JBLS, or “Java Battle.net Login Server” is a Java application compatible with the BNLS protocol which assists bots in connecting to classic Battle.net, primarily by remotely performing hash checks on the game files. This allows an emulation client’s end user to connect to official servers without having to have a copy of the game files on the device they are connecting from.
While I am not the original developer of the JBLS software, I was responsible for running one of the most popular instances of the software, and have kept it maintained throughout Blizzard’s recent changes.