深入理解HttpClient
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Httpclient的设计图:
HttpClient请求过程:
从Request发起,经过DelegatingHanlder处理后,进入InnerHandler,数据返回后再从Inner Handler 返回到Delegating Hanlder进行处理,最后返回结果。
从设计角度来讲,HttpClient库提供了强大的扩展性,使用者不需要任何继承即可完成对HttpClient的扩展(如果对设计模式熟悉,可以清楚的看出这里用到了装饰器模式)
先看案例:
LoggingHandler:将HttpClient请求、响应内容记录日志
public class LoggingHandler : DelegatingHandler { public LoggingHandler(HttpMessageHandler handler) : base(handler) { } protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken) { if (request.Content!=null) { Debug.WriteLine(await request.Content.ReadAsStringAsync()); } HttpResponseMessage response = await base.SendAsync(request, cancellationToken); if (response.Content!=null) { Debug.WriteLine(await response.Content.ReadAsStringAsync()); } return response; } }
HttpMessageHandler handler = new HttpClientHandler(); HttpClient client = new HttpClient(new LoggingHandler(handler));
代码说明:
1、HttpClient这个类本身并不会进行实际的网络请求收发处理,我们应将其理解成一个容器、一个中继者,实际的网络请求核心在HttpClientHanlder中,也就是前面图中对应的Inner Handler。
2、我们自己定义了一个LoggingHandler,这个类对应Delegating Handler 是我们自定义的、装饰在Inner Handler外的Handler
3、DelegatingHandler重载了SendAsync,在其内部调用了InnerHandler的SendAsync方法,如此我们便可以在实际请求发出,以及返回后进行各种统一的处理,总结起来仍是上面图中画出的,逐层调用。
4、HttpClientHandler、DelegatingHandler都是继承自HttpMessageHandler
HttpClientHandler:
public class HttpClientHandler : HttpMessageHandler {}
DelegatingHandler:
public abstract class DelegatingHandler : HttpMessageHandler {}
案例2:封装一个RetryHandler,目的是失败重试
RetryHandler:
public class RetryHandler : DelegatingHandler { private const int MAX_COUNT = 3; protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken) { HttpResponseMessage response = null; for (int i = 0; i < MAX_COUNT; i++) { response = await base.SendAsync(request, cancellationToken); if (response.IsSuccessStatusCode) { return response; } } return response; } }
HttpMessageHandler handler = new HttpClientHandler(); handler = new LoggingHandler(handler); handler = new RetryHandler(handler); HttpClient client = new HttpClient(handler);
在.NET Core中,如果使用HttpClientFactory创建HttpClient,如何添加HttpMessageHandler?
需要自己去实现接口IHttpMessageHandlerBuilderFilter
参考:https://blog.csdn.net/weixin_30236595/article/details/101066107
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