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