一、前言

前面分享了 .net core HttpClient 使用之掉坑解析（一），今天来分享自定义消息处理HttpMessageHandler和PrimaryHttpMessageHandler 的使用场景和区别

二、源代码阅读

2.1 核心消息管道模型图

先贴上一张核心MessageHandler 管道模型的流程图，图如下：HttpClient 中的HttpMessageHandler 负责主要核心的业务，HttpMessageHandler 是由MessageHandler 链表结构组成，形成一个消息管道模式；具体我们一起来看看源代码

2.2 Demo代码演示

再阅读源代码的时候我们先来看下下面注入HttpClient 的Demo 代码，代码如下:

services.AddHttpClient("test").ConfigurePrimaryHttpMessageHandler(provider =>{return new PrimaryHttpMessageHandler(provider);}).AddHttpMessageHandler(provider =>{return new LogHttpMessageHandler(provider);}).AddHttpMessageHandler(provider =>{return new Log2HttpMessageHandler(provider);});

上面代码中有两个核心扩展方法，分别是ConfigurePrimaryHttpMessageHandler和AddHttpMessageHandler,这两个方法大家可能会有疑问是做什么的呢？不错，这两个方法就是扩展注册自定义的HttpMessageHandler 如果不注册，会有默认的HttpMessageHandler,接下来我们分别来看下提供的扩展方法，如下图:图中提供了一系列的AddHttpMessageHandler 扩展方法和ConfigurePrimaryHttpMessageHandler的扩展方法。

2.3 AddHttpMessageHandler

我们来看看HttpClientBuilderExtensions中的其中一个AddHttpMessageHandler扩展方法，代码如下：

        /// <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;}

代码中把自定义的DelegatingHandler 方法添加到HttpMessageHandlerBuilderActions中，我们再来看看HttpClientFactoryOptions对象源代码，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; }}

源代码中有如下核心List:

 public IList<Action<HttpMessageHandlerBuilder>> HttpMessageHandlerBuilderActions { get; } = new List<Action<HttpMessageHandlerBuilder>>();

提供了HttpMessageHandlerBuilder HttpMessageHandler 的构造器列表对象，故，通过AddHttpMessageHandler可以添加一系列的消息构造器方法对象 我们再来看看这个消息构造器类，核心部分，代码如下：

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构造器中有两个核心属性PrimaryHandler 和AdditionalHandlers ，细心的同学可以发现AdditionalHandlers是一个IList<DelegatingHandler>列表，也就是说可以HttpClient 可以添加多个DelegatingHandler 即多个HttpMessageHandler 消息处理Handler 但是只能有一个PrimaryHandler Handler

同时HttpMessageHandlerBuilder提供了一个抽象的Build方法，还有一个CreateHandlerPipeline 方法，这个方法主要是把IList<DelegatingHandler> 和PrimaryHandler 构造成一个MessageHandler 链表结构（通过DelegatingHandler 的InnerHandler属性进行连接起来）

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;}

通过上面的HttpMessageHandlerBuilder 源代码分析ConfigurePrimaryHttpMessageHandler 方法主要是给Builder 中添加PrimaryHandler消息Handler

2.5 DefaultHttpMessageHandlerBuilder

我们知道在services.AddHttpClient() 方法中会注册默认的DefaultHttpMessageHandlerBuilder 消息构造器方法，它继承DefaultHttpMessageHandlerBuilder,那我们来看看它的源代码

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);}

代码中Build 会去调用HttpMessageHandlerBuilder 的CreateHandlerPipeline方法把HttpMessageHandler 构建成一个类似于链表的结构。到这里源代码已经分析完了，接下来我们来演示一个Demo，来证明上面的核心HttpMessageHandler 流程走向图

三、Demo演示证明

我们继续来看上面我的Demo代码：

services.AddHttpClient("test").ConfigurePrimaryHttpMessageHandler(provider =>{return new PrimaryHttpMessageHandler(provider);}).AddHttpMessageHandler(provider =>{return new LogHttpMessageHandler(provider);}).AddHttpMessageHandler(provider =>{return new Log2HttpMessageHandler(provider);});

代码中自定义了两个HttpMessageHandler和一个PrimaryHttpMessageHandler我们再来分别看看Log2HttpMessageHandler、LogHttpMessageHandler 和PrimaryHttpMessageHandler 代码，代码很简单就是SendAsync前后输出了Log信息，代码如下：自定义的PrimaryHttpMessageHandler 代码如下：

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 代码如下：

 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代码如下：

 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;}}

三个自定义Handler 代码已经完成，我们继续添加调用代码，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;}

现在我们运行访问接口，运行后的控制台Log 如下图：看到输出结果，大家有没有发现跟Asp.net core 中的中间件管道的运行图一样。

四、总结

HttpClient中HttpMessageHandler可以自定义多个，但是只能有一个PrimaryHttpMessageHandler如果添加多个只会被最后面添加的给覆盖；添加的一系列Handler 构成一个链式管道模型，并且PrimaryHttpMessageHandler 主的消息Handler 是在管道的最外层，也就是管道模型中的最后一道Handler。使用场景：我们可以通过自定义的MessageHandler 来动态加载请求证书，通过数据库的一些信息，在自定义的Handler 中加载注入对应的证书，这样可以起到动态加载支付证书作用，同时可以SendAsync 之前或者之后做一些自己的验证等相关业务，大家只需要理解它们的用途，自然知道它的强大作用，今天就分享到这里。

往期精彩回顾

• 【.net core】电商平台升级之微服务架构应用实战

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• Asp.Net Core 中IdentityServer4 授权中心之自定义授权模式

• Asp.Net Core 中IdentityServer4 授权流程及刷新Token

• Asp.Net Core 中IdentityServer4 实战之 Claim详解

• Asp.Net Core 中IdentityServer4 实战之角色授权详解

• Asp.Net Core 中间件应用实战中你不知道的那些事

• Asp.Net Core Filter 深入浅出的那些事-AOP

• Asp.Net Core EndPoint 终结点路由工作原理解读

• ASP.NET CORE 内置的IOC解读及使用

♥ 给个[在看]，是对我最大的支持 ♥