.net core HttpClient 使用之消息管道解析（二）
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
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
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