對于ADO.NET連接池，大家不會陌生。不過多次用過ADO.NET連接池的各位.NET程序員，卻不一定深入了解它，具體原因是：
ADO.NET中提供了連接池的功能，多數(shù)開發(fā)人員很少設置它，因為它是默認的。
界面設置如下圖：
關閉連接池也很簡單，在連接字符串如下：
Data Source=(local);Initial Catalog=AdventureWorks;Integrated Security=SSPI;Pooling=False;
但連接池的本質是什么樣的呢？
用Reflector，打開System.Data.SqlClient.SqlConnection的ConnectionString屬性的設置值的方法，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);
- }
- }
再連接 到紅色的GetConnectionPoolGroup方法，如下代碼
- 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是判斷是否存在連接字符串為connectionString的連接，存在返回到group，不存在就調(diào)用CreateConnectionOptions創(chuàng)建一個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;
- }
這段代碼放到連接池中，在這里，可能顯示的看到，ado.NET的連接池實質上是一個Dictionary<string, DbConnectionPoolGroup>泛型集合。
所謂的連接池，就是一個與連接對象Connection相關的集合，這不只是簡單的集合，而是有一定的機制在內(nèi)部。我們做開發(fā)時，可能建立Connection連接對象，關閉連接對象，有時候還調(diào)用Dispose來釋放連接。下次再用時，便重新實例化一個連接。但在池中的連接不隨連接對象的Close或Dispose而釋放。如果下次重新建立連接，連接字符串與前一次完全一模一樣，則連接池就會把上次可用的連接對象賦給連接去用。如果兩個連接字符串有一點不一樣，即使在某一個地方多一個空格，連接池也不會以為是相同的連接，這點微軟可能在內(nèi)部只直接去比較兩個字符串了，而不是比較連接數(shù)據(jù)庫字符串的鍵值互相匹配。
連接池的好處就是保留連接對象，防止下次重頭再來實例化一個連接對象。
- 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);
- }
（說明：可能找到結果后覺得非常簡單，但怎么找到結果的，卻是費了很大勁，幾乎是5個小時，所以相把找到結果的過程簡單說一下：
一開始用Reflector發(fā)現(xiàn)SqlConnection中有一個PoolGroup的屬性，于是就想在運行時候比較兩個SqlConnection對象的這個屬性，但由于這個屬性是的訪問修飾符是internal的，不能直接訪問，只有用反射，代碼（是經(jīng)過優(yōu)化的）如下：
然后在倒數(shù)第一行設置斷點，為比較poolGroup1和poolGroup2的不同，結果發(fā)現(xiàn)，當連接字符串一樣時，這兩個對象的_objectID相同，字符串有一點不同就會不同，這點說明連接池中是用字符串本身比較的，而不是字符串中鍵值對進行比較。同還發(fā)現(xiàn)當con1和con2的ConnectionString不賦值時這兩個對象都是null，由此說明關鍵是ConnectionString賦值上，所以才開始用Reflector查看這個屬性的賦值方法，才有上面的代碼。
