介绍

以一个简易版的数据库连接池的实现来说明一下
连接池的connection以队列来管理
getConnection的时候，如果队列中connection个数小于50，且暂时无可用的connection（个数为0或者peek看下头部需要先出那个元素还处于不可用状态），就新建连接并建立连接，开始一直新建到50个connection，就是_currentPoolSize =50
如果队列中connection个数大于等于50，且暂时无可用的connection（个数为0或者peek看下头部需要先出那个元素还处于不可用状态），就等着Monitor.Wait(_connectionPoolQueueLock)

returnConnection的时候，使用Monitor.Pulse(_connectionPoolQueueLock) 随机通知一个wait的线程可以继续getConnection了

using System;
using System.Collections.Generic;
using System.Data;
using System.Threading;
using Mono.Data.Sqlite;namespace demo.unity.sqlite
{public class SQLiteConnectionManager{private Queue<Tuple<SqliteConnection, DateTime>> _connectionPoolQueue;private object _connectionPoolQueueLock = new object();private const int maxPoolSize = 50;private volatile bool _disposed;private int _currentPoolSize;private readonly System.Timers.Timer _cleanupTimer = new System.Timers.Timer(10 * 60 * 1000);public SQLiteConnectionManager(){_connectionPoolQueue = new Queue<Tuple<SqliteConnection, DateTime>>(maxPoolSize);_cleanupTimer.Elapsed += _cleanupTimerElapsed;_cleanupTimer.AutoReset = true;_cleanupTimer.Start();}private void _cleanupTimerElapsed(object sender, System.Timers.ElapsedEventArgs e){lock (_connectionPoolQueueLock){while (_connectionPoolQueue.Count > 0 && (DateTime.UtcNow - _connectionPoolQueue.Peek().Item2).TotalMinutes > 15){var tup = _connectionPoolQueue.Dequeue();tup.Item1.Dispose();_currentPoolSize--;}}}private SqliteConnection _createNewConnection(SqliteConnectionStringBuilder builder){var connection = new SqliteConnection(builder.ConnectionString);connection.Open();return connection;}public SqliteConnection getConnection(SqliteConnectionStringBuilder builder){lock (_connectionPoolQueueLock){// count == 0 or queue.peek no use connectionwhile (_connectionPoolQueue.Count == 0  || _connectionPoolQueue.Peek().Item1.State != ConnectionState.Open){if (_disposed){throw new ObjectDisposedException("The DB connection pool is is already disposed");}if (_currentPoolSize < maxPoolSize){// create and open connectionvar connection = _createNewConnection(builder);_connectionPoolQueue.Enqueue( new Tuple<SqliteConnection, DateTime>(connection, DateTime.UtcNow));_currentPoolSize++;}else{Monitor.Wait(_connectionPoolQueueLock);}}return _connectionPoolQueue.Dequeue().Item1;}}public void returnConnection(SqliteConnection connection){if (connection == null){return;}lock (_connectionPoolQueueLock){_connectionPoolQueue.Enqueue(new Tuple<SqliteConnection, DateTime>(connection, DateTime.UtcNow));Monitor.Pulse(_connectionPoolQueueLock);}}public void dispose(){lock (_connectionPoolQueueLock){_disposed = true;while (_connectionPoolQueue.Count > 0){var tup  = _connectionPoolQueue.Dequeue();tup.Item1?.Dispose();_currentPoolSize--;}// wake up any waiting threadsMonitor.PulseAll(_connectionPoolQueueLock);}_cleanupTimer.Stop();_cleanupTimer.Dispose();}}
}