.NET CORE HttpClient使用 _.NET

自从HttpClient诞生依赖，它的使用方式一直备受争议， framework版本时代产生过相当多经典的错误使用案例，包括Tcp链接耗尽、DNS更改无感知等问题。有兴趣的同学自行查找研究。在.NETCORE版本中，提供了IHttpClientFactory用来创建HttpClient以解决之前的种种问题。那么我们一起看一下它的用法。
使用方式注：这种方式经测试貌似不适用控制台程序。【.NET CORE HttpClient使用】示例代码
public void ConfigureServices(IServiceCollection services){ //普通注入 serviceCollection.AddHttpClient();//命名注入serviceCollection.AddHttpClient(Constants.SERVICE_USERACCOUNT, (serviceProvider, c) =>{var configuration = serviceProvider.GetRequiredService<IConfiguration>();c.BaseAddress = new Uri(configuration.GetValue<string>("ServiceApiBaseAddress:UserAccountService")); }); //类型化客户端 services.AddHttpClient<TypedClientService>();}public class AccreditationService{ private IHttpClientFactory _httpClientFactory; private const string _officialAccreName = "manage/CommitAgencyOfficialOrder"; private const string _abandonAccUserName = "info/AbandonUserAccreditationInfo";public AccreditationService(IHttpClientFactory clientFactory){_httpClientFactory = clientFactory;}public async Task<string> CommitAgentOfficial(CommitAgencyOfficialOrderRequest request){//使用factory 创建httpclientvar httpClient = _httpClientFactory.CreateClient(Constants.SERVICE_ACCREDITATION);var response = await httpClient.PostAsJsonAsync(_officialAccreName, request);if (!response.IsSuccessStatusCode) return string.Empty;var result = await response.Content.ReadAsAsync<AccreditationApiResponse<CommitAgencyOfficialOrderResult>>();if (result.ReturnCode != "0") return string.Empty;return result.Data.OrderNo;}}命名化客户端方式直接注入的是HttpClient而非HttpClientFactory
public class TypedClientService{private HttpClient _httpClient;public TypedClientService(HttpClient httpClient){_httpClient = httpClient;}}Logging通过IHttpClientFactory创建的客户端默认记录所有请求的日志消息，并每个客户端的日志类别会包含客户端名称，例如，名为 MyNamedClient 的客户端记录类别为“System.Net.Http.HttpClient.MyNamedClient.LogicalHandler”的消息。
请求管道同framework时代的HttpClient一样支持管道处理。需要自定义一个派生自 DelegatingHandler 的类，并实现 SendAsync 方法。例如下面的例子
public class ValidateHeaderHandler : DelegatingHandler{protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request,CancellationToken cancellationToken){if (!request.Headers.Contains("X-API-KEY")){return new HttpResponseMessage(HttpStatusCode.BadRequest){Content = new StringContent("You must supply an API key header called X-API-KEY")};}return await base.SendAsync(request, cancellationToken);}}在AddHttpClient的时候注入进去
public void ConfigureServices(IServiceCollection services){services.AddTransient<ValidateHeaderHandler>();services.AddHttpClient("externalservice", c =>{// Assume this is an "external" service which requires an API KEYc.BaseAddress = new Uri("https://localhost:5001/");}).AddHttpMessageHandler<ValidateHeaderHandler>();}原理和生存周期IHttpClientFactory每次调用CreateHttpClient都会返回一个全新的HttpClient实例。而负责http请求处理的核心HttpMessageHandler将会有工厂管理在一个池中，可以重复使用，以减少资源消耗。HttpMessageHandler默认生成期为两分钟。可以在每个命名客户端上重写默认值:
public void ConfigureServices(IServiceCollection services){services.AddHttpClient("extendedhandlerlifetime").SetHandlerLifetime(TimeSpan.FromMinutes(5));}

文章插图

Polly支持Polly是一款为.NET提供恢复能力和瞬态故障处理的库，它的各种策略应用（重试、断路器、超时、回退等）。IHttpClientFactory增加了对其的支持，它的nuget包为： Microsoft.Extensions.Http.Polly 。注入方式如下：

public void ConfigureServices(IServiceCollection services){services.AddHttpClient<UnreliableEndpointCallerService>().AddTransientHttpErrorPolicy(p =>p.WaitAndRetryAsync(3, _ => TimeSpan.FromMilliseconds(600)));}

更详细的结合使用请参考： https://github.com/App-vNext/Polly/wiki/Polly-and-HttpClientFactory