Java Cookbook

Contacting a Server on the Applet Host

Problem

You want an applet to contact a socket-based server on the host from which it was loaded.

Solution

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.

Discussion

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; ...