自從HttpClient誕生依賴，它的使用方式一直備受爭議，framework版本時代產生過相當多經典的錯誤使用案例，包括Tcp鏈接耗盡、DNS更改無感知等問題。有興趣的同學自行查找研究。在.NETCORE版本中，提供了IHttpClientFactory用來創建HttpClient以解決之前的種種問題。那么我們一起看一下它的用法。

使用方式

注：這種方式經測試貌似不適用控制臺程序。

示例代碼

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"));
	});
	//類型化客戶端
	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 創建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;
   	}
}

命名化客戶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

通過IHttpClientFactory創建的客戶端默認記錄所有請求的日志消息，并每個客戶端的日志類別會包含客戶端名稱，例如，名為 MyNamedClient 的客戶端記錄類別為“System.Net.Http.HttpClient.MyNamedClient.LogicalHandler”的消息。

請求管道

同framework時代的HttpClient一樣支持管道處理。需要自定義一個派生自 DelegatingHandler 的類，并實現 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的時候注入進去

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每次調用CreateHttpClient都會返回一個全新的HttpClient實例。而負責http請求處理的核心HttpMessageHandler將會有工廠管理在一個池中，可以重復使用，以減少資源消耗。HttpMessageHandler默認生成期為兩分鐘。可以在每個命名客戶端上重寫默認值:

public void ConfigureServices(IServiceCollection services)
{           
    services.AddHttpClient("extendedhandlerlifetime")
        .SetHandlerLifetime(TimeSpan.FromMinutes(5));
}

Polly支持

Polly是一款為.NET提供恢復能力和瞬態故障處理的庫，它的各種策略應用（重試、斷路器、超時、回退等）。IHttpClientFactory增加了對其的支持，它的nuget包為： 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)));

}

更詳細的結合使用請參考： https://github.com/App-vNext/Polly/wiki/Polly-and-HttpClientFactory