What should a backend java developer know?

Question

Java

What is the body of knowledge of a good Java back-end developer? Theory, frameworks, standards, or something else (important in your opinion). The opinion of a more or less experienced developer in the backend and Java is interesting.

Answer 1

Core at a decent level, spring and javaEE at least in general terms, have an idea about JMS or analogues, about SQL and noSQL, have an idea about protocols and OSI levels, be able to work with multithreading and know the corresponding frameworks.
Actually, the question is very imprecise. There is no final set of knowledge, it all depends on the specifics, and on the level you are targeting.