If you have some complicated code and you don't want to store it in Jenkins configuration page, you can easily extract it to groovy file, save in folder and add this folder to class path. After that you will be able to import this class from your code. Example:

        > cd /var/lib/jenkins
        > mkdir groovy_scripts
        > echo "class Constants { public static final int TIMEOUT = 10; }" > groovy_scripts/Constants.groovy
    

Now you cad add /var/lib/jenkins/groovy_scripts folder to classpath and do something like this:

        import Constants
        log.info Constants.TIMEOUT