.net core HttpClient 使用之訊息管道解析（二）
阿新 • • 發佈：2020-05-25
## 一、前言
前面分享了 [.net core HttpClient 使用之掉坑解析（一）](https://www.cnblogs.com/jlion/p/12813692.html)，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：
```
///
/// Adds a delegate that will be used to create an additional message handler for a named
/// The An
///
/// The
public static IHttpClientBuilder AddHttpMessageHandler(this IHttpClientBuilder builder, Func configureHandler)
{
if (builder == null)
{
throw new ArgumentNullException(nameof(builder));
}
if (configureHandler == null)
{
throw new ArgumentNullException(nameof(configureHandler));
}
builder.Services.Configure(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：
```
///
/// An options class for configuring the default
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);
///
/// Gets a list of operations used to configure an
public IList> HttpMessageHandlerBuilderActions { get; } = new List>();
///
/// Gets a list of operations used to configure an
public IList> HttpClientActions { get; } = new List>();
///
/// Gets or sets the length of time that a
///
///
/// The default implementation of
///
/// 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
///
/// 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
///
///
public TimeSpan HandlerLifetime
{
get => _handlerLifetime;
set
{
if (value != Timeout.InfiniteTimeSpan && value < MinimumHandlerLifetime)
{
throw new ArgumentException(Resources.HandlerLifetime_InvalidValue, nameof(value));
}
_handlerLifetime = value;
}
}
///
/// The
public Func ShouldRedactHeaderValue { get; set; } = (header) => false;
///
///
/// Gets or sets a value that determines whether the false (default), a scope will be created, otherwise a scope will not be created.
///
///
/// This option is provided for compatibility with existing applications. It is recommended
/// to use the default setting for new applications.
///
///
///
///
/// The
///
/// When operations that are part of
///
public bool SuppressHandlerScope { get; set; }
}
```
原始碼中有如下核心List:
```
public IList> HttpMessageHandlerBuilderActions { get; } = new List>();
```
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
{
///
/// Gets or sets the name of the
///
/// The
public abstract string Name { get; set; }
///
/// Gets or sets the primary
public abstract HttpMessageHandler PrimaryHandler { get; set; }
///
/// Gets a list of additional
public abstract IList AdditionalHandlers { get; }
///
/// Gets an
///
/// This property is sensitive to the value of
/// true this
/// property will be a reference to the application's root service provider. If false
/// (default) this will be a reference to a scoped service provider that has the same
/// lifetime as the handler being created.
///
public virtual IServiceProvider Services { get; }
///
/// Creates an
///
/// An
public abstract HttpMessageHandler Build();
protected internal static HttpMessageHandler CreateHandlerPipeline(HttpMessageHandler primaryHandler, IEnumerable 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 ?? 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`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` 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 configureHandler)
{
if (builder == null)
{
throw new ArgumentNullException(nameof(builder));
}
if (configureHandler == null)
{
throw new ArgumentNullException(nameof(configureHandler));
}
builder.Services.Configure(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 AdditionalHandlers { get; } = new List();
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 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 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 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：
```
///
///
///
///
/// 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