All of the subclasses of AbstractConfig (that I cared to look at)
provide a public constructor that is not meant to be used.  At
the very least, these constructors should have javadoc along the lines of

    /**
     * Do not instantiate this class directly.
     *
     * @see ContentSection#getConfig()
     **/
    public ContentSectionConfig() {
         ...
    }


Ideally, the singletonness should be enforced rather than advised,
unless there is some fundamental reason why it can't be enforced.