Struts 2 and all depedencies, by default is compiled using JDK 1.5 and just can be used on at least JDK 1.5 environment.
But, for people who wish to use this current framework on environment below JDK 1.5, such as JDK 1.4 or 1.3, can use Retrotranslator (http://retrotranslator.sourceforge.net/) for backporting all struts 2 core, library and dependencies on current destination….
the steps for backporting, completely can be read at :