http基础知识总结

1.HTTP协议（HyperText Transfer Protocol，超文本传输协议）是因特网上应用最为广泛的一种网络传输协议，所有的WWW文件都必须遵守这个标准。

HTTP是一个基于TCP/IP通信协议来传递数据（HTML 文件, 图片文件, 查询结果等）。

Http通信概述

Http通信主要有两种方式POST方式和GET方式。前者通过Http消息实体发送数据给服务器，安全性高，数据传输大小没有限制，后者通过URL的查询字符串传递给服务器参数，以明文显示在浏览器地址栏，保密性差，最多传输2048个字符。但是GET请求并不是一无是处——GET请求大多用于查询（读取资源），效率高。POST请求用于注册、登录等安全性较高且向数据库中写入数据的操作。

除了POST和GET，http通信还有其他方式！请参见http请求的方法

编码前的准备

在进行编码之前，我们先创建一个Servlet，该Servlet接收客户端的参数(name和age)，并响应客户端。

@WebServlet(urlPatterns={"/demo.do"})
public class DemoServlet extends HttpServlet {

    private static final long serialVersionUID = 1L;

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        request.setCharacterEncoding("utf-8");
        response.setContentType("text/html;charset=utf-8");
        String name = request.getParameter("name");
        String age = request.getParameter("age");
        PrintWriter pw = response.getWriter();
        pw.print("您使用GET方式请求该Servlet。<br />" + "name = " + name + ",age = " + age);
        pw.flush();
        pw.close();
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        request.setCharacterEncoding("utf-8");
        response.setContentType("text/html;charset=utf-8");
        String name = request.getParameter("name");
        String age = request.getParameter("age");
        PrintWriter pw = response.getWriter();
        pw.print("您使用POST方式请求该Servlet。<br />" + "name = " + name + ",age = " + age);
        pw.flush();
        pw.close();
    }

}

使用JDK实现http通信

使用URLConnection实现GET请求

1. 实例化一个java.net.URL对象；
2. 通过URL对象的openConnection()方法得到一个java.net.URLConnection;
3. 通过URLConnection对象的getInputStream()方法获得输入流；
4. 读取输入流；
5. 关闭资源。

public void get() throws Exception{

    URL url = new URL("http://127.0.0.1/http/demo.do?name=Jack&age=10");
    URLConnection urlConnection = url.openConnection();                                                    // 打开连接
    BufferedReader br = new BufferedReader(new InputStreamReader(urlConnection.getInputStream(),"utf-8")); // 获取输入流
    String line = null;
    StringBuilder sb = new StringBuilder();
    while ((line = br.readLine()) != null) {
        sb.append(line + "\n");
    }

    System.out.println(sb.toString());
}

使用HttpURLConnection实现POST请求

java.net.HttpURLConnection是java.net.URL的子类，提供了更多的关于http的操作（getXXX 和 setXXX方法）。该类中定义了一系列的HTTP状态码：

public void post() throws IOException{

    URL url = new URL("http://127.0.0.1/http/demo.do");
    HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();

    httpURLConnection.setDoInput(true);
    httpURLConnection.setDoOutput(true);        // 设置该连接是可以输出的
    httpURLConnection.setRequestMethod("POST"); // 设置请求方式
    httpURLConnection.setRequestProperty("charset", "utf-8");

    PrintWriter pw = new PrintWriter(new BufferedOutputStream(httpURLConnection.getOutputStream()));
    pw.write("name=welcome");                   // 向连接中输出数据（相当于发送数据给服务器）
    pw.write("&age=14");
    pw.flush();
    pw.close();

    BufferedReader br = new BufferedReader(new InputStreamReader(httpURLConnection.getInputStream(),"utf-8"));
    String line = null;
    StringBuilder sb = new StringBuilder();
    while ((line = br.readLine()) != null) {    // 读取数据
        sb.append(line + "\n");
    }

    System.out.println(sb.toString());
}

使用httpclient进行http通信

httpclient大大简化了JDK中http通信的实现。

maven依赖：

<dependency>
    <groupId>org.apache.httpcomponents</groupId> <artifactId>httpclient</artifactId> <version>4.3.6</version> </dependency> 

GET请求

public void httpclientGet() throws Exception{

    // 创建HttpClient对象
    HttpClient client = HttpClients.createDefault();

    // 创建GET请求（在构造器中传入URL字符串即可）
    HttpGet get = new HttpGet("http://127.0.0.1/http/demo.do?name=admin&age=40");

    // 调用HttpClient对象的execute方法获得响应
    HttpResponse response = client.execute(get);

    // 调用HttpResponse对象的getEntity方法得到响应实体
    HttpEntity httpEntity = response.getEntity();

    // 使用EntityUtils工具类得到响应的字符串表示
    String result = EntityUtils.toString(httpEntity,"utf-8");
    System.out.println(result);
}

POST请求

public void httpclientPost() throws Exception{

    // 创建HttpClient对象
    HttpClient client = HttpClients.createDefault();

    // 创建POST请求
    HttpPost post = new HttpPost("http://127.0.0.1/http/demo.do");

    // 创建一个List容器，用于存放基本键值对（基本键值对即：参数名-参数值）
    List<BasicNameValuePair> parameters = new ArrayList<>();
    parameters.add(new BasicNameValuePair("name", "张三"));
    parameters.add(new BasicNameValuePair("age", "25"));

    // 向POST请求中添加消息实体
    post.setEntity(new UrlEncodedFormEntity(parameters, "utf-8"));

    // 得到响应并转化成字符串
    HttpResponse response = client.execute(post);
    HttpEntity httpEntity = response.getEntity();
    String result = EntityUtils.toString(httpEntity,"utf-8");
    System.out.println(result);
}