Java Web Start Information

JWS is no longer supported by Java 11+ so the following information is for legacy support ONLY.

To run JWS apps in Java 11+, try using OpenWebStart.

9/1/22 UPDATE: The current OpenWebStart (v1.6.0) unfortunately does not work with Java 18 due to deprecated features in Java 18.    Please see the current Canvas site for the course for alternative techniques for running the demos. This issue may be fixed in later versions of OpenWebStart.

OpenWebStart Tips and Traps:

Important: The errors that one encounters may differ depending on the version of one's operating system.

Always keep your operating system updated to the latest version to ensure that any issue encountered hasn't been eliminated or at least clarified (e.g. better error message) in the latest version of the operating system.

• Error: Unable to read JNLP file or Server XXX is not on the whitelist
  • This error is almost always a permissions problem of some sort where the local system is preventing OpenWebStart from using the downloaded JNLP file because it considers such downloads to be dangerous.
  • Possible solution:
    • Add the server to OpenWebStart's "whitelist" of servers from which it accepts downloads from.
      • Refs.:
        • The Definitive Guide To OpenWebStart: Server Whitelist
        • Frequently Asked Questions on OpenWebStart: How can I define a server whitelist for OWS?
    • Procedure:
      • Find and edit OpenWebStart's deployement.properties configuration file.
        • Mac: Macintosh HD/Users/your_username/.config/icedtead-web
          • Note: The .config folder may not be visible when browsing using Finder.
        • Windows: C:\Users\your_username\.config\icedtea-web
          • Note: The .config folder may not be visible when browsing using Windows Explorer.
      • Add the following line or append to it if it already exists:
        • deployment.security.whitelist = server_name
          • Use * as a wildcard for multiple possibilities for the server's name, e.g. *.clear.rice.edu will cover clear.rice.edu and www.clear.rice.edu.
            • e.g. deployment.security.whitelist = *.clear.rice.edu
          • Separate multiple servers by commas.
    • WARNING: The server whitelist is RESET whenever OpenWebStart is updated! The above procedure to add a server to the whitelist must be repeated if OpenWebStart is updated or reinstalled!
• Error: "Application is not initialized" or "Cannot find main class" (look in error details to find this)
  • This can be cause by OpenWebStart running a JVM that is not compatible with the JAR file downloaded by the JNLP file.
  • Possible solution:
    • Add a compatible JVM to OpenWebStart's configuration:
      1. Run thee "OpenWebStart Settings" program that was installed along with OpenWebStart.
      2. Go to the "JVM Manager" tab.
      3. Click the "Add local..." button.
      4. All the locally installed Java JVM installations should appear and be available for use when running a JNLP file.
      5. Click "OK" to close the OpenWebStart Settings app.

Java Web Start ("JWS") is a technology to launch Java applications directly from a web page.   JWS does not use the same mechanism as Java applets and in such, is able to get around the lack of applet support in many current browsers,  

Available JWS-based demos:

• Ballworld
• Lists
• Visitors:
  • Standard Visitors
  • Extended Visitors
    • ABC Music Player
    • Traditional Rotations-based 2-3-4 Tree Algorithms Demo
    • Self-balancing Trees Using Extended Visitors Demo
• RMI
  • Compute Engine

Troubleshooting JWS

One's computer may still need to be configured to properly run a JWS-launched application.   If you are having problems running JWS-launched applications, try these steps:

Add the site to the Java's "Exception Site List":

This will help keep Java from blocking the application from running by allowing applications from the launching site.   See the directions for "Application Blocked by Security Settings" in the "Tips and Traps" page of the Java Resources site.

Enable the Java console:

Enabling the Java console will allow you to see any error messages generated during the launch and/or running of a JWS application.   This can be very helpful in debugging what is going wrong.

(Linux only) OpenJDK doesn't support JWS:

The standard OpenJDK installation on Linux does not support JWS.   Add JWS support by also installing IcedTea-Web.

© 2021 by Stephen Wong