提交表单数据

在向Web服务器发送信息时，通常有两个命令会被用到：GET和POST。

在使用GET命令时，只需将参数附在URL的结尾处即可。格式如下

http://host/script?parameters

其中每个参数都具有“名字=值”的形式，而且这些参数之间用&字符分隔开。参数的值遵循以下规则。使用URL编码模式进行编码

• 保留字符A-Z、a-z、1-9以及. - * _
• 用+字符替换所有的空格。
• 将其他所有字符编码为UTF-8，并将每个字节都编码为%后面紧跟一个两位的十六进制数字。例如，若要发送街道名S. Main， 可以使用S%2e+Main，因为十六进制数2e是“.”的ASCII码值

使用POST命令时，并不需要在URL中添加任何参数，而是从URLConnection中获取输出流并将名-值对写入该流中。当然，仍然需要对这些值进行编码，并用&字符将它们隔开。过程如下

URL url= new URL("http://host/script");
URLConnection connection=url.openConnection();
connection.setDoOutput("true");
//设置可输出
PrintWriter out = new PrintWriter(connection.getOutputStream());
//得到输出流
out.print(name1+"="+URLEncoder.encode(value1,"UTF-8")+"&");
out.print(name2+"="+URLEncoder.encode(value2,"UTF-8"));
//发送数据
out.close();
//关闭输出流

最后，调用getInputStream方法读取服务器的响应，在读取响应的过程中有个问题，若脚本运行出现错误，那么connection.getInputStream()时就会抛出一个FileNotFoundException异常，但是此时服务器依然会向浏览器发送一个错误页面。为了捕获该异常，可以将URLConnection对象转型为HTTPURLConnection类并调用它的getErrorStream方法

InputStream err = ((HttpURLConnection)connection).getErrorStream()

URLConnection对象首先向服务器发送了一个请求头，当提交表单数据时，请求头必须包含：

Content-Type: application/x-www-form-urlencoded

而POST的请求头还必须包括内容的长度

Content-Length: 124

请求头必须以空白行结尾，紧跟其后的才是数据部分

示例程序如下

import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.net.*;
import java.util.*;
import javax.swing.*;

/**
 * This program demonstrates how to use the URLConnection class for a POST request.
 * @version 1.20 2007-06-25
 * @author Cay Horstmann
 */
public class PostTest
{
   public static void main(String[] args)
   {
      EventQueue.invokeLater(new Runnable()
         {
            public void run()
            {
               JFrame frame = new PostTestFrame();
               frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
               frame.setVisible(true);
            }
         });
   }
}

class PostTestFrame extends JFrame
{
   /**
    * Makes a POST request and returns the server response.
    * @param urlString the URL to post to
    * @param nameValuePairs a map of name/value pairs to supply in the request.
    * @return the server reply (either from the input stream or the error stream)
    */
   public static String doPost(String urlString, Map<String, String> nameValuePairs)
         throws IOException
   {
      URL url = new URL(urlString);
      URLConnection connection = url.openConnection();
      connection.setDoOutput(true);

      PrintWriter out = new PrintWriter(connection.getOutputStream());

      boolean first = true;
      for (Map.Entry<String, String> pair : nameValuePairs.entrySet())
      {
         if (first) first = false;
         else out.print('&');
         String name = pair.getKey();
         String value = pair.getValue();
         out.print(name);
         out.print('=');
         out.print(URLEncoder.encode(value, "UTF-8"));
      }

      out.close();

      Scanner in;
      StringBuilder response = new StringBuilder();
      try
      {
         in = new Scanner(connection.getInputStream());
      }
      catch (IOException e)
      {
         if (!(connection instanceof HttpURLConnection)) throw e;
         InputStream err = ((HttpURLConnection) connection).getErrorStream();
         if (err == null) throw e;
         in = new Scanner(err);
      }

      while (in.hasNextLine())
      {
         response.append(in.nextLine());
         response.append("\n");
      }

      in.close();
      return response.toString();
   }

   public PostTestFrame()
   {
      setTitle("PostTest");

      northPanel = new JPanel();
      add(northPanel, BorderLayout.NORTH);
      northPanel.setLayout(new GridLayout(0, 2));
      northPanel.add(new JLabel("Host: ", SwingConstants.TRAILING));
      final JTextField hostField = new JTextField();
      northPanel.add(hostField);
      northPanel.add(new JLabel("Action: ", SwingConstants.TRAILING));
      final JTextField actionField = new JTextField();
      northPanel.add(actionField);
      for (int i = 1; i <= 8; i++)
         northPanel.add(new JTextField());

      final JTextArea result = new JTextArea(20, 40);
      add(new JScrollPane(result));

      JPanel southPanel = new JPanel();
      add(southPanel, BorderLayout.SOUTH);
      JButton addButton = new JButton("More");
      southPanel.add(addButton);
      addButton.addActionListener(new ActionListener()
         {
            public void actionPerformed(ActionEvent event)
            {
               northPanel.add(new JTextField());
               northPanel.add(new JTextField());
               pack();
            }
         });

      JButton getButton = new JButton("Get");
      southPanel.add(getButton);
      getButton.addActionListener(new ActionListener()
         {
            public void actionPerformed(ActionEvent event)
            {
               result.setText("");
               final Map<String, String> post = new HashMap<String, String>();
               for (int i = 4; i < northPanel.getComponentCount(); i += 2)
               {
                  String name = ((JTextField) northPanel.getComponent(i)).getText();
                  if (name.length() > 0)
                  {
                     String value = ((JTextField) northPanel.getComponent(i + 1)).getText();
                     post.put(name, value);
                  }
               }
               new SwingWorker<Void, Void>()
                  {
                     protected Void doInBackground() throws Exception
                     {
                        try
                        {
                           String urlString = hostField.getText() + "/" + actionField.getText();
                           result.setText(doPost(urlString, post));
                        }
                        catch (IOException e)
                        {
                           result.setText("" + e);
                        }
                        return null;
                     }
                  }.execute();
            }
         });

      pack();
   }

   private JPanel northPanel;
}