As the Issue Manager – a liaison position between our tech support and development – you always discover interesting constellations under which Java applets that our web application uses simply won’t run. A bunch of customers with Internet Explorer 7 or 8 could not launch any of our applets, although other applets worked perfectly fine for them.
Internet research yielded no similar experiences this far, so hopefully this one will help if you are facing the same issues… the message is: keep your system clean!

These were the tests I ran:

• The applet checker on Sun’s Java Website returned a correct installation of the currently up to date 1.6.0_13 JRE
• Entering “javascript:navigator.javaEnabled();” in the URL box returned “true”

Still, our applets would not start, showing the non-applet alternative text instead.

Here’s how to get this to work in IE6, 7 and 8:
Go to Extras => Manage Add-ons => Activate/Deactivate
List all add-ons currently in use:

You can see that under “Currently deployed Add-ons” not all Sun Microsystems add-ons are activated. Activate all of them, this should help!

In our case the JRE 1.5 add-on was deactivated, which shouldn’t have interfered with the applet being executed using the later JRE, but it did.

What’s peculiar is the way the add-on shows three times. I suspect this happens when uninstalling and reinstalling different JRE versions.
It’s also not possible to remove the add-ons completely, you can only disable them (unless you fiddle with the registry, which I wouldn’t recommend to our clients).

You can prevent this situation by using JavaRa, a handy tool that helps you uninstall any outdated JRE scrap from your computer:
http://raproducts.org/javara.html

Microsoft boasts about its improved add-on handling in IE8 (see http://blogs.msdn.com/ie/archive/2008/03/20/add-on-management-improvements-in-internet-explorer-8.aspx), however it is still possible to end up with old JRE residues on your machine.
With this mystery solved, let’s see what Microsoft and Sun Microsystems have in stock in terms of future surprises!