ado.NET中提供了连接池的功能,多数开发人员很少设置它,因为它是默认的。
界面设置如下图:
 
关闭连接池也很简单,在连接字符串如下:
Data Source=(local);Initial Catalog=AdventureWorks;Integrated Security=SSPI;Pooling=False;
但连接池的本质是什么样的呢?
 
Reflector,打开System.Data.SqlClient.SqlConnectionConnectionString属性的设置值的方法,如下:
private void ConnectionString_Set(string value)
{
    DbConnectionOptions userConnectionOptions = null;
    DbConnectionPoolGroup group = this.ConnectionFactory.GetConnectionPoolGroup(value, null, ref userConnectionOptions);
    DbConnectionInternal innerConnection = this.InnerConnection;
    bool allowSetConnectionString = innerConnection.AllowSetConnectionString;
    if (allowSetConnectionString)
    {
        allowSetConnectionString= this.SetInnerConnectionFrom(DbConnectionClosedBusy.SingletonInstance, innerConnection);
        if (allowSetConnectionString)
        {
            this._userConnectionOptions = userConnectionOptions;
            this._poolGroup = group;
            this._innerConnection = DbConnectionClosedNeverOpened.SingletonInstance;
        }
    }
    if (!allowSetConnectionString)
    {
        throw ADP.OpenConnectionPropertySet("ConnectionString", innerConnection.State);
    }
    if (Bid.TraceOn)
    {
        string str = (userConnectionOptions != null) ? userConnectionOptions.UsersConnectionStringForTrace() : "";
        Bid.Trace("<prov.DbConnectionHelper.ConnectionString_Set|API> %d#, '%ls'\n", this.ObjectID, str);
    }
}
再连接 到红色的GetConnectionPoolGroup方法,如下代码
 internal DbConnectionPoolGroup GetConnectionPoolGroup(string connectionString, DbConnectionPoolGroupOptions poolOptions, ref DbConnectionOptions userConnectionOptions)
{
    DbConnectionPoolGroup group;
    if (ADP.IsEmpty(connectionString))
    {
        return null;
    }
    if (!this._connectionPoolGroups.TryGetValue(connectionString, out group) || (group.IsDisabled && (group.PoolGroupOptions != null)))
    {
        DbConnectionOptions options = this.CreateConnectionOptions(connectionString, userConnectionOptions);
        if (options == null)
        {
            throw ADP.InternalConnectionError(ADP.ConnectionError.ConnectionOptionsMissing);
        }
        string str = connectionString;
        if (userConnectionOptions == null)
        {
            userConnectionOptions = options;
            str = options.Expand();
            if (str != connectionString)
            {
                return this.GetConnectionPoolGroup(str, null, ref userConnectionOptions);
            }
        }
        if ((poolOptions == null) && ADP.IsWindowsNT)
        {
            if (group != null)
            {
                poolOptions = group.PoolGroupOptions;
            }
            else
            {
                poolOptions = this.CreateConnectionPoolGroupOptions(options);
            }
        }
        DbConnectionPoolGroup group2 = new DbConnectionPoolGroup(options, poolOptions) {
            ProviderInfo = this.CreateConnectionPoolGroupProviderInfo(options)
        };
        lock (this)
        {
            Dictionary<string, DbConnectionPoolGroup> dictionary = this._connectionPoolGroups;
            if (!dictionary.TryGetValue(str, out group))
            {
                Dictionary<string, DbConnectionPoolGroup> dictionary2 = new Dictionary<string, DbConnectionPoolGroup>(1 + dictionary.Count);
                foreach (KeyValuePair<string, DbConnectionPoolGroup> pair in dictionary)
                {
                    dictionary2.Add(pair.Key, pair.Value);
                }
                dictionary2.Add(str, group2);
                this.PerformanceCounters.NumberOfActiveConnectionPoolGroups.Increment();
                group = group2;
                this._connectionPoolGroups = dictionary2;
            }
            return group;
        }
    }
    if (userConnectionOptions == null)
    {
        userConnectionOptions = group.ConnectionOptions;
    }
    return group;
}
TryGetValue是判断是否存在连接字符串为connectionString的连接,存在返回到group,不存在就调用CreateConnectionOptions创建一个DbConnectionOptions,最后用
lock (this)
        {
            Dictionary<string, DbConnectionPoolGroup> dictionary = this._connectionPoolGroups;
            if (!dictionary.TryGetValue(str, out group))
            {
                Dictionary<string, DbConnectionPoolGroup> dictionary2 = new Dictionary<string, DbConnectionPoolGroup>(1 + dictionary.Count);
                foreach (KeyValuePair<string, DbConnectionPoolGroup> pair in dictionary)
                {
                    dictionary2.Add(pair.Key, pair.Value);
                }
                dictionary2.Add(str, group2);
                this.PerformanceCounters.NumberOfActiveConnectionPoolGroups.Increment();
                group = group2;
                this._connectionPoolGroups = dictionary2;
            }
            return group;
        }
这段代码放到连接池中,在这里,可能显示的看到,ado.NET的连接池实质上是一个Dictionary<string, DbConnectionPoolGroup>泛型集合。
所谓的连接池,就是一个与连接对象Connection相关的集合,这不只是简单的集合,而是有一定的机制在内部。我们做开发时,可能建立Connection连接对象,关闭连接对象,有时候还调用Dispose来释放连接。下次再用时,便重新实例化一个连接。但在池中的连接不随连接对象的CloseDispose而释放。如果下次重新建立连接,连接字符串与前一次完全一模一样,则连接池就会把上次可用的连接对象赋给连接去用。如果两个连接字符串有一点不一样,即使在某一个地方多一个空格,连接池也不会以为是相同的连接,这点微软可能在内部只直接去比较两个字符串了,而不是比较连接数据库字符串的键值互相匹配。
连接池的好处就是保留连接对象,防止下次重头再来实例化一个连接对象。
 
说明:可能找到结果后觉得非常简单,但怎么找到结果的,却是费了很大劲,几乎是5个小时,所以相把找到结果的过程简单说一下:
一开始用Reflector发现SqlConnection中有一个PoolGroup的属性,于是就想在运行时候比较两个SqlConnection对象的这个属性,但由于这个属性是的访问修饰符是internal的,不能直接访问,只有用反射,代码(是经过优化的)如下:
            string constr1 = "Data Source=(local);Initial Catalog=AdventureWorks;Integrated Security=SSPI;";
            string constr2 = "Data Source=(local);Initial Catalog=Pubs;Integrated Security=SSPI;";
            string AssMark = "System.Data,Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089";
            Assembly ass = Assembly.Load(AssMark);
 
            Type SqlConType = null;
            foreach (Type conType in ass.GetExportedTypes())
            {
                Console.WriteLine(conType .ToString ());
                if ("System.Data.SqlClient.SqlConnection" == conType.ToString())
                {
                    SqlConType = conType;
                }
            }
            if (SqlConType != null)
            {
                Type[] types1 = new Type[0];
                ConstructorInfo constructorInfoObj1 = SqlConType.GetConstructor(
                     BindingFlags.Instance | BindingFlags.Public, null,
                     CallingConventions.HasThis, types1, null);
                SqlConnection con1 = (SqlConnection)constructorInfoObj1.Invoke(null);
                con1.ConnectionString = constr1;           
                SqlConnection con2 = (SqlConnection)constructorInfoObj1.Invoke(null);
                con2.ConnectionString = constr2;
                PropertyInfo PI = SqlConType.GetProperty("PoolGroup", BindingFlags.Instance | BindingFlags.NonPublic);
                object poolGroup1 = PI.GetValue(con1, null);
                object poolGroup2 = PI.GetValue(con2, null); 
            }
然后在倒数第一行设置断点,为比较poolGroup1poolGroup2的不同,结果发现,当连接字符串一样时,这两个对象的_objectID相同,字符串有一点不同就会不同,这点说明连接池中是用字符串本身比较的,而不是字符串中键值对进行比较。同还发现当con1con2ConnectionString不赋值时这两个对象都是null,由此说明关键是ConnectionString赋值上,所以才开始用Reflector查看这个属性的赋值方法,才有上面的代码。)

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