As a computer scientist, I'm not sure whether I should be using footnotes or references (as in, those that go in "thebibliography" at the end) when referring to Javadocs. I think there is some valuable research even in the documentation of the Java standard library but I'm sure many will either gloss over these references or those who don't will probably already know about them. What is the appropriate style, in LNCS format?
The Springer instructions for authors for conference proceedings (including LNCS) don't give any guidance on what information should be put in footnotes, so you appear to be free to choose. I would suggest that if you are referring to a document then the easiest thing is to include it in the references as you would with any other document to which you refer.
Including it as a reference also means:
- it will be visible on the article webpage even to people who aren't logged in or don't have access to the full paper, and
- it will be linked to Crossref, which may benefit people who mine citation data.
If you're using BibTeX, you can use the
@manual reference type, but see also this answer on referencing software documentation for options.