一、前言
前面分享了 .net core HttpClient 使用之掉坑解析（一），今天來分享自定義訊息處理HttpMessageHandler和PrimaryHttpMessageHandler 的使用場景和區別
二、原始碼閱讀
2.1 核心訊息管道模型圖
先貼上一張核心MessageHandler 管道模型的流程圖，圖如下：
HttpClient 中的HttpMessageHandler 負責主要核心的業務，HttpMessageHandler 是由MessageHandler 連結串列結構組成，形成一個訊息管道模式；具體我們一起來看看原始碼
2.2 Demo程式碼演示
再閱讀原始碼的時候我們先來看下下面注入HttpClient 的Demo 程式碼，程式碼如下:
services.AddHttpClient("test")
.ConfigurePrimaryHttpMessageHandler(provider =>
{
return new PrimaryHttpMessageHandler(provider);
})
.AddHttpMessageHandler(provider =>
{
return new LogHttpMessageHandler(provider);
})
.AddHttpMessageHandler(provider =>
{
return new Log2HttpMessageHandler(provider);
});
上面程式碼中有兩個核心擴充套件方法，分別是ConfigurePrimaryHttpMessageHandler和AddHttpMessageHandler,這兩個方法大家可能會有疑問是做什麼的呢？
不錯，這兩個方法就是擴充套件註冊自定義的HttpMessageHandler 如果不註冊，會有預設的HttpMessageHandler,接下來我們分別來看下提供的擴充套件方法，如下圖:
圖中提供了一系列的AddHttpMessageHandler 擴充套件方法和ConfigurePrimaryHttpMessageHandler的擴充套件方法。
2.3 AddHttpMessageHandler
我們來看看HttpClientBuilderExtensions中的其中一個AddHttpMessageHandler擴充套件方法，程式碼如下：
/// <summary>
/// Adds a delegate that will be used to create an additional message handler for a named <see cref="HttpClient"/>.
/// </summary>
/// <param name="builder">The <see cref="IHttpClientBuilder"/>.</param>
/// <param name="configureHandler">A delegate that is used to create a <see cref="DelegatingHandler"/>.</param>
/// <returns>An <see cref="IHttpClientBuilder"/> that can be used to configure the client.</returns>
/// <remarks>
/// The <see paramref="configureHandler"/> delegate should return a new instance of the message handler each time it
/// is invoked.
/// </remarks>
public static IHttpClientBuilder AddHttpMessageHandler(this IHttpClientBuilder builder, Func<DelegatingHandler> configureHandler)
{
if (builder == null)
{
throw new ArgumentNullException(nameof(builder));
}
if (configureHandler == null)
{
throw new ArgumentNullException(nameof(configureHandler));
}
builder.Services.Configure<HttpClientFactoryOptions>(builder.Name, options =>
{
options.HttpMessageHandlerBuilderActions.Add(b => b.AdditionalHandlers.Add(configureHandler()));
});
return builder;
}
程式碼中把自定義的DelegatingHandler 方法新增到HttpMessageHandlerBuilderActions中，我們再來看看HttpClientFactoryOptions物件原始碼，如下：
/// <summary>
/// An options class for configuring the default <see cref="IHttpClientFactory"/>.
/// </summary>
public class HttpClientFactoryOptions
{
// Establishing a minimum lifetime helps us avoid some possible destructive cases.
//
// IMPORTANT: This is used in a resource string. Update the resource if this changes.
internal readonly static TimeSpan MinimumHandlerLifetime = TimeSpan.FromSeconds(1);
private TimeSpan _handlerLifetime = TimeSpan.FromMinutes(2);
/// <summary>
/// Gets a list of operations used to configure an <see cref="HttpMessageHandlerBuilder"/>.
/// </summary>
public IList<Action<HttpMessageHandlerBuilder>> HttpMessageHandlerBuilderActions { get; } = new List<Action<HttpMessageHandlerBuilder>>();
/// <summary>
/// Gets a list of operations used to configure an <see cref="HttpClient"/>.
/// </summary>
public IList<Action<HttpClient>> HttpClientActions { get; } = new List<Action<HttpClient>>();
/// <summary>
/// Gets or sets the length of time that a <see cref="HttpMessageHandler"/> instance can be reused. Each named
/// client can have its own configured handler lifetime value. The default value of this property is two minutes.
/// Set the lifetime to <see cref="Timeout.InfiniteTimeSpan"/> to disable handler expiry.
/// </summary>
/// <remarks>
/// <para>
/// The default implementation of <see cref="IHttpClientFactory"/> will pool the <see cref="HttpMessageHandler"/>
/// instances created by the factory to reduce resource consumption. This setting configures the amount of time
/// a handler can be pooled before it is scheduled for removal from the pool and disposal.
/// </para>
/// <para>
/// Pooling of handlers is desirable as each handler typically manages its own underlying HTTP connections; creating
/// more handlers than necessary can result in connection delays. Some handlers also keep connections open indefinitely
/// which can prevent the handler from reacting to DNS changes. The value of <see cref="HandlerLifetime"/> should be
/// chosen with an understanding of the application's requirement to respond to changes in the network environment.
/// </para>
/// <para>
/// Expiry of a handler will not immediately dispose the handler. An expired handler is placed in a separate pool
/// which is processed at intervals to dispose handlers only when they become unreachable. Using long-lived
/// <see cref="HttpClient"/> instances will prevent the underlying <see cref="HttpMessageHandler"/> from being
/// disposed until all references are garbage-collected.
/// </para>
/// </remarks>
public TimeSpan HandlerLifetime
{
get => _handlerLifetime;
set
{
if (value != Timeout.InfiniteTimeSpan && value < MinimumHandlerLifetime)
{
throw new ArgumentException(Resources.HandlerLifetime_InvalidValue, nameof(value));
}
_handlerLifetime = value;
}
}
/// <summary>
/// The <see cref="Func{T, R}"/> which determines whether to redact the HTTP header value before logging.
/// </summary>
public Func<string, bool> ShouldRedactHeaderValue { get; set; } = (header) => false;
/// <summary>
/// <para>
/// Gets or sets a value that determines whether the <see cref="IHttpClientFactory"/> will
/// create a dependency injection scope when building an <see cref="HttpMessageHandler"/>.
/// If <c>false</c> (default), a scope will be created, otherwise a scope will not be created.
/// </para>
/// <para>
/// This option is provided for compatibility with existing applications. It is recommended
/// to use the default setting for new applications.
/// </para>
/// </summary>
/// <remarks>
/// <para>
/// The <see cref="IHttpClientFactory"/> will (by default) create a dependency injection scope
/// each time it creates an <see cref="HttpMessageHandler"/>. The created scope has the same
/// lifetime as the message handler, and will be disposed when the message handler is disposed.
/// </para>
/// <para>
/// When operations that are part of <see cref="HttpMessageHandlerBuilderActions"/> are executed
/// they will be provided with the scoped <see cref="IServiceProvider"/> via
/// <see cref="HttpMessageHandlerBuilder.Services"/>. This includes retrieving a message handler
/// from dependency injection, such as one registered using
/// <see cref="HttpClientBuilderExtensions.AddHttpMessageHandler{THandler}(IHttpClientBuilder)"/>.
/// </para>
/// </remarks>
public bool SuppressHandlerScope { get; set; }
}
原始碼中有如下核心List:
public IList<Action<HttpMessageHandlerBuilder>> HttpMessageHandlerBuilderActions { get; } = new List<Action<HttpMessageHandlerBuilder>>();
提供了HttpMessageHandlerBuilder HttpMessageHandler 的構造器列表物件，故，通過AddHttpMessageHandler可以新增一系列的訊息構造器方法物件
我們再來看看這個訊息構造器類，核心部分，程式碼如下：
public abstract class HttpMessageHandlerBuilder
{
/// <summary>
/// Gets or sets the name of the <see cref="HttpClient"/> being created.
/// </summary>
/// <remarks>
/// The <see cref="Name"/> is set by the <see cref="IHttpClientFactory"/> infrastructure
/// and is public for unit testing purposes only. Setting the <see cref="Name"/> outside of
/// testing scenarios may have unpredictable results.
/// </remarks>
public abstract string Name { get; set; }
/// <summary>
/// Gets or sets the primary <see cref="HttpMessageHandler"/>.
/// </summary>
public abstract HttpMessageHandler PrimaryHandler { get; set; }
/// <summary>
/// Gets a list of additional <see cref="DelegatingHandler"/> instances used to configure an
/// <see cref="HttpClient"/> pipeline.
/// </summary>
public abstract IList<DelegatingHandler> AdditionalHandlers { get; }
/// <summary>
/// Gets an <see cref="IServiceProvider"/> which can be used to resolve services
/// from the dependency injection container.
/// </summary>
/// <remarks>
/// This property is sensitive to the value of
/// <see cref="HttpClientFactoryOptions.SuppressHandlerScope"/>. If <c>true</c> this
/// property will be a reference to the application's root service provider. If <c>false</c>
/// (default) this will be a reference to a scoped service provider that has the same
/// lifetime as the handler being created.
/// </remarks>
public virtual IServiceProvider Services { get; }
/// <summary>
/// Creates an <see cref="HttpMessageHandler"/>.
/// </summary>
/// <returns>
/// An <see cref="HttpMessageHandler"/> built from the <see cref="PrimaryHandler"/> and
/// <see cref="AdditionalHandlers"/>.
/// </returns>
public abstract HttpMessageHandler Build();
protected internal static HttpMessageHandler CreateHandlerPipeline(HttpMessageHandler primaryHandler, IEnumerable<DelegatingHandler> additionalHandlers)
{
// This is similar to https://github.com/aspnet/AspNetWebStack/blob/master/src/System.Net.Http.Formatting/HttpClientFactory.cs#L58
// but we don't want to take that package as a dependency.
if (primaryHandler == null)
{
throw new ArgumentNullException(nameof(primaryHandler));
}
if (additionalHandlers == null)
{
throw new ArgumentNullException(nameof(additionalHandlers));
}
var additionalHandlersList = additionalHandlers as IReadOnlyList<DelegatingHandler> ?? additionalHandlers.ToArray();
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);
}
// Checking for this allows us to catch cases where someone has tried to re-use a handler. That really won't
// work the way you want and it can be tricky for callers to figure out.
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;
}
return next;
}
}
HttpMessageHandlerBuilder構造器中有兩個核心屬性PrimaryHandler 和AdditionalHandlers ，細心的同學可以發現AdditionalHandlers是一個IList<DelegatingHandler>列表，也就是說可以HttpClient 可以新增多個DelegatingHandler 即多個HttpMessageHandler 訊息處理Handler 但是隻能有一個PrimaryHandler Handler
同時HttpMessageHandlerBuilder提供了一個抽象的Build方法，還有一個CreateHandlerPipeline 方法，這個方法主要是把IList<DelegatingHandler> 和PrimaryHandler 構造成一個MessageHandler 連結串列結構（通過DelegatingHandler 的InnerHandler屬性進行連線起來）
2.4 ConfigurePrimaryHttpMessageHandler
public static IHttpClientBuilder ConfigurePrimaryHttpMessageHandler(this IHttpClientBuilder builder, Func<HttpMessageHandler> configureHandler)
{
if (builder == null)
{
throw new ArgumentNullException(nameof(builder));
}
if (configureHandler == null)
{
throw new ArgumentNullException(nameof(configureHandler));
}
builder.Services.Configure<HttpClientFactoryOptions>(builder.Name, options =>
{
options.HttpMessageHandlerBuilderActions.Add(b => b.PrimaryHandler = configureHandler());
});
return builder;
}
通過上面的HttpMessageHandlerBuilder 原始碼分析ConfigurePrimaryHttpMessageHandler 方法主要是給Builder 中新增PrimaryHandler訊息Handler
2.5 DefaultHttpMessageHandlerBuilder
我們知道在services.AddHttpClient() 方法中會註冊預設的DefaultHttpMessageHandlerBuilder 訊息構造器方法，它繼承DefaultHttpMessageHandlerBuilder,那我們來看看它的原始碼
internal class DefaultHttpMessageHandlerBuilder : HttpMessageHandlerBuilder
{
public DefaultHttpMessageHandlerBuilder(IServiceProvider services)
{
Services = services;
}
private string _name;
public override string Name
{
get => _name;
set
{
if (value == null)
{
throw new ArgumentNullException(nameof(value));
}
_name = value;
}
}
public override HttpMessageHandler PrimaryHandler { get; set; } = new HttpClientHandler();
public override IList<DelegatingHandler> AdditionalHandlers { get; } = new List<DelegatingHandler>();
public override IServiceProvider Services { get; }
public override HttpMessageHandler Build()
{
if (PrimaryHandler == null)
{
var message = Resources.FormatHttpMessageHandlerBuilder_PrimaryHandlerIsNull(nameof(PrimaryHandler));
throw new InvalidOperationException(message);
}
return CreateHandlerPipeline(PrimaryHandler, AdditionalHandlers);
}
程式碼中Build 會去呼叫HttpMessageHandlerBuilder 的CreateHandlerPipeline方法把HttpMessageHandler 構建成一個類似於連結串列的結構。
到這裡原始碼已經分析完了，接下來我們來演示一個Demo，來證明上面的核心HttpMessageHandler 流程走向圖
三、Demo演示證明
我們繼續來看上面我的Demo程式碼：
services.AddHttpClient("test")
.ConfigurePrimaryHttpMessageHandler(provider =>
{
return new PrimaryHttpMessageHandler(provider);
})
.AddHttpMessageHandler(provider =>
{
return new LogHttpMessageHandler(provider);
})
.AddHttpMessageHandler(provider =>
{
return new Log2HttpMessageHandler(provider);
});
程式碼中自定義了兩個HttpMessageHandler和一個PrimaryHttpMessageHandler
我們再來分別看看Log2HttpMessageHandler、LogHttpMessageHandler 和PrimaryHttpMessageHandler 程式碼，程式碼很簡單就是SendAsync前後輸出了Log資訊，程式碼如下：
自定義的PrimaryHttpMessageHandler 程式碼如下：
public class PrimaryHttpMessageHandler: DelegatingHandler
{
private IServiceProvider _provider;
public PrimaryHttpMessageHandler(IServiceProvider provider)
{
_provider = provider;
InnerHandler = new HttpClientHandler();
}
protected async override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
{
System.Console.WriteLine("PrimaryHttpMessageHandler Start Log");
var response= await base.SendAsync(request, cancellationToken);
System.Console.WriteLine("PrimaryHttpMessageHandler End Log");
return response;
}
}
Log2HttpMessageHandler 程式碼如下：
public class Log2HttpMessageHandler : DelegatingHandler
{
private IServiceProvider _provider;
public Log2HttpMessageHandler(IServiceProvider provider)
{
_provider = provider;
//InnerHandler = new HttpClientHandler();
}
protected async override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
{
System.Console.WriteLine("LogHttpMessageHandler2 Start Log");
var response=await base.SendAsync(request, cancellationToken);
System.Console.WriteLine("LogHttpMessageHandler2 End Log");
return response;
}
}
LogHttpMessageHandler程式碼如下：
public class LogHttpMessageHandler : DelegatingHandler
{
private IServiceProvider _provider;
public LogHttpMessageHandler(IServiceProvider provider)
{
_provider = provider;
//InnerHandler = new HttpClientHandler();
}
protected async override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
{
System.Console.WriteLine("LogHttpMessageHandler Start Log");
var response=await base.SendAsync(request, cancellationToken);
System.Console.WriteLine("LogHttpMessageHandler End Log");
return response;
}
}
三個自定義Handler 程式碼已經完成，我們繼續新增呼叫程式碼，如下：
/// <summary>
///
/// </summary>
/// <param name="url"></param>
/// <returns></returns>
public async Task<string> GetBaiduAsync(string url)
{
var client = _clientFactory.CreateClient("test");
var result = await client.GetStringAsync(url);
return result;
}
現在我們執行訪問介面，執行後的控制檯Log 如下圖：
看到輸出結果，大家有沒有發現跟Asp.net core 中的中介軟體管道的執行圖一樣。
四、總結
HttpClient中HttpMessageHandler可以自定義多個，但是隻能有一個PrimaryHttpMessageHandler如果新增多個只會被最後面新增的給覆蓋；新增的一系列Handler 構成一個鏈式管道模型，並且PrimaryHttpMessageHandler 主的訊息Handler 是在管道的最外層，也就是管道模型中的最後一道Handler。
使用場景：我們可以通過自定義的MessageHandler 來動態載入請求證書，通過資料庫的一些資訊，在自定義的Handler 中載入注入對應的證書，這樣可以起到動態載入支付證書作用，同時可以SendAsync 之前或者之後做一些自己的驗證等相關業務，大家只需要理解它們的用途，自然知道它的強大作用，今天就分享到這裡