微软的 ASP.NET Web API 是一个轻量级的web框架,可用来构建基于 http 无状态的rest服务,异常是一种运行时错误,异常处理是一种处理运行时错误的技术,每一个开发者都应该知道如何处理 Web API 中的异常,并且在 Action 中使用合适的 错误码
和 错误信息
进行包装。
WebAPI 中的 HttpResponseException
你可以在 Action 中使用 HttpResponseException 来包装指定的 HttpCode 和 HttpMessage,如下例子所示:
public Employee GetEmployee(int id)
{Employee emp = employeeRepository.Get(id);if (emp == null){var response = new HttpResponseMessage(HttpStatusCode.NotFound){Content = new StringContent("Employee doesn't exist", System.Text.Encoding.UTF8, "text/plain"),StatusCode = HttpStatusCode.NotFound}throw new HttpResponseException(response);}return emp;
}
如果你的 Action 返回的是 IHttpActionResult,那么可将 GetEmployee()
方法修改如下:
public IHttpActionResult GetEmployee(int id)
{Employee emp = employeeRepository.Get(id);if (emp == null){var response = new HttpResponseMessage(HttpStatusCode.NotFound){Content = new StringContent("Employee doesn't exist", System.Text.Encoding.UTF8, "text/plain"),StatusCode = HttpStatusCode.NotFound}throw new HttpResponseException(response);}return Ok(emp);
}
从上面的代码可以看出,错误码 和 错误消息 都赋给了 Response 对象,然后包装到了 HttpResponseException 进行返回。
WebAPI 中使用 HttpError
除了直接实例化 HttpResponseMessage 类,还可以使用 Request.CreateErrorResponse()
快捷的创建 HttpResponseMessage 类,如下代码所示:
public IActionResult GetEmployee(int id)
{Employee emp = employeeRepository.Get(id);if (emp == null){string message = "Employee doesn't exist";throw new HttpResponseException(Request.CreateErrorResponse(HttpStatusCode.NotFound, message));}return Ok(emp);
}
WebAPI 中使用 异常过滤器
异常过滤器是一种可以在 WebAPI 中捕获那些未得到处理的异常的过滤器,要想创建异常过滤器,你需要实现 IExceptionFilter 接口,不过这种方式比较麻烦,更快捷的方法是直接继承 ExceptionFilterAttribute 并重写里面的 OnException()
方法即可,这是因为 ExceptionFilterAttribute 类本身就实现了 IExceptionFilter 接口,如下代码所示:
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, Inherited = true, AllowMultiple = true)]public abstract class ExceptionFilterAttribute : FilterAttribute, IExceptionFilter, IFilter{protected ExceptionFilterAttribute();public virtual void OnException(HttpActionExecutedContext actionExecutedContext);public virtual Task OnExceptionAsync(HttpActionExecutedContext actionExecutedContext, CancellationToken cancellationToken);}
下面的代码片段展示了如何通过重写 ExceptionFilterAttribute.OnException()
方法来创建一个自定义异常过滤器,请注意下面的代码是如何捕获在 Action 中抛出的异常,并将捕获到的异常转换为 HttpStatusResponse 实体,然后塞入合适的 httpcode 和 httpmessage,如下代码所示:
public class CustomExceptionFilter : ExceptionFilterAttribute{public override void OnException(HttpActionExecutedContext actionExecutedContext){HttpStatusCode status = HttpStatusCode.InternalServerError;String message = String.Empty;var exceptionType = actionExecutedContext.Exception.GetType();if (exceptionType == typeof(UnauthorizedAccessException)){message = "Access to the Web API is not authorized.";status = HttpStatusCode.Unauthorized;}else if (exceptionType == typeof(DivideByZeroException)){message = "Internal Server Error.";status = HttpStatusCode.InternalServerError;}else{message = "Not found.";status = HttpStatusCode.NotFound;}actionExecutedContext.Response = new HttpResponseMessage(){Content = new StringContent(message, System.Text.Encoding.UTF8, "text/plain"),StatusCode = status};base.OnException(actionExecutedContext);}}
接下来将自定义的异常过滤器添加到 HttpConfiguration 全局集合中,如下代码所示:
public static void Register(HttpConfiguration config){config.MapHttpAttributeRoutes();config.Routes.MapHttpRoute(name: "DefaultApi",routeTemplate: "api/{controller}/{id}",defaults: new { id = RouteParameter.Optional });config.Formatters.Remove(config.Formatters.XmlFormatter);config.Filters.Add(new CustomExceptionFilter());}
除了将自定义异常设置到全局上,你还可以缩小粒度到 Controller 或者 Action 级别上,下面的代码分别展示了如何将其控制在 Action 和 Controller 上。
[DatabaseExceptionFilter]
public class EmployeesController : ApiController
{//Some code
}[CustomExceptionFilter]public IEnumerable<string> Get(){throw new DivideByZeroException(); }
ASP.NET Web API 提供了强大的 HttpResponseException 来包装异常信息,默认情况下,当 WebAPI 中抛出异常,系统默认使用 Http StateCode = 500
作为回应,也即:Internal Server Error.
,场景就来了,如果你会用 HttpResponseException 的话,就可以改变这种系统默认行为,自定义错误码和错误信息让结果更加清晰语义化。
译文链接:https://www.infoworld.com/article/2994111/how-to-handle-errors-in-aspnet-web-api.html