緊接着前文Asp.net web Api源碼分析-如何獲取IHttpHandler 我們已經得到了HttpControllerHandler實例，它是一個IHttpAsyncHandler類型，我們來看看它的BeginProcessRequest方法是如何實現：
protected virtual IAsyncResult BeginProcessRequest(HttpContextBase httpContextBase, AsyncCallback callback, object state)
{
HttpRequestMessage request = httpContextBase.GetHttpRequestMessage() ?? ConvertRequest(httpContextBase);
request.Properties[HttpPropertyKeys.HttpRouteDataKey] = _routeData;
Task responseBodyTask = _server.Value.SendAsync(request, CancellationToken.None)
.Then(response => ConvertResponse(httpContextBase, response, request));
TaskWrapperAsyncResult result = new TaskWrapperAsyncResult(responseBodyTask, state);
if (callback != null)
{
if (result.IsCompleted)
{
result.CompletedSynchronously = true;
callback(result);
}
else
{
result.CompletedSynchronously = false;
responseBodyTask.ContinueWith(_ =>
{
callback(result);
});
}
}
return result;
}
我想這個方法的大致邏輯大家一看也就明白了，這里我們只關心 HttpRequestMessage request = httpContextBase.GetHttpRequestMessage() ?? ConvertRequest(httpContextBase);這句，創建HttpRequestMessage實例，有關 HttpRequestMessage的一些介紹大家可以參考MVC4 WebAPI(二)——Web API工作方式
其中GetHttpRequestMessage方法的實現非常簡單：
internal static readonly string HttpRequestMessageKey = "MS_HttpRequestMessage";
public static HttpRequestMessage GetHttpRequestMessage(this HttpContextBase context)
{
return context.Items[HttpRequestMessageKey] as HttpRequestMessage;
}
public static void SetHttpRequestMessage(this HttpContextBase context, HttpRequestMessage request)
{
context.Items.Add(HttpRequestMessageKey, request);
}
所以這里的GetHttpRequestMessage並沒有真正創建HttpRequestMessage，ConvertRequest方法才是真正創建HttpRequestMessage的地方。
internal static HttpRequestMessage ConvertRequest(HttpContextBase httpContextBase)
{
Contract.Assert(httpContextBase != null);
HttpRequestBase requestBase = httpContextBase.Request;
HttpMethod method = HttpMethodHelper.GetHttpMethod(requestBase.HttpMethod);
Uri uri = requestBase.Url;
HttpRequestMessage request = new HttpRequestMessage(method, uri);
IHostBufferPolicySelector policySelector = _bufferPolicySelector.Value;
bool isInputBuffered = policySelector == null ? true : policySelector.UseBufferedInputStream(httpContextBase);
Stream inputStream = isInputBuffered
? requestBase.InputStream
: httpContextBase.ApplicationInstance.Request.GetBufferlessInputStream();
request.Content = new StreamContent(inputStream);
foreach (string headerName in requestBase.Headers)
{
string[] values = requestBase.Headers.GetValues(headerName);
AddHeaderToHttpRequestMessage(request, headerName, values);
}
request.Properties.Add(HttpContextBaseKey, httpContextBase);
request.Properties.Add(HttpPropertyKeys.RetrieveClientCertificateDelegateKey, _retrieveClientCertificate);
request.Properties.Add(HttpPropertyKeys.IsLocalKey, new Lazy<bool>(() => requestBase.IsLocal));
request.Properties.Add(HttpPropertyKeys.IncludeErrorDetailKey, new Lazy<bool>(() => !httpContextBase.IsCustomErrorEnabled));
return request;
}
其中
HttpRequestBase requestBase = httpContextBase.Request;
HttpMethod method = HttpMethodHelper.GetHttpMethod(requestBase.HttpMethod);
Uri uri = requestBase.Url;
HttpRequestMessage request = new HttpRequestMessage(method, uri);
這幾句代碼很簡單也很好明白，我想我就不多說了，而下面的幾句代碼頁很好理解
IHostBufferPolicySelector policySelector = _bufferPolicySelector.Value;
bool isInputBuffered = policySelector == null ? true : policySelector.UseBufferedInputStream(httpContextBase);
Stream inputStream = isInputBuffered
? requestBase.InputStream
: httpContextBase.ApplicationInstance.Request.GetBufferlessInputStream();
request.Content = new StreamContent(inputStream);
isInputBuffered是否使用輸入緩存，這里默認返回true，這幾句主要是 獲取請求的輸入流isInputBuffered? requestBase.InputStream: httpContextBase.ApplicationInstance.Request.GetBufferlessInputStream();， 然后設置HttpRequestMessage的Content屬性，
foreach (string headerName in requestBase.Headers)
{
string[] values = requestBase.Headers.GetValues(headerName);
AddHeaderToHttpRequestMessage(request, headerName, values);
}
這句也很好明白，就是把Request的header信息按需添加到HttpRequestMessage的Content.Headers里面， 最后在HttpRequestMessage的Properties屬性中添加一些暫存信息。這里的Properties屬性沒得說它是 IDictionary<string, object>類型。
現在我們好好分析一下這里面的一些細節的東西。
首先我們來看看 IHostBufferPolicySelector policySelector = _bufferPolicySelector.Value;這一句，這里的_bufferPolicySelector是一個延遲加載對象，其定義如下：
private static readonly Lazy<IHostBufferPolicySelector> _bufferPolicySelector =
new Lazy<IHostBufferPolicySelector>(() => GlobalConfiguration.Configuration.Services.GetHostBufferPolicySelector());
在GlobalConfiguration類中有這么一句
config.Services.Replace(typeof(IHostBufferPolicySelector), new WebHostBufferPolicySelector());這里我們可以知道_bufferPolicySelector.Value其實就是一個WebHostBufferPolicySelector實例，該實例的UseBufferedInputStream方法返回true，表示使用輸入緩存。
大家應該還記得前面在說HttpConfiguration的構造函數有這么一句吧：
Services = new DefaultServices(this); 其中Services是ServicesContainer類型。
DefaultServices的代碼比較多但是還是比較好理解的，
private readonly ReaderWriterLockSlim _cacheLock = new ReaderWriterLockSlim();
private readonly Dictionary<Type, object[]> _cacheMulti = new Dictionary<Type, object[]>();
private readonly Dictionary<Type, object> _cacheSingle = new Dictionary<Type, object>();
private readonly Dictionary<Type, object> _defaultServicesSingle = new Dictionary<Type, object>();
private readonly Dictionary<Type, List<object>> _defaultServicesMulti = new Dictionary<Type, List<object>>();
public DefaultServices(HttpConfiguration configuration)
{
if (configuration == null)
{
throw Error.ArgumentNull("configuration");
}
_configuration = configuration;
// Initialize the dictionary with all known service types, even if the list for that service type is
// empty, because we will throw if the developer tries to read or write unsupported types.
SetSingle<IActionValueBinder>(new DefaultActionValueBinder());
SetSingle<IApiExplorer>(new ApiExplorer(configuration));
SetSingle<IAssembliesResolver>(new DefaultAssembliesResolver());
SetSingle<IBodyModelValidator>(new DefaultBodyModelValidator());
SetSingle<IContentNegotiator>(new DefaultContentNegotiator());
SetSingle<IDocumentationProvider>(null); // Missing
SetMultiple<IFilterProvider>(new ConfigurationFilterProvider(),
new ActionDescriptorFilterProvider());
SetSingle<IHostBufferPolicySelector>(null);
SetSingle<IHttpActionInvoker>(new ApiControllerActionInvoker());
SetSingle<IHttpActionSelector>(new ApiControllerActionSelector());
SetSingle<IHttpControllerActivator>(new DefaultHttpControllerActivator());
SetSingle<IHttpControllerSelector>(new DefaultHttpControllerSelector(configuration));
SetSingle<IHttpControllerTypeResolver>(new DefaultHttpControllerTypeResolver());
SetSingle<ITraceManager>(new TraceManager());
SetSingle<ITraceWriter>(null);
// This is a priority list. So put the most common binders at the top.
SetMultiple<ModelBinderProvider>(new TypeConverterModelBinderProvider(),
new TypeMatchModelBinderProvider(),
new KeyValuePairModelBinderProvider(),
new ComplexModelDtoModelBinderProvider(),
new ArrayModelBinderProvider(),
new DictionaryModelBinderProvider(),
new CollectionModelBinderProvider(),
new MutableObjectModelBinderProvider());
SetSingle<ModelMetadataProvider>(new DataAnnotationsModelMetadataProvider());
SetMultiple<ModelValidatorProvider>(new DataAnnotationsModelValidatorProvider(),
new DataMemberModelValidatorProvider(),
new InvalidModelValidatorProvider());
// This is an ordered list,so put the most common providers at the top.
SetMultiple<ValueProviderFactory>(new QueryStringValueProviderFactory(),
new RouteDataValueProviderFactory());
ModelValidatorCache validatorCache = new ModelValidatorCache(new Lazy<IEnumerable<ModelValidatorProvider>>(() => this.GetModelValidatorProviders()));
configuration.RegisterForDispose(validatorCache);
SetSingle<IModelValidatorCache>(validatorCache);
_serviceTypesSingle = new HashSet<Type>(_defaultServicesSingle.Keys);
_serviceTypesMulti = new HashSet<Type>(_defaultServicesMulti.Keys);
// Reset the caches and the known dependency scope
ResetCache();
}
private void SetSingle<T>(T instance) where T : class
{
_defaultServicesSingle[typeof(T)] = instance;
}
private void SetMultiple<T>(params T[] instances) where T : class
{
var x = (IEnumerable<object>)instances;
_defaultServicesMulti[typeof(T)] = new List<object>(x);
}
private void ResetCache()
{
_cacheLock.EnterWriteLock();
try
{
_cacheSingle.Clear();
_cacheMulti.Clear();
_lastKnownDependencyResolver = _configuration.DependencyResolver;
}
finally
{
_cacheLock.ExitWriteLock();
}
}
public override object GetService(Type serviceType)
{
if (serviceType == null)
{
throw Error.ArgumentNull("serviceType");
}
if (!_serviceTypesSingle.Contains(serviceType))
{
throw Error.Argument("serviceType", SRResources.DefaultServices_InvalidServiceType, serviceType.Name);
}
// Invalidate the cache if the dependency scope has switched
if (_lastKnownDependencyResolver != _configuration.DependencyResolver)
{
ResetCache();
}
object result;
_cacheLock.EnterReadLock();
try
{
if (_cacheSingle.TryGetValue(serviceType, out result))
{
return result;
}
}
finally
{
_cacheLock.ExitReadLock();
}
// Get the service from DI, outside of the lock. If we're coming up hot, this might
// mean we end up creating the service more than once.
object dependencyService = _configuration.DependencyResolver.GetService(serviceType);
_cacheLock.EnterWriteLock();
try
{
if (!_cacheSingle.TryGetValue(serviceType, out result))
{
result = dependencyService ?? _defaultServicesSingle[serviceType];
_cacheSingle[serviceType] = result;
}
return result;
}
finally
{
_cacheLock.ExitWriteLock();
}
}
public override IEnumerable<object> GetServices(Type serviceType)
{
if (serviceType == null)
{
throw Error.ArgumentNull("serviceType");
}
if (!_serviceTypesMulti.Contains(serviceType))
{
throw Error.Argument("serviceType", SRResources.DefaultServices_InvalidServiceType, serviceType.Name);
}
// Invalidate the cache if the dependency scope has switched
if (_lastKnownDependencyResolver != _configuration.DependencyResolver)
{
ResetCache();
}
object[] result;
_cacheLock.EnterReadLock();
try
{
if (_cacheMulti.TryGetValue(serviceType, out result))
{
return result;
}
}
finally
{
_cacheLock.ExitReadLock();
}
// Get the service from DI, outside of the lock. If we're coming up hot, this might
// mean we end up creating the service more than once.
IEnumerable<object> dependencyServices = _configuration.DependencyResolver.GetServices(serviceType);
_cacheLock.EnterWriteLock();
try
{
if (!_cacheMulti.TryGetValue(serviceType, out result))
{
result = dependencyServices.Where(s => s != null)
.Concat(_defaultServicesMulti[serviceType])
.ToArray();
_cacheMulti[serviceType] = result;
}
return result;
}
finally
{
_cacheLock.ExitWriteLock();
}
}
在我們的日常開發中會遇到2中情況，一種是一個接口對應着一個具體實現類（Dictionary<Type, object> _defaultServicesSingle），還有一種情況是一個接口對應着多個實現類（Dictionary<Type, List<object>> _defaultServicesMulti），所以這里的DefaultServices實際就是把一個接口和一個或則多個實例關聯起來。
現在我們來看看ServicesContainer的GetHostBufferPolicySelector()方法實現，
public static IHostBufferPolicySelector GetHostBufferPolicySelector(this ServicesContainer services)
{
return services.GetService<IHostBufferPolicySelector>();
}
private static TService GetService<TService>(this ServicesContainer services)
{
if (services == null)
{
throw Error.ArgumentNull("services");
}
return (TService)services.GetService(typeof(TService));
}
這里其實還是調用的是DefaultServices的GetService方法，這里的IHostBufferPolicySelector接口 和實例是一一對應的，首先在_cacheSingle中根據type來查找實例，如果找到這直接返回，如果沒有找到就根 據 _configuration.DependencyResolver.GetService(serviceType)方法來找實例，如果 _cacheSingle key中不含type，這把該type和實例添加到_cacheSingle中來，
if (!_cacheSingle.TryGetValue(serviceType, out result))
{
result = dependencyService ?? _defaultServicesSingle[serviceType];
_cacheSingle[serviceType] = result;
}
如果前面的 _configuration.DependencyResolver.GetService(serviceType)方法返回null，這里就用默認的_defaultServicesSingle中type對應的實例。
而HttpConfiguration的DependencyResolver屬性如下：
private IDependencyResolver _dependencyResolver = EmptyResolver.Instance;
public IDependencyResolver DependencyResolver
{
get { return _dependencyResolver; }
set
{
if (value == null)
{
throw Error.PropertyNull();
}
_dependencyResolver = value;
}
}
看見HttpConfiguration的DependencyResolver默認是EmptyResolver.Instance，其GetService實現如下：
public object GetService(Type serviceType)
{
return null;
}
到這里我們的_bufferPolicySelector的創建就說完了。DependencyResolver這個東東在mvc里面也有，微軟做 這個東東，無非就是讓我們在需要的時候設置自己的類型實例。可能是自己接觸面比較窄，在工作中我還沒遇到要設置DependencyResolver這個 屬性的需求。
下面我們來看看添加header時候用到一個 AddHeaderToHttpRequestMessage(request, headerName, values);方法，主要的實現代碼如下：
private static void AddHeaderToHttpRequestMessage(HttpRequestMessage httpRequestMessage, string headerName, string[] headerValues)
{
if (!httpRequestMessage.Headers.TryAddWithoutValidation(headerName, headerValues))
{
httpRequestMessage.Content.Headers.TryAddWithoutValidation(headerName, headerValues);
}
}
這里的HttpRequestHeaders、HttpContentHeaders 都繼承於HttpHeaders，TryAddWithoutValidation的實現在HttpHeaders類中
public bool TryAddWithoutValidation(string name, IEnumerable<string> values)
{
if (values == null)
{
throw new ArgumentNullException("values");
}
if (!this.TryCheckHeaderName(name))
{
return false;
}
HeaderStoreItemInfo orCreateHeaderInfo = this.GetOrCreateHeaderInfo(name, false);
foreach (string str in values)
{
AddValue(orCreateHeaderInfo, str ?? string.Empty, StoreLocation.Raw);
}
return true;
}
這里的具體是怎么添加的我們也就忽略它吧。
到這里我想我們大家對HttpRequestMessage的創建應該比較清楚了吧，直接實例 化 HttpRequestMessage request = new HttpRequestMessage(method, uri);，然后主要設置它的Content屬性和添加Content.Headers成員。