This is because your class files were compiled with a Java compiler not compatible with JVM in use within tomcat. If you compiled on the same machine on which you have tomcat running, you must be having 2 JDKs (or a JDK and another JRE) with different versions. Solution to this is:
On Linux:
1. Find tomcat bin folder and open Catalina.sh in it (on my machine it is /usr/share/tomcat6/bin). You can also search for Catalina.sh directly (find / -namecatalina.sh).
2. Add correct JAVA_HOME to it and then restart tomcat (add it after all initial comments statements end)
export JAVA_HOME=/opt/java/jdk1.6.0_21/
On windows, you can make the same change in catalina.bat and it should work.
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