NullPointerException at sun.applet.AppletPanel.findAppletJDKLevel

Question

Hi,
we have a online HTML page containing an <applet> and different <iframe> definitions. By clicking a HTML link or calling Applet.getAppletContext().showDocument(), other HTML pages containing other applets are loaded into the referenced frame. All applets have identical archive and codebase attributes, allowing them to communicate with each other.
This works actually very well. The problem is that sometimes newly loaded applets do not start, and a red cross appears in the inner frame. In all cases, the following exception appears in the java console (which does not happen if everything works fine):
java.lang.NullPointerException
at sun.applet.AppletPanel.findAppletJDKLevel(Unknown Source)
at sun.applet.AppletPanel.createApplet(Unknown Source)
at sun.plugin.AppletViewer.createApplet(Unknown Source)
at sun.applet.AppletPanel.runLoader(Unknown Source)
at sun.applet.AppletPanel.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
We experienced this on customer clients using
- Windows XP or Vista
- SUN Java 1.6 (e.g. Java 1.6.0_02), Java 1.5 worked so far
- Firefox2, IE6, IE7
Any ideas?
We use the JarIndex mechanism to enable dynamical class loading, could that cause problems?

Answer 1

After installing JRE1.6.0_02, the problem occured now as well, but didn't occur under the previously JRE1.5.0_07 version.
For me, it seems to be some classloader related timing problem and occurs even in the following scenario:
I have a main frame m, containing an inner frame a with applet A and an empty inner frame b.
1. at startup, A gets loaded into a. A starts well.
2. A loads a second Applet B via <applet> into b. B starts well.
3. Click HTML button within m loads a third Applet C via <applet> into b. Using Java 1.6, B does sometimes not start (see first post)
4. Click HTML button within m loads B via <applet> into b. This works.
5. Click HTML button within m loads C via <applet> into b. This works sometimes, sometimes not.
So, the class needed at that moment (whatever that is) is actually available but cannot be loaded properly at first start for some reason.
As said before, codebase and archive tags are exactly the same. Javascript is not used at all. I couldn't set up a test environment to simulate this effect, so I'm sorry that I couldn't supply any code example.
Does anyone could give me a hint please?