += (strColumn[i] + " = '" + strValue[i] + "', ");
}
strSql += (strColumn[strColumn.Length - 1] + " = '" + strValue[strValue.Length - 1] + "' ");
concreteDbCommand.CommandText = strSql;
concreteDbCommand.ExecuteNonQuery();
concreteDbTrans.Commit();
}
catch
{
concreteDbTrans.Rollback();
throw;
}
finally
{
concreteDbConn.Close();
}
}
/// <summary>
/// 条件更新操作
/// </summary>
/// <param name="strTableName">表名</param>
/// <param name="strColumn">列名组</param>
/// <param name="strValue">值组</param>
/// <param name="strCondition">条件</param>
public static void Update(string strTableName, string[] strColumn, object[] strValue, string strCondition)
{
Factory factory = Factory.GetInstance();
AbstractDbFactory abstractDbFactory = factory.CreateInstance();
IDbConnection concreteDbConn = abstractDbFactory.CreateConnection();
concreteDbConn.ConnectionString = abstractDbFactory.GetConnectionString();
concreteDbConn.Open();
IDbCommand concreteDbCommand = abstractDbFactory.CreateCommand();
IDbTransaction concreteDbTrans = abstractDbFactory.CreateTransaction(concreteDbConn);
concreteDbCommand.Connection = concreteDbConn;
concreteDbCommand.Transaction = concreteDbTrans;
try
{
string strSql = "UPDATE " + strTableName + " SET ";
for(int i = 0; i < strColumn.Length - 1; i++)
{
strSql += (strColumn[i] + " = '" + strValue[i] + "', ");
}
strSql += (strColumn[strColumn.Length - 1] + " = '" + strValue[strValue.Length - 1] + "' " + " WHERE " + strCondition);
concreteDbCommand.CommandText = strSql;
concreteDbCommand.ExecuteNonQuery();
concreteDbTrans.Commit();
}
catch
{
concreteDbTrans.Rollback();
throw;
}
finally
{
concreteDbConn.Close();
}
}
}
}
最后一步，在Web.config中的根结点configuration下增加一些关于数据库连接字符串的变量：
<appSettings>
<add key="DatabaseType" value="SqlServer" />
<add key="SqlServerServer" value="flyingyoko" />
<add key="SqlServerDatabase" value="test" />
<add key="SqlServerUid" value="sa" />
<add key="SqlServerPwd" value="****" />
<add key="OleDbProvider" value="Microsoft.jet.oledb.4.0" />
<add key="OleDbDataSource" value="D:\test.mdb" />
<add key="OdbcDriver" value="Microsoft Access Driver (*.mdb)" />
<add key="OdbcDBQ" value="d:\test.mdb" />
</appSettings>
现在一切OK，大家可以通过改变Web.config中的变量来使用不同的数据库连接方式(SqlServer专用连接、OleDb连接和Odbc连接)连接不同的数据库，同时整个使用仍通过DbAccess，不受任何影响。
attention:（原始资料出自何处已忘V_V） 
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