Frequently Asked Questions

 The Java Filter
 How It Works

Can I force my clients/employees/students to use the Java Filter?

No. Everyone is responsible for the setup of their browsers. If people choose to turn on Java support on their browsers, they are exposed to potential security risks. If people choose to tamper with their browsers and turn off the Java sandbox, they open the door for all kinds of attacks. Likewise, people can choose not to install the Java Filter. You as a content provider/site administrator can do little against this.

Can I find out whether my applet is running on a Java Filter-enabled browser?

Yes. Here is a piece a code that shows how to do it:

public boolean testJavaFilter() {
 try {
  Class c = Class.forName(“edu.princeton.cs.sip.urlfilter.URLFilter”);
  return true;
 } catch(Exception e) {
  return false;

How do I specify that all applets from my corporate Web server should be allowed, but no applet from the outside world?

Look for the file _urlfilter.dat. (Use the Find option in the Start menu to locate it. Its exact location depends on the setup of your system.) Change it to say something like this:


With this setup, the user will never be asked whether or not to download a specific applet. All decisions are based on the entries in the database file. Please read the documentation on how to use the Java Filter.

The installation fails!

If the installation stops with an error, make sure that
- your browser is not running,
- you have write access to the Java class library of your browser,
and try again.

The installation seems to succeed, but it still doesn’t work.

If the installation program can’t find any browser, it will do nothing, but still report that the installation “succeeded” (this can be considered a bug :-). One reason why it couldn’t find your browser might be that you have other versions of the browser installed on the same system. For example, if you have installed the preview releases of Netscape Communicator, the installation program might not be able to find Netscape Navigator.

Workaround: Reinstall Navigator 3.01 and then reinstall the Java Filter.

Will there be a version for the Macintosh?

We don’t have the resources to develop a version for the Macintosh right now. Please contact us if you want to donate a fast Macintosh with C++ and Java development environments :-)