UnsupportedClassVersionError

I just got the following error when trying to start Tomcat for an Eclipse project:

java.lang.UnsupportedClassVersionError: Bad version number in .class file

It turns out my Tomcat installation seemed to be using an older version of Java to run. To fix this I went into Eclipse Windows->Preferences->Java and made sure that both the "Installed JRE" and "Compiler" tags had the same Java version (Java 6).

I also checked that in the Project preferences under "MyEclipse->Application Servers->Tomcat->MyEclipseTomcat->JDK" that I had the correct JDK selected (6.0.03, in my case).

I also deleted the Java 5 JDK (even though it wasn't selected) and (for good measure) deleted all my .class files... but this was probably a bit of overkill.

Following a complete rebuild it all worked again... Phew!