As a practical matter, how does a plugin developer find this stuff? Just surfing the massive javadoc? Are there example plugins out there? The tutorials are not that useful for exploring the java API.
It’s a combination of http://docs.atlassian.com, google, java docs, tutorials, Atlas Camp presentation… nah, who am I kidding: the only reliable way to understand what is going on in Server land is the host product source code. You get access to the sources of all Atlassian products with a starter license. It’s the best $10,-- investment you can make.