[Day06] Asp.Net重要物件HttpApplication(二) 建置執行管道

internal override void BuildSteps(WaitCallback stepCallback ) {
	ArrayList steps = new ArrayList();
	HttpApplication app = _application;
	steps.Add(new ValidateRequestExecutionStep(app));
	steps.Add(new ValidatePathExecutionStep(app));

	if (urlMappingsEnabled)
		steps.Add(new UrlMappingsExecutionStep(app)); 

	app.CreateEventExecutionSteps(HttpApplication.EventBeginRequest, steps);
	app.CreateEventExecutionSteps(HttpApplication.EventAuthenticateRequest, steps);
	app.CreateEventExecutionSteps(HttpApplication.EventDefaultAuthentication, steps);
	app.CreateEventExecutionSteps(HttpApplication.EventPostAuthenticateRequest, steps);
	app.CreateEventExecutionSteps(HttpApplication.EventAuthorizeRequest, steps);
	app.CreateEventExecutionSteps(HttpApplication.EventPostAuthorizeRequest, steps);
	app.CreateEventExecutionSteps(HttpApplication.EventResolveRequestCache, steps);
	app.CreateEventExecutionSteps(HttpApplication.EventPostResolveRequestCache, steps);
	steps.Add(new MapHandlerExecutionStep(app));     // map handler
	app.CreateEventExecutionSteps(HttpApplication.EventPostMapRequestHandler, steps);
	app.CreateEventExecutionSteps(HttpApplication.EventAcquireRequestState, steps);
	app.CreateEventExecutionSteps(HttpApplication.EventPostAcquireRequestState, steps);
	app.CreateEventExecutionSteps(HttpApplication.EventPreRequestHandlerExecute, steps);
	steps.Add(app.CreateImplicitAsyncPreloadExecutionStep()); // implict async preload step
	steps.Add(new CallHandlerExecutionStep(app));  // execute handler
	app.CreateEventExecutionSteps(HttpApplication.EventPostRequestHandlerExecute, steps);
	app.CreateEventExecutionSteps(HttpApplication.EventReleaseRequestState, steps);
	app.CreateEventExecutionSteps(HttpApplication.EventPostReleaseRequestState, steps);
	steps.Add(new CallFilterExecutionStep(app));  // filtering
	app.CreateEventExecutionSteps(HttpApplication.EventUpdateRequestCache, steps);
	app.CreateEventExecutionSteps(HttpApplication.EventPostUpdateRequestCache, steps);
	_endRequestStepIndex = steps.Count;
	app.CreateEventExecutionSteps(HttpApplication.EventEndRequest, steps);
	steps.Add(new NoopExecutionStep()); // the last is always there

	_execSteps = new IExecutionStep[steps.Count];
	steps.CopyTo(_execSteps);
}

如果在Web.Config設定urlMappingsEnabled就會實施UrlMappingsModule.UrlMappingRewritePath(UrlRerwite)
MapHandlerExecutionStep：找尋匹配HttpHandler物件

CreateEventExecutionSteps 載入事件

下面程式碼可以看到CreateEventExecutionSteps方法透過eventIndex去事件集合查找註冊事件,並把事件寫入ArrayList steps集合中.

private void CreateEventExecutionSteps(Object eventIndex, ArrayList steps) {
    // async
    AsyncAppEventHandler asyncHandler = AsyncEvents[eventIndex];

    if (asyncHandler != null) {
        asyncHandler.CreateExecutionSteps(this, steps);
    }

    // sync
    EventHandler handler = (EventHandler)Events[eventIndex];

    if (handler != null) {
        Delegate[] handlers = handler.GetInvocationList();

        for (int i = 0; i < handlers.Length; i++)  {
            steps.Add(new SyncEventExecutionStep(this, (EventHandler)handlers[i]));
        }
    }
}

為了建立管道最後可看到steps.CopyTo(_execSteps);把建立的管道Step複製到_execSteps集合中

private IExecutionStep[] _execSteps;

internal override void BuildSteps(WaitCallback stepCallback ) {
    ArrayList steps = new ArrayList();
    //.....其他程式碼
    _execSteps = new IExecutionStep[steps.Count];
    steps.CopyTo(_execSteps);
}

steps最後把所有註冊事件Copy到ApplicationStepManager的private IExecutionStep[] _execSteps集合中提供ResumeSteps方法呼叫使用.

HttpApplication事件集合

這兩個欄位集合乘載我們註冊的Asp.net事件

EventHandlerList 同步使用事件.
AsyncAppEventHandlersTable 非同步使用事件.

private EventHandlerList _events;
protected EventHandlerList Events {
    get {
        if (_events == null) {
            _events = new EventHandlerList();
        }
        return _events;
    }
}


private AsyncAppEventHandlersTable _asyncEvents;
private AsyncAppEventHandlersTable AsyncEvents {
    get {
        if (_asyncEvents == null)
            _asyncEvents = new AsyncAppEventHandlersTable();
        return _asyncEvents;
    }
}

用其中一個事件舉例

PostMapRequestHandler提供擴充的事件註冊點，透過AddSyncEventHookup把事件加入集合中.

object key:此事件識別資訊(每個事件都有自己的Object),如PostMapRequestHandler事件傳入EventPostMapRequestHandler物件.
Delegate handler:使用者撰寫事件方法.
RequestNotification notification:屬於哪種分群.

public event EventHandler PostMapRequestHandler {
    add { AddSyncEventHookup(EventPostMapRequestHandler, value, RequestNotification.MapRequestHandler, true); }
    remove { RemoveSyncEventHookup(EventPostMapRequestHandler, value, RequestNotification.MapRequestHandler); }
}

其他10幾個事件使用方式大同小異,這裡就不一一介紹了

IExecutionStep介面

IExecutionStep這個介面，裡面最重要的方法是void Execute();來執行注冊的事件方法.

// interface to represent one execution step
internal interface IExecutionStep {
	void Execute();
	bool CompletedSynchronously { get;}
	bool IsCancellable { get; }
}

在BuildSteps方法中可以看到，全部事件轉成IExecutionStep介面放入_execSteps待被執行IExecutionStep集合列表.

_execSteps是一個物件區域變數，提供internal override void ResumeSteps(Exception error)呼叫使用.

ResumeSteps方法呼叫IExecutionStep物件

ResumeSteps這個方法做了許多事情,我下面只保留ResumeSteps方法如何去呼叫IExecutionStep物件的Execute方法

private int _currentStepIndex;
 

[System.Diagnostics.DebuggerStepperBoundaryAttribute]
internal override void ResumeSteps(Exception error) {
	bool appCompleted = false;
	bool stepCompletedSynchronously = true;
	HttpApplication app = _application;
	CountdownTask appInstanceConsumersCounter = app.ApplicationInstanceConsumersCounter;
	HttpContext context = app.Context;
	ThreadContext threadContext = null;
	AspNetSynchronizationContextBase syncContext = context.SyncContext;
	
	try {
		using (syncContext.AcquireThreadLock()) {
			try {
				threadContext = app.OnThreadEnter();
			}
			catch (Exception e) {
				if (error == null)
					error = e;
			}

			try {
				try {
					for (; ; ) {
						// record error

						if (syncContext.Error != null) {
							error = syncContext.Error;
							syncContext.ClearError();
						}

						if (error != null) {
							app.RecordError(error);
							error = null;
						}

						// check for any outstanding async operations

						if (syncContext.PendingCompletion(_resumeStepsWaitCallback)) {
							// wait until all pending async operations complete
							break;
						}

						// advance to next step

						if (_currentStepIndex < _endRequestStepIndex && (context.Error != null || _requestCompleted)) {
							// end request
							context.Response.FilterOutput();
							_currentStepIndex = _endRequestStepIndex;
						}
						else {
							_currentStepIndex++;
						}

						if (_currentStepIndex >= _execSteps.Length) {
							appCompleted = true;
							break;
						}

						// execute the current step

						_numStepCalls++;          // count all calls

						// enable launching async operations before each new step
						syncContext.Enable();

						// call to execute current step catching thread abort exception
						error = app.ExecuteStep(_execSteps[_currentStepIndex], ref stepCompletedSynchronously);

						// unwind the stack in the async case
						if (!stepCompletedSynchronously)
							break;

						_numSyncStepCalls++;      // count synchronous calls
					}
				}
				finally {
					if (appCompleted) {
						// need to raise OnRequestCompleted while within the ThreadContext so that things like User, CurrentCulture, etc. are available
						context.RaiseOnRequestCompleted();
					}

					if (threadContext != null) {
						try {
							threadContext.DisassociateFromCurrentThread();
						}
						catch {
						}
					}
				}
			}
			catch { // Protect against exception filters
				throw;
			}

		}   // using

		if (appCompleted) {

			context.RaiseOnPipelineCompleted();

			context.Unroot();

			app.AsyncResult.Complete((_numStepCalls == _numSyncStepCalls), null, null);
			app.ReleaseAppInstance();
		}
	}
	finally {
		if (appInstanceConsumersCounter != null) {
			appInstanceConsumersCounter.MarkOperationCompleted(); // ResumeSteps call complete
		}
	}
}

上面程式碼最核心的片段在

if (_currentStepIndex < _endRequestStepIndex && (context.Error != null || _requestCompleted)) {

    context.Response.FilterOutput();
    _currentStepIndex = _endRequestStepIndex;
}
else {
    _currentStepIndex++;
}

if (_currentStepIndex >= _execSteps.Length) {
    appCompleted = true;
    break;
}

// execute the current step
_numStepCalls++;         
// enable launching async operations before each new step
syncContext.Enable();
// call to execute current step catching thread abort exception
error = app.ExecuteStep(_execSteps[_currentStepIndex], ref stepCompletedSynchronously);

_currentStepIndex這個欄位表示取得當前需要跑事件Index(從之前IExecutionStep[]集合取得),每次執完都會_currentStepIndex++
有一個無限迴圈for (; ; )一直在跑除非兩種情況才會終止迴圈.
1. _currentStepIndex >= _execSteps.Length代表全部事件都跑完了.
2. 判斷context.Error != null執行得過程是否有出錯,如果有就終止繼續執行.
透過HttpApplication.ExecuteStep方法執行前面註冊的事件
bool appCompleted來判斷目前是否執行完全部事件.
只要還有事件就呼叫ExecuteStep,把當前事件傳入(_execSteps[_currentStepIndex])

HttpApplication的ExecuteStep

這邊蠻有趣一件事情是ExecuteStep方法回傳一個Exception物件當作這次執行成功或失敗,而ExecuteStep執行過程是主要是呼叫ExecuteStepImpl方法來呼叫step.Execute();

internal Exception ExecuteStep(IExecutionStep step, ref bool completedSynchronously) {
    Exception error = null;

    try {
        try {
            if (step.IsCancellable) {
                _context.BeginCancellablePeriod();  // request can be cancelled from this point

                try {
                    ExecuteStepImpl(step);
                }
                finally {
                    _context.EndCancellablePeriod();  // request can be cancelled until this point
                }

                _context.WaitForExceptionIfCancelled();  // wait outside of finally
            }
            else {
                ExecuteStepImpl(step);
            }

            if (!step.CompletedSynchronously) {
                completedSynchronously = false;
                return null;
            }
        }
        catch (Exception e) {
            error = e;
            if (e is ThreadAbortException &&
                ((Thread.CurrentThread.ThreadState & ThreadState.AbortRequested) == 0))  {
                error = null;
                _stepManager.CompleteRequest();
            }
        }
    }
    catch (ThreadAbortException e) {
        if (e.ExceptionState != null && e.ExceptionState is CancelModuleException) {
            CancelModuleException cancelException = (CancelModuleException)e.ExceptionState;

            if (cancelException.Timeout) {
                // Timed out
                error = new HttpException(SR.GetString(SR.Request_timed_out),
                                    null, WebEventCodes.RuntimeErrorRequestAbort);
                PerfCounters.IncrementCounter(AppPerfCounter.REQUESTS_TIMED_OUT);
            }
            else {
                // Response.End
                error = null;
                _stepManager.CompleteRequest();
            }

            Thread.ResetAbort();
        }
    }

    completedSynchronously = true;
    return error;
}

private void ExecuteStepImpl(IExecutionStep step) {
    if(_stepInvoker != null) {
        bool stepCalled = false;

        _stepInvoker.Invoke(() => {
            if (!stepCalled) {
                stepCalled = true;
                step.Execute();
            }
        });

        if (!stepCalled) {
            step.Execute();
        }
    } else {
        step.Execute();
    }
}

重要的兩個IExecutionStep

MapHandlerExecutionStep:透過HttpApplication.MapHttpHandler方法取得使用HttpHandler(透過IHttpHandlerFactory和XML註冊表來完成)
CallHandlerExecutionStep:取得使用HttpHandler依照非同步或同步HttpHandler執行相對應呼叫(先判斷是否非同步)