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C#中HttpWebRequest、WebClient、HttpClient的使用详解

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HttpWebRequest:

命名空间: System.Net,这是.NET创建者最初开发用于使用HTTP请求的标准类。使用HttpWebRequest可以让开发者控制请求/响应流程的各个方面,如 timeouts, cookies, headers, protocols。另一个好处是HttpWebRequest类不会阻塞UI线程。例如,当您从响应很慢的API服务器下载大文件时,您的应用程序的UI不会停止响应。HttpWebRequest通常和WebResponse一起使用,一个发送请求,一个获取数据。HttpWebRquest更为底层一些,能够对整个访问过程有个直观的认识,但同时也更加复杂一些。

     //POST方法
    public static string HttpPost(string Url, string postDataStr)
    {
      HttpWebRequest request = (HttpWebRequest)WebRequest.Create(Url);
      request.Method = "POST";
      request.ContentType = "application/x-www-form-urlencoded";
      Encoding encoding = Encoding.UTF8;
      byte[] postData = encoding.GetBytes(postDataStr);
      request.ContentLength = postData.Length;
      Stream myRequestStream = request.GetRequestStream();
      myRequestStream.Write(postData, 0, postData.Length);
      myRequestStream.Close();
      HttpWebResponse response = (HttpWebResponse)request.GetResponse();
      Stream myResponseStream = response.GetResponseStream();
      StreamReader myStreamReader = new StreamReader(myResponseStream, encoding);
      string retString = myStreamReader.ReadToEnd();
      myStreamReader.Close();
      myResponseStream.Close();

      return retString;
    }
    //GET方法
    public static string HttpGet(string Url, string postDataStr)
    {
      HttpWebRequest request = (HttpWebRequest)WebRequest.Create(Url + (postDataStr == "" ? "" : "?") + postDataStr);
      request.Method = "GET";
      request.ContentType = "text/html;charset=UTF-8";
      HttpWebResponse response = (HttpWebResponse)request.GetResponse();
      Stream myResponseStream = response.GetResponseStream();
      StreamReader myStreamReader = new StreamReader(myResponseStream, Encoding.GetEncoding("utf-8"));
      string retString = myStreamReader.ReadToEnd();
      myStreamReader.Close();
      myResponseStream.Close();
      return retString;
    }

WebClient:

命名空间System.Net,WebClient是一种更高级别的抽象,是HttpWebRequest为了简化最常见任务而创建的,使用过程中你会发现他缺少基本的header,timeoust的设置,不过这些可以通过继承httpwebrequest来实现。相对来说,WebClient比WebRequest更加简单,它相当于封装了request和response方法,不过需要说明的是,Webclient和WebRequest继承的是不同类,两者在继承上没有任何关系。使用WebClient可能比HttpWebRequest直接使用更慢(大约几毫秒),但却更为简单,减少了很多细节,代码量也比较少。

public class WebClientHelper
  {
    public static string DownloadString(string url)
    {
      WebClient wc = new WebClient();
      //wc.BaseAddress = url;  //设置根目录
      wc.Encoding = Encoding.UTF8;  //设置按照何种编码访问,如果不加此行,获取到的字符串中文将是乱码
      string str = wc.DownloadString(url);
      return str;
    }
    public static string DownloadStreamString(string url)
    {
      WebClient wc = new WebClient();
      wc.Headers.Add("User-Agent", "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/76.0.3809.132 Safari/537.36");
      Stream objStream = wc.OpenRead(url);
      StreamReader _read = new StreamReader(objStream, Encoding.UTF8);  //新建一个读取流,用指定的编码读取,此处是utf-8
      string str = _read.ReadToEnd();
      objStream.Close();
      _read.Close();
      return str;
    }

    public static void DownloadFile(string url, string filename)
    {
      WebClient wc = new WebClient();
      wc.DownloadFile(url, filename);   //下载文件
    }

    public static void DownloadData(string url, string filename)
    {
      WebClient wc = new WebClient();
      byte [] bytes = wc.DownloadData(url);  //下载到字节数组
      FileStream fs = new FileStream(filename, FileMode.Create);
      fs.Write(bytes, 0, bytes.Length); 
      fs.Flush();
      fs.Close();
    }

    public static void DownloadFileAsync(string url, string filename)
    {
      WebClient wc = new WebClient();
      wc.DownloadFileCompleted += DownCompletedEventHandler;
      wc.DownloadFileAsync(new Uri(url), filename);
      Console.WriteLine("下载中。。。");
    }
    private static void DownCompletedEventHandler(object sender, AsyncCompletedEventArgs e)
    {
      Console.WriteLine(sender.ToString());  //触发事件的对象
      Console.WriteLine(e.UserState);
      Console.WriteLine(e.Cancelled);
      Console.WriteLine("异步下载完成!");
    }

    public static void DownloadFileAsync2(string url, string filename)
    {
      WebClient wc = new WebClient();
      wc.DownloadFileCompleted += (sender, e) =>
      {
        Console.WriteLine("下载完成!");
        Console.WriteLine(sender.ToString());
        Console.WriteLine(e.UserState);
        Console.WriteLine(e.Cancelled);
      };
      wc.DownloadFileAsync(new Uri(url), filename);
      Console.WriteLine("下载中。。。");
    }
  }

HttpClient:

HttpClient是.NET4.5引入的一个HTTP客户端库,其命名空间为 System.Net.Http ,.NET 4.5之前我们可能使用WebClient和HttpWebRequest来达到相同目的。HttpClient利用了最新的面向任务模式,使得处理异步请求非常容易。它适合用于多次请求操作,一般设置好默认头部后,可以进行重复多次的请求,基本上用一个实例可以提交任何的HTTP请求。HttpClient有预热机制,第一次进行访问时比较慢,所以不应该用到HttpClient就new一个出来,应该使用单例或其他方式获取HttpClient的实例

单例模式:

单例模式(Singleton Pattern)这种类型的设计模式属于创建型模式,它提供了一种创建对象的最佳方式。

这种模式涉及到一个单一的类,该类负责创建自己的对象,同时确保只有单个对象被创建。这个类提供了一种访问其唯一的对象的方式,可以直接访问,不需要实例化该类的对象。

单例创建步骤:1、定义静态私有对象;2、定义私有构造函数;3、提供公共获取对象方法;

单例模式一般分为两种实现模式:懒汉模式、饿汉模式(以下为Java代码实现)

懒汉模式:  默认不会实例化,什么时候用什么时候new

public class Singleton { 
  private static Singleton instance = null; 
  private Singleton (){} 
 
  public static Singleton getInstance() { 
  if (instance == null) { 
    instance = new Singleton(); 
  } 
  return instance; 
  } 
}

这种方式是最基本的实现方式,这种实现最大的问题就是不支持多线程。因为没有加锁 synchronized,所以严格意义上它并不算单例模式。
这种方式 lazy loading 很明显,不要求线程安全,在多线程不能正常工作。

饿汉模式:  类初始化时,会立即加载该对象,线程天生安全,调用效率高

public class Singleton { 
  private static Singleton instance = new Singleton(); 
  private Singleton (){} 
  public static Singleton getInstance() { 
  return instance; 
  } 
}

双检锁/双重校验锁(DCL,即 double-checked locking):这种方式采用双锁机制,安全且在多线程情况下能保持高性能

public class Singleton { 
  private volatile static Singleton singleton; 
  private Singleton (){} 
  public static Singleton getSingleton() { 
  if (singleton == null) { 
    synchronized (Singleton.class) { 
    if (singleton == null) { 
      singleton = new Singleton(); 
    } 
    } 
  } 
  return singleton; 
  } 
}

HttpClient:

public class HttpClientHelper
  {
    private static readonly object LockObj = new object();
    private static HttpClient client = null;
    public HttpClientHelper() {
      GetInstance();
    }
    public static HttpClient GetInstance()
    {

      if (client == null)
      {
        lock (LockObj)
        {
          if (client == null)
          {
            client = new HttpClient();
          }
        }
      }
      return client;
    }
    public async Task<string> PostAsync(string url, string strJson)//post异步请求方法
    {
      try
      {
        HttpContent content = new StringContent(strJson);
        content.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("application/json");
        //由HttpClient发出异步Post请求
        HttpResponseMessage res = await client.PostAsync(url, content);
        if (res.StatusCode == System.Net.HttpStatusCode.OK)
        {
          string str = res.Content.ReadAsStringAsync().Result;
          return str;
        }
        else
          return null;
      }
      catch (Exception ex)
      {
        return null;
      }
    }

    public string Post(string url, string strJson)//post同步请求方法
    {
      try
      {
        HttpContent content = new StringContent(strJson);
        content.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("application/json");
        //client.DefaultRequestHeaders.Connection.Add("keep-alive");
        //由HttpClient发出Post请求
        Task<HttpResponseMessage> res = client.PostAsync(url, content);
        if (res.Result.StatusCode == System.Net.HttpStatusCode.OK)
        {
          string str = res.Result.Content.ReadAsStringAsync().Result;
          return str;
        }
        else
          return null;
      }
      catch (Exception ex)
      {
        return null;
      }
    }

    public string Get(string url)
    {
      try
      {
        var responseString = client.GetStringAsync(url);
        return responseString.Result;
      }
      catch (Exception ex)
      {
        return null;
      }
    }

  }

HttpClient有预热机制,第一次请求比较慢;可以通过初始化前发送一次head请求解决:

_httpClient = new HttpClient() { BaseAddress = new Uri(BASE_ADDRESS) };

    //帮HttpClient热身
    _httpClient.SendAsync(new HttpRequestMessage {
        Method = new HttpMethod("HEAD"), 
        RequestUri = new Uri(BASE_ADDRESS + "/") })
      .Result.EnsureSuccessStatusCode();

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