前言
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
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httpclient
var 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 KEY
c.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));
}
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
