Use the method getCodeBase( )
to retrieve a
URL for the applet host, and call the
URL’s getHost( ). Use this to construct a
client socket.
For very good security reasons, applets are not permitted network access to servers on hosts other than the one from which the applet was loaded.
To reach a server on the download host, call the applet method
getCodeBase( ), which yields a URL for the applet
host. Call this URL’s getHost( )
method to get the hostname. Finally, use the hostname to open a
client socket (see Section 15.2). For example:
URL u = getCodeBase( ); String host = u.getHost( ); Socket s = new Socket(host , MY_SERVER_PORT);
Of course, in real code you wouldn’t create all those temporary variables:
Socket s = new Socket(getCodeBase().getHost( ), MY_SERVER_PORT);
And, of course, you will need error handling. Example 17-2 shows an applet that constructs a sort of login dialog and passes the results to a socket-based server on the applet host, using exactly this technique. Figure 17-1 shows the screen display.
Example 17-2. SocketApplet.java
/** Initialize the GUI nicely. */
public void init( ) {
Label aLabel;
setLayout(new GridBagLayout( ));
int LOGO_COL = 1;
int LABEL_COL = 2;
int TEXT_COL = 3;
int BUTTON_COL = 1;
GridBagConstraints gbc = new GridBagConstraints( );
gbc.weightx = 100.0; gbc.weighty = 100.0;
gbc.gridx = LABEL_COL; gbc.gridy = 0;
gbc.anchor = GridBagConstraints.EAST;
add(aLabel = new Label("Name:", Label.CENTER), gbc);
gbc.anchor = GridBagConstraints.CENTER;
gbc.gridx = TEXT_COL; gbc.gridy = 0;
add(nameTF=new TextField(10), gbc);
gbc.gridx = LABEL_COL; gbc.gridy = 1;
gbc.anchor = GridBagConstraints.EAST;
add(aLabel = new Label("Password:", Label.CENTER), gbc);
gbc.anchor = GridBagConstraints.CENTER;
gbc.gridx = TEXT_COL; gbc.gridy = 1;
add(passTF=new TextField(10), gbc);
passTF.setEchoChar(*);
gbc.gridx = LABEL_COL; gbc.gridy = 2;
gbc.anchor = GridBagConstraints.EAST;
add(aLabel = new Label("Domain:", Label.CENTER), gbc);
gbc.anchor = GridBagConstraints.CENTER;
gbc.gridx = TEXT_COL; gbc.gridy = 2;
add(domainTF=new TextField(10), gbc);
sendButton = new Button("Send data");
gbc.gridx = BUTTON_COL; gbc.gridy = 3;
gbc.gridwidth = 3;
add(sendButton, gbc);
whence = getCodeBase( );
// Now the action begins...
sendButton.addActionListener(new ActionListener( ) {
public void actionPerformed(ActionEvent evt) {
String name = nameTF.getText( );
if (name.length( ) == 0) {
showStatus("Name required");
return;
}
String domain = domainTF.getText( );
if (domain.length( ) == 0) {
showStatus("Domain required");
return;
}
showStatus("Connecting to host " + whence.getHost( ) +
" as " + nameTF.getText( ));
try {
Socket s = new Socket(getCodeBase().getHost( ),
SocketServer.PORT);
PrintWriter pf = new PrintWriter(s.getOutputStream( ), true);
// send login name
pf.println(nameTF.getText( ));
// passwd
pf.println(passTF.getText( ));
// and domain
pf.println(domainTF.getText( ));
BufferedReader is = new BufferedReader(
new InputStreamReader(s.getInputStream( )));
String response = is.readLine( );
showStatus(response);
} catch (IOException e) {
showStatus("ERROR: " + e.getMessage( ));
}
}
});
}