【发布时间】：2018-10-01 21:32:53
【问题描述】：
这是一个关于 MySql Connection 的对象池设计模式的问题。实现这种设计模式的目的是在需要创建多个实例时重用相同的连接/对象。下面的代码是用 c# 编写的一类对象池设计模式。
public abstract class ObjectPool<T>
{
private long expirationTime;
private Dictionary<T, long> Running, Idle;
//Constructor
public ObjectPool()
{
expirationTime = 30000; //30 seconds
Running = new Dictionary<T, long>();
Idle = new Dictionary<T, long>();
}
protected internal abstract T create();
public abstract Boolean validate(T o);
public abstract void expire(T o);
public virtual T GetObject
{
get
{
//lock the first thread
//make sure first thread is executed before the second thread
lock (this)
{
long now = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();
T t;
if (Idle.Count > 0)
{
IEnumerator<T> element = Idle.Keys.GetEnumerator();
while (element.MoveNext())
{
t = element.Current;
//unlocked[t] -> Get the VALUE asscoiated with the specified KEY
if ((now - Idle[t]) > expirationTime)
{
//object has expired
System.Diagnostics.Debug.WriteLine("object has expired");
Idle.Remove(t);
expire(t);
//to obtain the default value of a Generic Type
t = default(T);
}
else
{
if (validate(t))
{
System.Diagnostics.Debug.WriteLine("Connection is still open. used back the same object");
Idle.Remove(t);
Running.Add(t, now);
return (t);
}
else
{
//object failed validation
System.Diagnostics.Debug.WriteLine("object has failed validation");
Idle.Remove(t);
expire(t);
t = default(T);
}
}
}
}
// no objects avaialable, create a new one
System.Diagnostics.Debug.WriteLine("no object is available, creating a new one");
t = create();
Running[t] = now;
return (t);
}
}
}
public virtual void releaseObject(T t)
{
lock (this)
{
Running.Remove(t);
Idle[t] = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds();
System.Diagnostics.Debug.WriteLine("release object");
}
}
}
上面的代码是对象池设计模式的实现，每次通过调用GetObject这个函数创建一个新的实例，下面的类是一个重写ObjectPooling的方法并与MySql集成的类
class DatabaseConnection : ObjectPool<MySqlConnection>
{
private String ConnectionString =
"datasource=127.0.0.1;" +
"port=3306;" +
"username=root;" +
"password=;" +
"database=mesa;" +
"Integrated Security = SSPI;"+
"Min Pool Size = 1;" +
"Max Pool Size = 9;" +
"Connection Lifetime = 600;";
public DatabaseConnection() : base()
{
}
public override void expire(MySqlConnection o)
{
try
{
o.Close();
System.Diagnostics.Debug.WriteLine("Closing Database Connection Successful");
}
catch (MySql.Data.MySqlClient.MySqlException ex)
{
System.Diagnostics.Debug.WriteLine("Closing Database Connection Unsuccessful");
MessageBox.Show("Error " + ex.Number + " has occurred: " + ex.Message,
"MySql Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
}
throw new NotImplementedException();
}
public override bool validate(MySqlConnection o)
{
System.Diagnostics.Debug.WriteLine("Validating");
try
{
if (o.State.ToString() == "Open")
{
return true;
}
}
catch (MySql.Data.MySqlClient.MySqlException ex)
{
System.Diagnostics.Debug.WriteLine("Database cannot check current state");
MessageBox.Show("Error " + ex.Number + " has occurred: " + ex.Message,
"MySql Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
return false;
}
throw new NotImplementedException();
}
protected internal override MySqlConnection create()
{
try
{
MySqlConnection mySqlConnection = new MySqlConnection(ConnectionString);
mySqlConnection.Open();
System.Diagnostics.Debug.WriteLine("Database Connection Successful");
return (mySqlConnection);
}
catch (MySql.Data.MySqlClient.MySqlException ex)
{
MessageBox.Show("Error " + ex.Number + " has occurred: " + ex.Message,
"MySql Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
return null;
}
throw new NotImplementedException();
}
}
上面的类说明了三个覆盖方法，“create”将在连接过期时创建一个新实例，“expire”这是在连接已过期时关闭连接。最后一个功能是验证当前连接是否打开。现在，下面的代码是一个与对象池和数据库连接相关联的普通类。
class InputValidation
{
private DatabaseConnection databaseConnection = new DatabaseConnection();
//Constructor
public InputValidation()
{
}
//Destructor
~InputValidation()
{
//databaseConnection.expire();
}
public void CheckForExistingPT_NUM1(String pt)
{
MySqlConnection connection1 = databaseConnection.GetObject;
try
{
string sql = "SELECT * FROM carrier";
MySqlCommand cmd = new MySqlCommand(sql, connection1);
MySqlDataReader reader = cmd.ExecuteReader();
while (reader.Read())
{
string someStringFromColumnZero = reader.GetString(0);
string someStringFromColumnOne = reader.GetString(1);
System.Diagnostics.Debug.WriteLine(someStringFromColumnZero + "," + someStringFromColumnOne);
}
reader.Close();
}
catch (MySql.Data.MySqlClient.MySqlException ex)
{
MessageBox.Show("Error " + ex.Number + " has occurred: " + ex.Message,
"Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
}
databaseConnection.releaseObject(connection1);
}
public void CheckForExistingPT_NUM2(String pt)
{
MySqlConnection connection1 = databaseConnection.GetObject;
try
{
string sql = "SELECT * FROM carrier";
MySqlCommand cmd = new MySqlCommand(sql, connection1);
MySqlDataReader reader = cmd.ExecuteReader();
while (reader.Read())
{
string someStringFromColumnZero = reader.GetString(0);
string someStringFromColumnOne = reader.GetString(1);
System.Diagnostics.Debug.WriteLine(someStringFromColumnZero + "," + someStringFromColumnOne);
}
reader.Close();
}
catch (MySql.Data.MySqlClient.MySqlException ex)
{
MessageBox.Show("Error " + ex.Number + " has occurred: " + ex.Message,
"Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
}
databaseConnection.releaseObject(connection1);
}
}
现在，从CheckForExistingPT_NUM1(String pt) 和CheckForExistingPT_NUM2(String pt) 这两个函数可以看出，实际上这两个函数只是使用同一个连接与数据库进行交互，而不是关闭连接并重新打开一个新连接。下面是实现所有逻辑的主类函数。
private void AliasButton_Click(object sender, EventArgs e)
{
String PT_NUM = PTNumTextBox.Text;
InputValidation inputValidation = new InputValidation();
inputValidation.CheckForExistingPT_NUM1(PT_NUM);
inputValidation.CheckForExistingPT_NUM2(PT_NUM);
}
下面是输出框中打印出来的一行信息
no object is available, creating a new one
The thread 0x994 has exited with code 0 (0x0).
Database Connection Successful
801BRWUE,BCM88682CA1KFSBG
U10BRT7E,BCM3450KMLG
release object
Validating
Connection is still open. used back the same object
801BRWUE,BCM88682CA1KFSBG
U10BRT7E,BCM3450KMLG
release object
这里的问题是没有创建实例时连接将保持空闲。从输出框中可以看到，connection2 仍然处于空闲状态，创建新实例时连接将被销毁，如何关闭所有空闲连接？谢谢
【问题讨论】：
标签： c# mysql visual-studio object-pooling