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。

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