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Java Tip 46: Use Java 1.2's Authenticator class
Learn to access password-protected URLs with Java 1.2
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When surfing the 'Net using your favorite browser you encounter a URL that requires authentication from a proxy or HTTP server and are presented with the all-too-familiar screen demanding your username and password:
Accessing a URL like http://www.lombard.com/cgi-bin/Quotes/quote from a browser is not a problem because you can provide a
username and password yourself. But from a Java program, a FileNotFoundException is thrown when you try to read from an InputStream associated with the URL.
With Java 1.0 or 1.1, you can avoid this by posting the appropriate authentication string at connection time, but only if
you know beforehand that the URL is protected. (Authorization: Basic username:password, where the basic authentication fields are encoded in base 64.) If you don't forsee that a document is protected, you won't
even be able to read the content. (See "Java Tip 47: URL authentication revisited" for the solution to accessing password-protected URLs with Java 1.1 applets and applications.) Thankfully, Java 1.2 adds
an Authenticator class to the java.net package that makes accessing password-protected URLs as easy as can be.
Now, in Java 1.2, you simply install an Authenticator, with Authenticator.setDefault(). Then, when authentication is necessary, the installed Authenticator's getPasswordAuthentication() method is called, and you return a PasswordAuthentication instance with the appropriate username and password. It's that easy.
The steps are shown below.
Step 1: Install Authenticator
Authenticator.setDefault (new MyAuthenticator ());
Step 2: Create Authenticator subclass
class MyAuthenticator extends Authenticator { protected PasswordAuthentication getPasswordAuthentication() {
return new PasswordAuthentication ("username", "password");
}
}
The following program shows off this behavior in its entirety, offering its own authentication prompt dialog.
import java.io.*;
import java.net.*;
import java.awt.*;
import java.awt.event.*;
public class URLPassword extends Frame {
private TextField tf = new TextField();
private TextArea ta = new TextArea();
public URLPassword() {
super ("URL Password");
// Install Authenticator
Authenticator.setDefault (new MyAuthenticator ());
// Setup screen
add (tf, BorderLayout.NORTH);
ta.setEditable(false);
add (ta, BorderLayout.CENTER);
tf.addActionListener (new ActionListener() {
public void actionPerformed (ActionEvent e) {
String s = tf.getText();
if (s.length() != 0)
ta.setText (fetchURL (s));
}
});
addWindowListener (new WindowAdapter() {
public void windowClosing (WindowEvent e) {
dispose();
System.exit(0);
}
});
}
private String fetchURL (String urlString) {
StringWriter sw = new StringWriter();
PrintWriter pw = new PrintWriter(sw);
try {
URL url = new URL (urlString);
InputStream content = (InputStream)url.getContent();
BufferedReader in =
new BufferedReader (new InputStreamReader (content));
String line;
while ((line = in.readLine()) != null) {
pw.println (line);
}
} catch (MalformedURLException e) {
pw.println ("Invalid URL");
} catch (IOException e) {
pw.println ("Error reading URL");
}
return sw.toString();
}
public static void main (String args[]) {
Frame f = new URLPassword();
f.setSize(300, 300);
f.setVisible (true);
}
class MyAuthenticator extends Authenticator {
protected PasswordAuthentication getPasswordAuthentication() {
final Dialog jd = new Dialog (URLPassword.this, "Enter password", true);
jd.setLayout (new GridLayout (0, 1));
Label jl = new Label (getRequestingPrompt());
jd.add (jl);
TextField username = new TextField();
username.setBackground (Color.lightGray);
jd.add (username);
TextField password = new TextField();
password.setEchoChar ('*');
password.setBackground (Color.lightGray);
jd.add (password);
Button jb = new Button ("OK");
jd.add (jb);
jb.addActionListener (new ActionListener() {
public void actionPerformed (ActionEvent e) {
jd.dispose();
}
});
jd.pack();
jd.setVisible(true);
return new PasswordAuthentication (username.getText(), password.getText());
}
}
}
A follow-up tip - From Luis Blanco Gomez
Line 85 of the code, which is the return argument of the MyAuthenticator class, displays an error:
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