使用JAVA实现http通信详解

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

实例化一个java.net.URL对象；
通过URL对象的openConnection()方法得到一个java.net.URLConnection;
通过URLConnection对象的getInputStream()方法获得输入流；
读取输入流；
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);
}

HttpClient是Apache Jakarta Common下的子项目，用来提供高效的、最新的、功能丰富的支持HTTP协议的客户端编程工具包，并且它支持HTTP协议最新的版本和建议。HttpClient已经应用在很多的项目中，比如Apache Jakarta上很著名的另外两个开源项目Cactus和HTMLUnit都使用了HttpClient。