ASP NET Web API之訊息 攔截 處理
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標題相當難取，內容也許和您想的不一樣，而且網上已經有很多這方面的資料了，我不過是在實踐過程中作下記錄。廢話少說，直接開始。
Exception
當服務端丟擲未處理異常時，most exceptions are translated into an HTTP response with status code 500, Internal Server Error.當然我們也可以丟擲一個特殊的異常HttpResponseException，它將被直接寫入響應流，而不會被轉成500。
public Product GetProduct(int id){ Product item = repository.Get(id); if (item == null) { throw new HttpResponseException(HttpStatusCode.NotFound); } return item;}
有時要對服務端異常做一封裝，以便對客戶端隱藏具體細節，或者統一格式，那麼可建立一繼承自System.Web.Http.Filters.ExceptionFilterAttribute的特性，如下：
public class APIExceptionFilterAttribute : ExceptionFilterAttribute{ public override void OnException(HttpActionExecutedContext context) {//業務異常 if (context.Exception is BusinessException) { context.Response = new HttpResponseMessage { StatusCode = System.Net.HttpStatusCode.ExpectationFailed }; BusinessException exception = (BusinessException)context.Exception; context.Response.Headers.Add("BusinessExceptionCode", exception.Code); context.Response.Headers.Add("BusinessExceptionMessage", exception.Message); } //其它異常 else { context.Response = new HttpResponseMessage { StatusCode = System.Net.HttpStatusCode.InternalServerError }; } }}
然後將該Attribute應用到action或controller，或者GlobalConfiguration.Configuration.Filters.Add(new APIExceptionFilterAttribute());使之應用於所有action（If you use the "ASP.NET MVC 4 Web Application" project template to create your project, put your Web API configuration code inside the WebApiConfig class, which is located in the App_Start folder:config.Filters.Add(newProductStore.NotImplExceptionFilterAttribute());）。當然，在上述程式碼中，我們也可以在OnException方法中直接丟擲HttpResponseException，效果是一樣的。
Note: Something to have in mind is that the ExceptionFilterAttribute will be ignored if the ApiController action method throws a HttpResponseException；If something goes wrong in the ExceptionFilterAttribute and an exception is thrown that is not of type HttpResponseException, a formatted exception will be thrown with stack trace etc to the client.
如果要返回給客戶端的不僅僅是一串字串，比如是json物件，那麼可以使用HttpError這個類。
以上知識主要來自Exception Handling in ASP.NET Web API。
ActionFilterAttribute、ApiControllerActionInvoker
有時要在action執行前後做額外處理，那麼ActionFilterAttribute和ApiControllerActionInvoker就派上用場了。比如客戶端請求發過來的引數為使用者令牌字串token，我們要在action執行之前先將其轉為action引數列表中對應的使用者編號ID，如下：
public class TokenProjectorAttribute : ActionFilterAttribute{ private string _userid = "userid"; public string UserID { get { return _userid; } set { _userid = value; } } public override void OnActionExecuting(HttpActionContext actionContext) { if (!actionContext.ActionArguments.ContainsKey(UserID)) { //引數列表中不存在userid，寫入日誌 //…… var response = new HttpResponseMessage(); response.Content = new StringContent("使用者資訊轉換異常."); response.StatusCode = HttpStatusCode.Conflict; //在這裡為了不繼續走流程，要throw出來，才會立馬返回到客戶端 throw new HttpResponseException(response); } //userid系統賦值 actionContext.ActionArguments[UserID] = actionContext.Request.Properties["shumi_userid"]; base.OnActionExecuting(actionContext); } public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext) { base.OnActionExecuted(actionExecutedContext); }}
ActionFilterAttribute如何應用到action，和前面的ExceptionFilterAttribute類似。
ApiControllerActionInvoker以上述Exception為例：
public class ServerAPIControllerActionInvoker : ApiControllerActionInvoker{ public override Task<HttpResponseMessage> InvokeActionAsync(HttpActionContext actionContext, CancellationToken cancellationToken) { //對actionContext做一些預處理 //…… var result = base.InvokeActionAsync(actionContext, cancellationToken); if (result.Exception != null && result.Exception.GetBaseException() != null) { var baseException = result.Exception.GetBaseException(); if (baseException is BusinessException) { return Task.Run<HttpResponseMessage>(() => { var response = new HttpResponseMessage(HttpStatusCode.ExpectationFailed); BusinessException exception = (BusinessException)baseException; response.Headers.Add("BusinessExceptionCode", exception.Code); response.Headers.Add("BusinessExceptionMessage", exception.Message); return response; }); } else { return Task.Run<HttpResponseMessage>(() => new HttpResponseMessage(HttpStatusCode.InternalServerError)); } } return result; }}
然後註冊至GlobalConfiguration.Configuration.Services中。由於ApiControllerActionInvoker乃是影響全域性的，所以若要對部分action進行包裝處理，應該優先選擇ActionFilterAttribute。
DelegatingHandler
前面的攔截都發生在請求已被路由至對應的action後發生，有一些情況需要在路由之前就做預先處理，或是在響應流返回過程中做後續處理，這時我們就要用到DelegatingHandler。比如對請求方的身份驗證，當驗證未通過時直接返回錯誤資訊，否則進行後續呼叫。
public class AuthorizeHandler : DelegatingHandler{ private static IAuthorizer _authorizer = null; static AuthorizeHandler() { } protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken) { if (request.Method == HttpMethod.Post) { var querystring = HttpUtility.ParseQueryString(request.RequestUri.Query); var formdata = request.Content.ReadAsFormDataAsync().Result; if (querystring.AllKeys.Intersect(formdata.AllKeys).Count() > 0) { return SendError("請求引數有重複.", HttpStatusCode.BadRequest); } } //請求方身份驗證 AuthResult result = _authorizer.AuthRequest(request); if (!result.Flag) { return SendError(result.Message, HttpStatusCode.Unauthorized); } request.Properties.Add("shumi_userid", result.UserID); return base.SendAsync(request, cancellationToken); } private Task<HttpResponseMessage> SendError(string error, HttpStatusCode code) { var response = new HttpResponseMessage(); response.Content = new StringContent(error); response.StatusCode = code; return Task<HttpResponseMessage>.Factory.StartNew(() => response); }}
參考資料：
- ASP.NET Web API Exception Handling
- Implementing message handlers to track your ASP.NET Web API usage
- MVC4 WebAPI(二)——Web API工作方式
- Asp.Net MVC及Web API框架配置會碰到的幾個問題及解決方案
轉載請註明原文出處：http://www.cnblogs.com/newton/p/3238082.html
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