How to open Javadoc JAR file in web browser
- Rename the file multiplayer-server-plugin-1.13-javadoc.jar to multiplayer-server-plugin-1.13-javadoc.zip
- Use your favourite unzip tool (WinRAR / WinZIP) to extract it, now you have a folder multiplayer-server-plugin-1.13-javadoc
- Double click index.html will open the index page on your default web browser.
How to generate Javadoc from a source JAR?
Running the command javadoc:
javadoc --ignore-source-errors -encoding UTF-8 -sourcepath "multiplayer-server-plugin-1.13-sources.jar" -d "multiplayer-server-plugin-1.13-javadoc" -subpackages 
