SharePoint利用HttpModule的Init方法实现全局初始化

 　　　　private static bool HasAppStarted = false;
        private readonly static object _syncObject = new object();

        public void Init(HttpApplication context)
        {
            // Below is an example of how you can handle LogRequest event and provide 
            // custom logging implementation for it
            context.LogRequest += new EventHandler(OnLogRequest);
            context.BeginRequest +=
            (new EventHandler(this.Application_BeginRequest));
            context.EndRequest +=
                (new EventHandler(this.Application_EndRequest));


            if (!HasAppStarted)
            {
                lock (_syncObject)
                {
                    　　　　　　　　　　　　//最后一道铁闸，在高并发情况下也会只执行一次，此处必须加上判断　　　　　　　　　　　　if (!HasAppStarted)
                    {
                        // Run application StartUp code here
                        //ThreadPool.QueueUserWorkItem(new WaitCallback(ClearExpireData));
                        Thread thread = new Thread(ClearExpireData);
                        thread.IsBackground = true;
                        thread.Start();
　　　　　　　　　　　　　　 //将标志位设置为True，就再也不会执行了
                        HasAppStarted = true;
                    }
                }
            }
        }

 context.LogRequest += new EventHandler(OnLogRequest);
            context.BeginRequest +=
            (new EventHandler(this.Application_BeginRequest));
            context.EndRequest +=
                (new EventHandler(this.Application_EndRequest));