Http学习之使用HttpURLConnection发送post和get请求（3）

    public static String sessionId = "";  
    public static void sendLoginRequest() throws IOException {
        URL loginUrl = new URL("http://xxx");
        HttpURLConnection connection = (HttpURLConnection) loginUrl.openConnection();
  
  
        // Output to the connection. Default is
        // false, set to true because post
        // method must write something to the
        // connection
        // 设置是否向connection输出，因为这个是post请求，参数要放在
        // http正文内，因此需要设为true
        connection.setDoOutput(true);
        // Read from the connection. Default is true.
        connection.setDoInput(true);
        // Set the post method. Default is GET
        connection.setRequestMethod("POST");
        // Post cannot use caches
        // Post 请求不能使用缓存
        connection.setUseCaches(false);
  
        // This method takes effects to
        // every instances of this class.
        // URLConnection.setFollowRedirects是static函数，作用于所有的URLConnection对象。
        // connection.setFollowRedirects(true);
       
        // This methods only
        // takes effacts to this
        // instance.
        // URLConnection.setInstanceFollowRedirects是成员函数，仅作用于当前函数
        connection.setInstanceFollowRedirects(false);
       
        // Set the content type to urlencoded,
        // because we will write
        // some URL-encoded content to the
        // connection. Settings above must be set before connect!
        // 配置本次连接的Content-type，配置为application/x-www-form-urlencoded的
        // 意思是正文是urlencoded编码过的form参数，下面我们可以看到我们对正文内容使用URLEncoder.encode
        // 进行编码
        connection.setRequestProperty("Content-Type",
               "application/x-www-form-urlencoded");
        // 连接，从postUrl.openConnection()至此的配置必须要在connect之前完成，
        // 要注意的是connection.getOutputStream会隐含的进行connect。
        connection.connect();
  
        DataOutputStream out = new DataOutputStream(connection
               .getOutputStream());
  
          // 要传的参数
        String content = URLEncoder.encode("username", "UTF-8") + "="
               + URLEncoder.encode("XXX", "UTF-8");
        content += "&" + URLEncoder.encode("password", "UTF-8") + "="
               + URLEncoder.encode("XXXX", "UTF-8");
  
        // DataOutputStream.writeBytes将字符串中的16位的unicode字符以8位的字符形式写道流里面
        out.writeBytes(content);
  
        out.flush();
        out.close(); // flush and close
  
           //Get Session ID
        String key = "";
        if (connection != null) {
            for (int i = 1; (key = connection.getHeaderFieldKey(i)) != null; i++) {
               if (key.equalsIgnoreCase("set-cookie")) {
                   sessionId = connection.getHeaderField(key);
                   sessionId = sessionId.substring(0, sessionId.indexOf(";"));
               }
            }
        }     
        connection.disconnect();
     }