Because the applications are installed on a Citrix server the users do not have write permissions to HKEY_LOCAL_MACHINE so that was another complication.

After a lot of monitoring with process monitor the general process of how a particular Java version loads in Internet Explorer became clear to me.

Java adds an addon to IE called ssv.dll, you can see this trough Tools | Manage Add-ons | Enable or Disable Add-ons:

In Internet Explorer there is a setting under Tools | Internet Options | Advanced | Java (Sun). This activates Sun Java instead of the Microsoft version:

This setting corresponds to a registry key:

This setting (when activated) sets the UseJava2Iexplorer value under the path specified in the RegPath key:

So my first try was to change the RegPath value to Software\JavaSoft\Java Plug-in\1.5.0_11 but that doesn’t work. Problem is that IE has already loaded the Add-on ssv.dll from version 1.6 and we now make the registry point to 1.5. So we need to load the Add-on from version 1.5 as well.

So next question: where is this add-in loaded?

I searched in the registry for ssv.dll and found it in several places:

[HKEY_CLASSES_ROOT\CLSID\{5852F5ED-8BF4-11D4-A245-0080C6F74284}\InprocServer32]
@=”C:\\Program Files\\Java\\jre1.5.0_11\\bin\\JavaWebStart.dll”
“ThreadingModel”=”Apartment”

[HKEY_CLASSES_ROOT\CLSID\{761497BB-D6F0-462C-B6EB-D4DAF1D92D43}\InprocServer32]
@=”C:\\Program Files\\Java\\jre1.5.0_11\\bin\\ssv.dll”
“ThreadingModel”= “Apartment”

It’s interesting to know that HKEY_CLASSES_ROOT is actually a merged view of HKEY_CURRENT_USER\Software\Classes and HKEY_LOCAL_MACHINE\Software\Classes. In this case the actual values were in the HKLM branch. So I began experimenting by adding the same keys in HKCU and modifying it’s values.

After some experimenting I came up with 2 reg files that can be imported that contain only HKEY_CURRENT_USER keys. Maybe these reg files can be tweaked further but I verified the correct working by Java version check on http://www.javatester.org/version.html.

So simply importing a reg file can switch between Java versions! In the production environment I will make a particular version default based upon Group membership and/or the application that is started. The actual change will be done by script and maybe the user will be offered a nice “switch java version” tool. But the principle is ofcourse to make the proper registry entries under HKCU.

You can download the registry files below, please note that I used Java versions 1.5.011 and 1.6.07. So the reg files are based on those versions!

PS Sun offers older versions download on a special page.