I sometimes find the Java setup on my various Apple devices to be a mystery.
Recently, I was trying to get a Java applet to run in the same way on 2 iMacs and my MacBook Air. The applet is a simple vpn client from Juniper that lets me access a Citrix Desktop from any Mac that I can install the Citrix receiver client on so I can work on 'Company stuff' from a large screen iMac when I'm sat at home or from my MacBook when I'm on the road (it works fine over 3/4G).
The first thing is that you have to do some configuring of both Java and Safari to get the applet to run at all.
Once that was all done, I could log in from all my Macs, fire up the applet and establish a secure connection.
On two of the Macs, as soon as I fired up the Citrix app, the Java vpn window would show 'error'. The console showed a Java crash. But on the third Mac, everything worked fine. I made sure that the Safari and Java preferences were set the same on each machine but still no joy. Then I remembered that I had done some Java development in the past and installed various jdks from Oracle so I ran:
in Terminal on each machine. I keep everything up to date via the Java control panel (currently 1.7xx soon to be 1.8) so was surprised to see this:
That was on the working Mac. Then I remembered the difference between 'System' Java, Java plugins, and Java development kits. Simply put, you can have multiple versions of Java in different places. What was happening on the not-working Macs was that the jdk versions were being used, and the Juniper vpn client won't work with them.
To fix things for the moment I simply removed the jdk folders.
And then checked that the reported version of Java was 1.6 on each Mac. Web applets still use the up to date, secure version 1.7 plugin.
[crarko adds: I believe Oracle has said that eventually Java will no longer support applets at all, on any platform.]