ASP.NET Core 中HttpClientFactory如何使用

# ASP.NET Core 中HttpClientFactory如何使用

## 引言

在現代Web開發中，HTTP請求是應用程序與外部服務通信的核心方式。在ASP.NET Core中，`HttpClient`是常用的HTTP客戶端工具，但直接使用`HttpClient`可能導致資源泄漏和性能問題。`HttpClientFactory`作為ASP.NET Core 2.1引入的官方解決方案，提供了高效、可管理的方式來創建和配置`HttpClient`實例。本文將詳細介紹`HttpClientFactory`的使用方法、工作原理以及最佳實踐。

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## 目錄

1. **為什么需要HttpClientFactory**
2. **HttpClientFactory的核心功能**
3. **基本使用方法**
4. **命名客戶端與類型化客戶端**
5. **配置HTTP請求**
6. **處理故障與重試策略**
7. **集成Polly實現彈性策略**
8. **性能優化與最佳實踐**
9. **總結**

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## 1. 為什么需要HttpClientFactory

### 1.1 HttpClient的直接使用問題
直接實例化`HttpClient`可能導致以下問題：
- **資源耗盡**：未正確釋放`HttpClient`會導致套接字泄漏
- **DNS更新問題**：默認`HttpClient`會無限緩存DNS記錄
- **缺乏集中管理**：難以統一配置和監控所有HTTP請求

### 1.2 HttpClientFactory的優勢
- **生命周期管理**：自動管理`HttpClient`及其處理器
- **DNS刷新**：定期回收連接避免DNS緩存問題
- **集中配置**：統一配置日志、重試策略等
- **彈性支持**：輕松集成Polly等庫

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## 2. HttpClientFactory的核心功能

### 2.1 主要組件
- `IHttpClientFactory`：創建`HttpClient`的核心接口
- `HttpMessageHandler`：底層HTTP消息處理器管道
- `IHttpMessageHandlerFactory`：高級場景下的處理器工廠

### 2.2 工作流程
```mermaid
graph TD
    A[HttpClientFactory] --> B[創建HttpClient]
    B --> C[配置Handler管道]
    C --> D[應用中間件]
    D --> E[發送請求]

3. 基本使用方法

3.1 注冊服務

首先在Program.cs中添加服務：

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddHttpClient();
var app = builder.Build();

3.2 基礎使用示例

在控制器中注入使用：

public class MyController : Controller
{
    private readonly IHttpClientFactory _httpClientFactory;

    public MyController(IHttpClientFactory httpClientFactory)
    {
        _httpClientFactory = httpClientFactory;
    }

    public async Task<IActionResult> GetData()
    {
        var client = _httpClientFactory.CreateClient();
        var response = await client.GetAsync("https://api.example.com/data");
        // 處理響應...
    }
}

4. 命名客戶端與類型化客戶端

4.1 命名客戶端(Named Client)

// 注冊
builder.Services.AddHttpClient("GitHub", client => 
{
    client.BaseAddress = new Uri("https://api.github.com/");
    client.DefaultRequestHeaders.Add("Accept", "application/vnd.github.v3+json");
});

// 使用
var client = _httpClientFactory.CreateClient("GitHub");

4.2 類型化客戶端(Typed Client)

推薦的生產環境用法：

// 定義客戶端類型
public class GitHubService
{
    private readonly HttpClient _httpClient;
    
    public GitHubService(HttpClient httpClient)
    {
        _httpClient = httpClient;
        _httpClient.BaseAddress = new Uri("https://api.github.com/");
    }
    
    public async Task<string> GetUserAsync(string username)
    {
        return await _httpClient.GetStringAsync($"/users/{username}");
    }
}

// 注冊
builder.Services.AddHttpClient<GitHubService>();

// 使用
[ApiController]
public class MyController : Controller
{
    private readonly GitHubService _gitHubService;
    
    public MyController(GitHubService gitHubService)
    {
        _gitHubService = gitHubService;
    }
}

5. 配置HTTP請求

5.1 常用配置項

builder.Services.AddHttpClient("ConfiguredClient", client =>
{
    client.BaseAddress = new Uri("https://api.example.com");
    client.Timeout = TimeSpan.FromSeconds(30);
    client.DefaultRequestHeaders.Add("User-Agent", "MyApp");
    client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
});

5.2 自定義HttpMessageHandler

builder.Services.AddHttpClient("CustomHandlerClient")
    .ConfigurePrimaryHttpMessageHandler(() => new HttpClientHandler
    {
        AllowAutoRedirect = false,
        UseProxy = false
    });

6. 處理故障與重試策略

6.1 基本重試策略

builder.Services.AddHttpClient("RetryClient")
    .AddTransientHttpErrorPolicy(policy => 
        policy.WaitAndRetryAsync(3, retryAttempt => 
            TimeSpan.FromSeconds(Math.Pow(2, retryAttempt))));

6.2 高級策略組合

var retryPolicy = HttpPolicyExtensions
    .HandleTransientHttpError()
    .OrResult(msg => msg.StatusCode == System.Net.HttpStatusCode.TooManyRequests)
    .WaitAndRetryAsync(/*...*/);

var circuitBreakerPolicy = Policy.Handle<HttpRequestException>()
    .CircuitBreakerAsync(5, TimeSpan.FromSeconds(30));

builder.Services.AddHttpClient("ResilientClient")
    .AddPolicyHandler(retryPolicy)
    .AddPolicyHandler(circuitBreakerPolicy);

7. 集成Polly實現彈性策略

7.1 安裝必要包

dotnet add package Microsoft.Extensions.Http.Polly

7.2 完整彈性策略示例

var retryPolicy = Policy<HttpResponseMessage>
    .Handle<HttpRequestException>()
    .OrResult(r => (int)r.StatusCode >= 500)
    .WaitAndRetryAsync(3, retryAttempt => 
        TimeSpan.FromSeconds(Math.Pow(2, retryAttempt)));

var timeoutPolicy = Policy.TimeoutAsync<HttpResponseMessage>(10);

builder.Services.AddHttpClient("PollyClient")
    .AddPolicyHandler(retryPolicy)
    .AddPolicyHandler(timeoutPolicy);

8. 性能優化與最佳實踐

8.1 重要建議

1. 避免手動創建HttpClient：始終通過工廠獲取
2. 合理設置超時：根據場景調整Timeout
3. 復用HttpClient：類型化客戶端是首選
4. 監控HTTP調用：集成日志和指標

8.2 性能對比

9. 總結

HttpClientFactory作為ASP.NET Core的HTTP客戶端解決方案，提供了： - 可靠的資源管理 - 靈活的配置方式 - 強大的彈性策略支持 - 優異的性能表現

通過命名客戶端、類型化客戶端與Polly策略的組合，開發者可以構建健壯的HTTP通信層。建議在新項目中始終使用HttpClientFactory，并在舊項目中進行遷移。

附錄：常見問題解答

Q: 如何處理自定義證書驗證？

builder.Services.AddHttpClient("SecureClient")
    .ConfigurePrimaryHttpMessageHandler(() => new HttpClientHandler
    {
        ServerCertificateCustomValidationCallback = (msg, cert, chain, errors) => 
            cert.Issuer == "CustomCA"
    });

Q: 如何記錄HTTP請求日志？

builder.Services.AddHttpClient("LoggingClient")
    .AddHttpMessageHandler(() => new LoggingHandler());

public class LoggingHandler : DelegatingHandler
{
    protected override async Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request, CancellationToken cancellationToken)
    {
        // 記錄請求日志
        var response = await base.SendAsync(request, cancellationToken);
        // 記錄響應日志
        return response;
    }
}

注意：實際項目應根據具體需求調整配置參數和策略組合。 “`