How to open Javadoc JAR file in web browser
- Rename the file MockBukkit-v1.13-spigot-0.2.0-javadoc.jar to MockBukkit-v1.13-spigot-0.2.0-javadoc.zip
- Use your favourite unzip tool (WinRAR / WinZIP) to extract it, now you have a folder MockBukkit-v1.13-spigot-0.2.0-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 "MockBukkit-v1.13-spigot-0.2.0-sources.jar" -d "MockBukkit-v1.13-spigot-0.2.0-javadoc" -subpackages 
