This is the Javadoc for Jenkins core. Multiple versions are available: The latest weekly release, as well as the most recent LTS baselines. Unless a plugin requires very recent functionality, it should generally use one of the LTS baselines for its Jenkins core dependency.
Stapler is the framework used by Jenkins to route requests to the objects handling them. Learn more.
Stapler Javadoc on the Stapler site
This site collects and the Javadoc for the newest releases of all Jenkins plugins, if available.