深入理解HttpClient

介绍：

HttpClient用于发送http请求，默认情况下内部使用HttpWebRequest发送请求，这一行为可以通过构造函数中指定HttpMessageHandler参数来使用不同的通道

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，也就是下面图中对应的Inner Handler。

HttpClient构造函数可以接受一个HttpMessageHandler类型的参数

HttpClientHandler：HttpClientHandler继承自HttpMessageHandler，用于设置cookie、代理、证书、Headlers等属性

DelegatingHandler：抽象类，DelegatingHandler继承自HttpMessageHandler，DelegatingHandler内部有一个属性InnerHandler，http请求都是通过InnerHandler发送。通过继承此类可实现类似HttpModule的功能

HttpClient继承自HttpMessageInvoker，从名字可以看出HttpClient是一个调用器，它是对HttpMessageHandler的一个调用，HttpClient本身不提供Http请求功能

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）

Httpclient的设计图：

   

案例1：

设置cookie、Headers、代理、证书等，通过设置HttpClientHandler属性实现

HttpClientHandler httpClientHandler = new HttpClientHandler();
            httpClientHandler.CookieContainer.Add(new Cookie("id", "1"));
            //httpClientHandler.Proxy = null;
            //httpClientHandler.Credentials = null;
            HttpClient httpClient = new HttpClient(httpClientHandler);

案例2：日志

将HttpClient请求、响应内容记录日志，通过继承DelegatingHandler抽象类实现

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

5、必须给LoggingHandler传入HttpClientHandler，因为最终都是通过HttpClientHandler发送请求，如果不传会抛异常

HttpClientHandler：

public class HttpClientHandler : HttpMessageHandler
{}

DelegatingHandler：

public abstract class DelegatingHandler : HttpMessageHandler
{}

案例3：重试

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);

案例4：ASP.NET Core中指定DelegatingHandler、HttpClientHandler

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddHttpClient("client1", client => { client.BaseAddress = new System.Uri("https://www.baidu.com"); })
                .AddHttpMessageHandler(() => new RetryHandler())//设置DelegatingHandler
                .ConfigurePrimaryHttpMessageHandler(() => new System.Net.Http.HttpClientHandler()
                {//设置HttpClientHandler，也就是InnerHandler
                    UseCookies = false
                });
        }

HttpResponseMessage：

HttpClient请求的返回类型，内部包含Headlers、Content、StatusCode、IsSuccessStatusCode等属性

其中Content属性是HttpContent类型，可转成对应类型，获取Content数据

性能隐患：

HttpClient有一个性能隐患，当要发送大量的http请求，如果频繁的创建HttpClient对象，会频繁建立TCP连接，连接不够用时会出现timeout，所以应该避免频繁创建HttpClient对象，如下使用静态变量+长连接。

private static readonly HttpClient _httpClient;

        static ApiLogger()
        {
            _httpClient = new HttpClient();
            _httpClient.Timeout = new TimeSpan(0, 0, 10);
            _httpClient.DefaultRequestHeaders.Connection.Add("keep-alive");
        }

上面这种方式有一个缺点，会缓存Dns，所以.net core中应该使用HttpClientFactory创建HttpClient

HttpClient扩展方法：

public static class HttpClientEx 
    {
        //GZIP压缩
        //var handler = new HttpClientHandler() { AutomaticDecompression = DecompressionMethods.GZip }
        //HttpClient client = new HttpClient(handler);

        /// <summary>
        /// http://www.it165.net/pro/html/201306/6052.html
        /// GET获取HTTP信息
        /// </summary>
        /// <param name="url"></param>
        /// <returns></returns>
        public static string Get(this HttpClient client,string url)
        {
            return client.GetStringAsync(url).Result;
        }
        /// <summary>
        /// POST空的HTTP信息
        /// http://www.cnblogs.com/xishuai/p/aspnet_mvc_web_api_httpclient_json_frombody_post.html
        /// </summary>
        /// <param name="url"></param>
        /// <returns></returns>
        public static string Post(this HttpClient client,string url)
        {
            string responseText = null;
            using (StringContent content = new StringContent(string.Empty))
            {
                content.Headers.ContentType = new MediaTypeHeaderValue("application/x-www-form-urlencoded");
                using (HttpResponseMessage responseMessage = client.PostAsync(url, content).Result)
                {
                    responseMessage.EnsureSuccessStatusCode();//确保请求成功
                    responseText = responseMessage.Content.ReadAsStringAsync().Result;

                }
            }
            return responseText;
        }
        /// <summary>
        /// http://www.haogongju.net/art/1642652
        /// POST方式发送信息(x-www-form-urlencoded字典格式)
        /// </summary>
        /// <param name="url"></param>
        /// <param name="dict">x-www-form-urlencoded字典格式</param>
        /// <param name="isException">false表示抛出异常，true表示返回异常错误信息</param>
        /// <returns></returns>
        public static string Post(this HttpClient client,string url, Dictionary<string, string> dict)
        {
            string responseText = null;
            using (FormUrlEncodedContent content = new FormUrlEncodedContent(dict))
            {
                //content.Headers.ContentType = new MediaTypeHeaderValue("application/x-www-form-urlencoded");
                using (HttpResponseMessage responseMessage = client.PostAsync(url, content).Result)
                {
                    responseMessage.EnsureSuccessStatusCode();
                    responseText = responseMessage.Content.ReadAsStringAsync().Result;

                }
            }
            return responseText;
        }
        /// <summary>
        /// http://www.cnblogs.com/xishuai/p/aspnet_mvc_web_api_httpclient_json_frombody_post.html
        /// POST方式发送信息(raw文本格式)
        /// </summary>
        /// <param name="url"></param>
        /// <param name="raw">raw文本格式</param>
        /// <param name="isException">false表示抛出异常，true表示返回异常错误信息</param>
        /// <returns></returns>
        public static string Post(this HttpClient client,string url, string raw)
        {
            string responseText = null;
            using (StringContent content = new StringContent(raw))
            {
                content.Headers.ContentType = new MediaTypeHeaderValue("application/json");
                using (HttpResponseMessage responseMessage = client.PostAsync(url, content).Result)
                {
                    responseMessage.EnsureSuccessStatusCode();
                    responseText = responseMessage.Content.ReadAsStringAsync().Result;

                }
            }
            return responseText;
        }

        

    }

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，看下一篇

参考：

https://www.cnblogs.com/Herzog3/p/6128822.html

https://www.cnblogs.com/Leo_wl/p/3439512.html

https://docs.microsoft.com/en-us/aspnet/web-api/overview/advanced/http-message-handlers

https://blog.csdn.net/weixin_30236595/article/details/101066107

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