12.5.1 Using the GET Method

Using the GET method is pretty easy. All we have to do is create a URL pointing to a CGI program and use a simple convention to tack on the encoded name=value pairs that make up our data. For example, the following code snippet opens a URL to a CGI program called login.cgi on the server myhost and passes it two name=value pairs. It also prints whatever text the CGI sends back:

    URL url = new URL(
        "http://myhost/cgi-bin/login.cgi?Name=Pat&Password=foobar");

    BufferedReader bin = new BufferedReader (
        new InputStreamReader( url.openStream() ));

    String line;
    while ( (line = bin.readLine()) != null )  
        System.out.println( line );

Finally, it's important to point out that we have skipped a step here. This example works because our name=value pairs happen to be simple text. If any "non-printable" or special characters (including "?" or "&") are in the data itself, they have to be encoded first. The java.net.URLEncoder class provides a utility for encoding the data. We'll show how to use it in the next example.

12.5.2 Using the POST Method

Let's create an applet that acts like a simple form. It presents two text fields, Name and Password, and a Post button that sends the data to a specified URL:

import java.net.*;
import java.io.*;
import java.awt.*;
import java.awt.event.*;

public class Post extends java.applet.Applet implements ActionListener {
    TextField nameField, passwordField;

    GridBagConstraints constraints = new GridBagConstraints();
    void addGB( Component component, int x, int y  ) {
        constraints.gridx = x;  constraints.gridy = y;
        add ( component, constraints );
    }
    public void init() {
        Button postButton = new Button("Post");
        postButton.addActionListener( this );
        setLayout( new GridBagLayout() );
        addGB( new Label("Name:"),              0,0 );
        addGB( nameField = new TextField(20),   1,0 );
        addGB( new Label("Password:"),          0,1 );
        addGB( passwordField = new TextField(20),1,1 );
        Panel p = new Panel();
        p.add ( postButton );
        constraints.gridwidth = 2;
        addGB( p, 0,2 );
    }
    public void actionPerformed(ActionEvent e) {
        postData();
    }

    private void postData() {
        StringBuffer sb = new StringBuffer();
        sb.append( URLEncoder.encode("Name") + "=" );
        sb.append( URLEncoder.encode(nameField.getText()) );
        sb.append( "&" + URLEncoder.encode("Password") + "=" );
        sb.append( URLEncoder.encode(passwordField.getText()) );
        String formData = sb.toString();
        try {
            URL url = new URL( getParameter("postURL") );
            HttpURLConnection urlcon = (HttpURLConnection)url.openConnection();
            urlcon.setRequestMethod("POST");
            urlcon.setRequestProperty("Content-type", 
                "application/x-www-form-urlencoded");
            urlcon.setDoOutput(true);
            urlcon.setDoInput(true);
            PrintWriter pout = new PrintWriter( new OutputStreamWriter(
                urlcon.getOutputStream(), "8859_1"), true );
            pout.print( formData );
            pout.flush();

            // read results...
            if ( urlcon.getResponseCode() != HttpURLConnection.HTTP_OK ) {
                System.out.println("Bad post...");
                return;
            }
            InputStream in = urlcon.getInputStream();
            // ...

        } catch (MalformedURLException e) {
            System.out.println("Bad postURL");      
        } catch (IOException e2) {
            System.out.println("I/O error: "+e2);
        }
    }
}

<param name="postURL" value="http://myhost/cgi-bin/post-login.cgi" >

Next we set up the connection to the CGI program. In our previous example, we didn't have to do anything special to send the data, because the request was made by the Web browser for us. Here, we have to carry some of the weight of talking to the remote Web server. Fortunately, the HttpURLConnection object does most of the work for us; we just have to tell it that we want to do a POST to the URL and the type of data we are sending. We ask for the URLConnection object using the URL's openConnection() method. We know that we are using HTTP, so we should be able to cast it to an HttpURLConnection type, which has the support we need.

Next we use setRequestMethod() to tell the connection we want to POST. We also use setRequestProperty() to set the "Content-Type" field of our HTTP request to the appropriate type--in this case, the proper MIME type for encoded form data. (This helps the CGI sort out what we're sending.) Finally, we use the setDoOutput() and setDoInput() methods to tell the connection that we want to send and receive stream data. We get an output stream from the connection with getOutputStream() and create a PrintWriter so we can easily write our encoded data.

After we post the data, we call getResponseCode() to check the HTTP response code from the server to see if it indicates the POST was successful. Other response codes (defined as constants in HttpURLConnection) indicate various failures. At the end, we indicate where we could read back the response. I haven't implemented anything; you can add your own code if you want.

Although form-encoded data is the most common, other types of communications are possible. We could have used the input and output streams to exchange arbitrary data with the CGI.

12.5.3 What About HTTPS?

HTTPS is the standard HTTP protocol run over SSL (Secure Socket Layer) sockets, which use public-key encryption techniques to encrypt the data sent. Most browsers currently come with built-in support for HTTPS (or raw SSL sockets). Therefore, if the server supports HTTPS, you can use the browser to send and receive secure data simply by specifying the https: protocol in your URLs.