ASP.NET Web API之消息[拦截]处理

拼搏现实的明天。 2021-11-16 09:48 221阅读 0赞

标题相当难取，内容也许和您想的不一样，而且网上已经有很多这方面的资料了，我不过是在实践过程中作下记录。废话少说，直接开始。

**Exception**

--------------------

当服务端抛出未处理异常时，most exceptions are translated into an HTTP response with status code 500, Internal Server Error.当然我们也可以抛出一个特殊的异常HttpResponseException，它将被直接写入响应流，而不会被转成500。

public Product GetProduct(int id)
    {
        Product item = repository.Get(id);
        if (item == null)
        {
            throw new HttpResponseException(HttpStatusCode.NotFound);
        }
        return item;
    }

有时要对服务端异常做一封装，以便对客户端隐藏具体细节，或者统一格式，那么可创建一继承自System.Web.Http.Filters.ExceptionFilterAttribute的特性，如下：

public class APIExceptionFilterAttribute : ExceptionFilterAttribute
    {
        public override void OnException(HttpActionExecutedContext context)
        {
            //业务异常
            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);
            }
            //其它异常
            else
            {
                context.Response = new HttpResponseMessage { StatusCode = System.Net.HttpStatusCode.InternalServerError };
            }
        }
    }

然后将该Attribute应用到action或controller，或者GlobalConfiguration.Configuration.Filters.Add(new APIExceptionFilterAttribute());使之应用于所有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());）。当然，在上述代码中，我们也可以在OnException方法中直接抛出HttpResponseException，效果是一样的。

***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.***

如果要返回给客户端的不仅仅是一串字符串，比如是json对象，那么可以使用**HttpError**这个类。

以上知识主要来自[Exception Handling in ASP.NET Web API][]。

**ActionFilterAttribute、ApiControllerActionInvoker**

--------------------

有时要在action执行前后做额外处理，那么ActionFilterAttribute和ApiControllerActionInvoker就派上用场了。比如客户端请求发过来的参数为用户令牌字符串token，我们要在action执行之前先将其转为action参数列表中对应的用户编号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))
            {
                //参数列表中不存在userid，写入日志
                //……
                var response = new HttpResponseMessage();
                response.Content = new StringContent("用户信息转换异常.");
                response.StatusCode = HttpStatusCode.Conflict;
                //在这里为了不继续走流程，要throw出来，才会立马返回到客户端
                throw new HttpResponseException(response);
            }
            //userid系统赋值
            actionContext.ActionArguments[UserID] = actionContext.Request.Properties["shumi_userid"];
            base.OnActionExecuting(actionContext);
        }
    
        public override void OnActionExecuted(HttpActionExecutedContext actionExecutedContext)
        {
            base.OnActionExecuted(actionExecutedContext);
        }
    }

ActionFilterAttribute如何应用到action，和前面的ExceptionFilterAttribute类似。

ApiControllerActionInvoker以上述Exception为例：

public class ServerAPIControllerActionInvoker : ApiControllerActionInvoker
    {
        public override Task<HttpResponseMessage> InvokeActionAsync(HttpActionContext actionContext, CancellationToken cancellationToken)
        {
            //对actionContext做一些预处理
            //……
            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;
        }
    }

然后注册至GlobalConfiguration.Configuration.Services中。由于ApiControllerActionInvoker乃是影响全局的，所以若要对部分action进行包装处理，应该优先选择ActionFilterAttribute。

**DelegatingHandler**

--------------------

前面的拦截都发生在请求已被路由至对应的action后发生，有一些情况需要在路由之前就做预先处理，或是在响应流返回过程中做后续处理，这时我们就要用到DelegatingHandler。比如对请求方的身份验证，当验证未通过时直接返回错误信息，否则进行后续调用。

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("请求参数有重复.", HttpStatusCode.BadRequest);
                }
            }
            //请求方身份验证
            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);
        }
    }

参考资料：

*  [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][MVC4 WebAPI_Web API]
 *  [Asp.Net MVC及Web API框架配置会碰到的几个问题及解决方案][Asp.Net MVC_Web API]

转载请注明原文出处：[http://www.cnblogs.com/newton/p/3238082.html][http_www.cnblogs.com_newton_p_3238082.html]

[Exception Handling in ASP.NET Web API]: http://www.asp.net/web-api/overview/web-api-routing-and-actions/exception-handling
[ASP.NET Web API Exception Handling]: http://weblogs.asp.net/fredriknormen/archive/2012/06/11/asp-net-web-api-exception-handling.aspx
[Implementing message handlers to track your ASP.NET Web API usage]: http://www.strathweb.com/2012/05/implementing-message-handlers-to-track-your-asp-net-web-api-usage/
[MVC4 WebAPI_Web API]: http://www.cnblogs.com/wk1234/archive/2012/05/07/2486872.html
[Asp.Net MVC_Web API]: http://www.cnblogs.com/xqin/archive/2013/05/31/3109569.html
[http_www.cnblogs.com_newton_p_3238082.html]: http://www.cnblogs.com/newton/p/3238082.html