The JavaDoc of sun.text.RuleBasedBreakIterator contains a link to "Text Boundary Analysis in Java" by Richard Gillam which describes its implementation details:

http://www.ibm.com/java/education/boundaries/boundaries.html

This link isn't online anymore since at least 2001, but fortunately its original content can still be found trough archive.org at https://web.archive.org/web/20001117163900/http://www.ibm.com/java/education/boundaries/boundaries.html

Replace the naked link text with the title and author of the publication and the link target with the new link at the ICU project which is still available: https://icu-project.org/docs/papers/text_boundary_analysis_in_java/