Hardcoding passwords in source code as Example 19.4 does is, to say the least, a very bad idea. If a password is required, you should ask the user for it at runtime. Furthermore, when the user types the password, it should not be displayed on the screen. Ideally, it should not even be transmitted in clear text across the network, though in fact many current POP clients and servers do exactly that. (IMAP tends to be a little more secure.)
When you open a connection to a message store, the JavaMail API
allows you to provide a javax.mail.Authenticator
object that it can use to get the username and password.
Authenticator
is an abstract class:
public abstract class Authenticator extends Object
When the provider needs to know a username or password, it calls back
to the getPasswordAuthentication( )
method in a user-defined
subclass of Authenticator. This returns a
PasswordAuthentication
object containing this information:
protected PasswordAuthentication getPasswordAuthentication( )
Note
These two classes are almost exactly the same as the
java.net.Authenticator and
java.net.PasswordAuthentication classes discussed
in Chapter 7. However, those classes are available
only in Java 1.2 and later. To make the JavaMail API work in Java
1.1, Sun had to duplicate their functionality in the
javax.mail package. Sun could have included
java.net.Authenticator and
java.net.PasswordAuthentication in
mail.jar, but that would have meant that the
JavaMail API could not be certified as 100% Pure Java. However,
everything you learned about
java.net.Authenticator and
java.netPasswordAuthentication in Chapter 7 is true of
javax.mail.Authenticator and
javax.mailPasswordAuthentication in this chapter.
The only thing you have to watch out for is that if you import both
java.net.* and javax.mail.* in
a class, then your source code will have to use fully qualified names
like java.net.Authenticator instead of short names
like Authenticator.
To add runtime password authentication to your programs, you subclass
Authenticator and override
getPasswordAuthentication( ) with a method that
knows how to securely ask the user for a password. One useful tool
for this process is the JPasswordField component
from Swing. Example 19.5 demonstrates a Swing-based
Authenticator subclass that brings up a dialog to
ask the user for his username and
password.
Example 19-5. A GUI Authenticator
import javax.mail.*;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class MailAuthenticator extends Authenticator {
private JDialog passwordDialog = new JDialog(new JFrame( ), true);
private JLabel mainLabel = new JLabel(
"Please enter your user name and password: ");
private JLabel userLabel = new JLabel("User name: ");
private JLabel passwordLabel = new JLabel("Password: ");
private JTextField usernameField = new JTextField(20);
private JPasswordField passwordField = new JPasswordField(20);
private JButton okButton = new JButton("OK");
public MailAuthenticator( ) {
this("");
}
public MailAuthenticator(String username) {
Container pane = passwordDialog.getContentPane( );
pane.setLayout(new GridLayout(4, 1));
pane.add(mainLabel);
JPanel p2 = new JPanel( );
p2.add(userLabel);
p2.add(usernameField);
usernameField.setText(username);
pane.add(p2);
JPanel p3 = new JPanel( );
p3.add(passwordLabel);
p3.add(passwordField);
pane.add(p3);
JPanel p4 = new JPanel( );
p4.add(okButton);
pane.add(p4);
passwordDialog.pack( );
ActionListener al = new HideDialog( );
okButton.addActionListener(al);
usernameField.addActionListener(al);
passwordField.addActionListener(al);
}
class HideDialog implements ActionListener {
public void actionPerformed(ActionEvent e) {
passwordDialog.hide( );
}
}
public PasswordAuthentication getPasswordAuthentication( ) {
passwordDialog.show( );
// getPassword( ) returns an array of chars for security reasons.
// We need to convert that to a String for
// the PasswordAuthentication( ) constructor.
String password = new String(passwordField.getPassword( ));
String username = usernameField.getText( );
// Erase the password in case this is used again.
// The provider should cache the password if necessary.
passwordField.setText("");
return new PasswordAuthentication(username, password);
}
}Most of this code is just for handling the GUI. Figure 19.4 shows the rather simple dialog box this produces.
Interestingly, JPasswordField takes more pains to
be secure than does PasswordAuthentication.
JPasswordField stores passwords as an array of
chars so that when you’re done with the password, you can
overwrite it with nulls. This means that the password exists in
memory for less time and is less likely to be accidentally swapped
out to disk and left there in a virtual memory system. However,
PasswordAuthentication stores passwords as
strings, which are immutable and therefore may be unintentionally
stored on the disk.
Modifying the POP client to support this style of authentication is
straightforward, as Example 19.6 demonstrates. We
replace the hardcoded username and password with nulls and pass an
instance of MailAuthenticator as the second
argument to connect( ). The only other change is
that we call System.exit( ) at the end of the
main( ) method, since the program will no longer
exit when the main( ) method returns once the AWT
thread has been started.
Example 19-6. A POP Client That Asks the User for the Password as Necessary
import javax.mail.*;
import javax.mail.internet.*;
import java.util.*;
import java.io.*;
public class SecurePOP3Client {
public static void main(String[] args) {
Properties props = new Properties( );
String host = "utopia.poly.edu";
String provider = "pop3";
try {
// Connect to the POP3 server
Session session = Session.getDefaultInstance(props,
new MailAuthenticator( ));
Store store = session.getStore(provider);
store.connect(host, null, null);
// Open the folder
Folder inbox = store.getFolder("INBOX");
if (inbox == null) {
System.out.println("No INBOX");
System.exit(1);
}
inbox.open(Folder.READ_ONLY);
// Get the messages from the server
Message[] messages = inbox.getMessages( );
for (int i = 0; i < messages.length; i++) {
System.out.println("------------ Message " + (i+1)
+ " ------------");
messages[i].writeTo(System.out);
}
// Close the connection
// but don't remove the messages from the server
inbox.close(false);
store.close( );
}
catch (Exception e) {
e.printStackTrace( );
}
// since we brought up a GUI returning from main( ) won't exit
System.exit(0);
}
}