.NET Core 3.0之深入原始碼理解HttpClientFactory(二)

 

寫在前面

上一篇文章討論了通過在ConfigureServices中呼叫services.AddHttpClient()方法，並基於此進一步探討了DefaultHttpClientFactory是如何建立HttpClient例項和HttpMessageHandler例項的，並瞭解了DefaultHttpClientFactory內部維護者一個定時器和兩個HttpMessageHandler物件集合，以定期清理無效的 HttpMessageHandler物件，詳細的內容可以點選連結跳轉，接下來我會接著前一篇文章繼續展開相關討論。

詳細介紹

HttpMessageHandlerBuilder

該類是一個抽象類，起到生成器的作用，可用於用於配置HttpMessageHandler例項。HttpMessageHandlerBuilder會在ServiceCollection中被註冊為Transient服務。呼叫方要為每個要建立的HttpMessageHandler例項檢索一個新例項。實現者應該確保每個例項都只使用一次。

HttpMessageHandlerBuilder裡面有三個比較重要的屬性：

/// <summary>
/// 主HttpMessageHandler例項
/// </summary>
public abstract HttpMessageHandler PrimaryHandler { get; set; }

/// <summary>
/// 這個是一個附加例項，用於配置HttpClient管道
/// </summary>
public abstract IList<DelegatingHandler> AdditionalHandlers { get; }

/// <summary>
/// 可用於從依賴項注入容器解析服務的IServiceProvider
/// </summary>
public virtual IServiceProvider Services { get; }

這三個屬性意味著每個HttpMessageHandlerBuilder都需要維護自身的HttpMessageHandler例項和管道。

其內部還有一個抽象方法：

public abstract HttpMessageHandler Build();

當然，內部最核心的方法就是管道的建立過程了，需要傳入主派生類自身的HttpMessageHandler和管道列表物件。它會將primaryHandler例項付給管道列表的第一個Item的InnerHandler，其他物件會依此後移，這也為我們自定義HttpMessageHandler(各種中介軟體)提供了無限可能。

相關實現如下：

var next = primaryHandler;
for (var i = additionalHandlersList.Count - 1; i >= 0; i--)
{
    var handler = additionalHandlersList[i];
    if (handler == null)
    {
        var message = Resources.FormatHttpMessageHandlerBuilder_AdditionalHandlerIsNull(nameof(additionalHandlers));
        throw new InvalidOperationException(message);
    }

    if (handler.InnerHandler != null)
    {
        var message = Resources.FormatHttpMessageHandlerBuilder_AdditionHandlerIsInvalid(
            nameof(DelegatingHandler.InnerHandler),
            nameof(DelegatingHandler),
            nameof(HttpMessageHandlerBuilder),
            Environment.NewLine,
            handler);
        throw new InvalidOperationException(message);
    }

    handler.InnerHandler = next;
    next = handler;
}

接下來我們看一下HttpMessageHandlerBuilder一個派生類DefaultHttpMessageHandlerBuilder，其建構函式會傳入IServiceProvider例項，我們的自定義操作也可以參照這個類。

關於Build方法的實現如下，比較簡單主要是呼叫了CreateHandlerPipeline方法：

public override HttpMessageHandler Build()
{
    if (PrimaryHandler == null)
    {
        var message = Resources.FormatHttpMessageHandlerBuilder_PrimaryHandlerIsNull(nameof(PrimaryHandler));
        throw new InvalidOperationException(message);
    }

    return CreateHandlerPipeline(PrimaryHandler, AdditionalHandlers);
}

ITypedHttpClientFactory

這是一個抽象工廠，該元件可以使用給定邏輯名稱的自定義配置建立型別化HttpClient例項，與命名方式建立HttpClient具有相同的的功能。型別化客戶端可能用於單個後端終結點，並封裝此終結點的所有處理邏輯。 另一個優勢是它們使用 DI 被注入到應用中需要的位置，下一篇文章會再次討論相關功能。

我們首先看一下呼叫方式：

public static IHttpClientBuilder AddHttpClient<TClient>(this IServiceCollection services)
    where TClient : class
{
    if (services == null)
    {
        throw new ArgumentNullException(nameof(services));
    }

    AddHttpClient(services);

    var name = TypeNameHelper.GetTypeDisplayName(typeof(TClient), fullName: false);
    var builder = new DefaultHttpClientBuilder(services, name);
    builder.AddTypedClient<TClient>();
    return builder;
}

可以看出此處的呼叫與普通的HttpClient沒有什麼太大區別，只是增加了一個泛型標記，而且該型別沒有特殊的要求，只要是個類就行。其內部依然呼叫AddHttpClient(services），但它呼叫了另一個擴充套件方法，如下所示：

public static IHttpClientBuilder AddTypedClient<TClient>(this IHttpClientBuilder builder)
    where TClient : class
{
    if (builder == null)
    {
        throw new ArgumentNullException(nameof(builder));
    }

    builder.Services.AddTransient<TClient>(s =>
    {
        var httpClientFactory = s.GetRequiredService<IHttpClientFactory>();
        var httpClient = httpClientFactory.CreateClient(builder.Name);

        var typedClientFactory = s.GetRequiredService<ITypedHttpClientFactory<TClient>>();
        return typedClientFactory.CreateClient(httpClient);
    });

    return builder;
}

可以看到最終的程式碼呼叫了ITypedHttpClientFactory的CreateClient方法，Microsoft.Extensions.Http包中有一個預設的ITypedHttpClientFactory派生類，DefaultTypedHttpClientFactory<TClient>，該類提供了了建構函式用於接收IServiceProvider例項，以及一個內部類宣告的快取物件，該物件十分重要，它被註冊為singleton型別，已達到全域性使用，並可以充當相關例項啟用時的物件池。它也允許它的外部類註冊為transient，這樣它就不會在應用根服務提供程式上被關掉了。

相關程式碼如下：

public TClient CreateClient(HttpClient httpClient)
{
    if (httpClient == null)
    {
        throw new ArgumentNullException(nameof(httpClient));
    }

    return (TClient)_cache.Activator(_services, new object[] { httpClient });
}

內部快取物件：

public class Cache
{
    private readonly static Func<ObjectFactory> _createActivator = () => ActivatorUtilities.CreateFactory(typeof(TClient), new Type[] { typeof(HttpClient), });

    private ObjectFactory _activator;
    private bool _initialized;
    private object _lock;

    public ObjectFactory Activator => LazyInitializer.EnsureInitialized(
        ref _activator,
        ref _initialized,
        ref _lock,
        _createActivator);
}

最後我們看一下原始碼中提供的範例：

class ExampleClient
{
    private readonly HttpClient _httpClient;
    private readonly ILogger _logger;
    // typed clients can use constructor injection to access additional services
    public ExampleClient(HttpClient httpClient, ILogger<ExampleClient> logger)
    {
        _httpClient = httpClient;
        _logger = logger;
    }
    // typed clients can expose the HttpClient for application code to call directly
    public HttpClient HttpClient => _httpClient;
    // typed clients can also define methods that abstract usage of the HttpClient
    public async Task SendHelloRequest()
    {
        var response = await _httpClient.GetAsync("/helloworld");
        response.EnsureSuccessStatusCode();
    }
}
//This sample shows how to consume a typed client from an ASP.NET Core middleware.
public void Configure(IApplicationBuilder app, ExampleClient exampleClient)
{
    app.Run(async (context) =>
    {
        var response = await _exampleClient.GetAsync("/helloworld");
        await context.Response.WriteAsync("Remote server said: ");
        await response.Content.CopyToAsync(context.Response.Body);
    });
}
//This sample shows how to consume a typed client from an ASP.NET Core MVC Controller.
public class HomeController : ControllerBase(IApplicationBuilder app, ExampleClient exampleClient)
{
    private readonly ExampleClient _exampleClient;
    public HomeController(ExampleClient exampleClient)
    {
        _exampleClient = exampleClient;
    }
    public async Task<IActionResult> Index()
    {
        var response = await _exampleClient.GetAsync("/helloworld");
        var text = await response.Content.ReadAsStringAsync();
        return Content("Remote server said: " + text, "text/plain");
    };
}