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JRuby With Wrong Java

[ed: This page needs revision because some other Linux distros have the same problem as Ubuntu. What we might want to add is proper instructions for getting a hotspot-based JVM on those Linux versions, either as OpenJDK (like RedHat's "icedtea" builds) or a Sun build.]

You may find that even though you have an up-to-date JDK installed, JRuby does not work on Ubuntu. When running jruby -v, you might get exceptions from Java. The reason for these errors might be that, even though you have an up-to-date JDK installed, Ubuntu is not using it by default. You need to update your configuration to use the proper JDK. See https://help.ubuntu.com/community/Java for more information on this issue.

On Ubuntu 7.10, I ran

 java -version

which said that I was running Java version 1.5.0 when I knew I had Java version 1.6 installed. I then ran:

 sudo update-alternatives --config java

I picked the option /usr/lib/jvm/java-6-sun/jre/bin/java. Following that, running java -version gave me

 java version "1.6.0_03"
 Java(TM) SE Runtime Environment (build 1.6.0_03-b05)
 Java HotSpot(TM) Server VM (build 1.6.0_03-b05, mixed mode)

And JRuby then worked fine.