There will always be things that I do not understand

Code found in JAXB

/**
     * Makes sure that we are running with 2.1 JAXB API,
     * and report an error if not.
     */
    static {
        try {
            XmlSchema s = null;
            s.location();
        } catch (NullPointerException e) {
            // as epxected
        } catch (NoSuchMethodError e) {
            // this is not a 2.1 API. Where is it being loaded from?
            Messages res;
            if(XmlSchema.class.getClassLoader()==null)
                res = Messages.INCOMPATIBLE_API_VERSION_MUSTANG;
            else
                res = Messages.INCOMPATIBLE_API_VERSION;

            throw new LinkageError( res.format(
                Which.which(XmlSchema.class),
                Which.which(ModelBuilder.class)
            ));
        }
    }