Should know Java8 and Java * knowledge is big benefit
Generics and It Benefits
Exception handling.
hashmap vs Concurrent hashmap
Spring
Should have knowledge on How Spring works and it concepts, container spring lifecycle and its features.
Addon: Spring dependency injection, different injection ways, lazy vs eager initialization, aop, properties injection, transactions etc. Worked on multiple modules apart from core for rest services, JPA, MVC etc.
Should have knowledge on Spring Boot Micro Services
Middleware
Queue & Topics
point to point comm
publisher-subscriber model
be it Tibco or MQ series anything should be fine etc
Misc
Should have knowledge in Multi-threading(synchronization, wait/notify, ain thread spawning and waiting for child threads to complete and then terminate (thread join or countdown latch concepts)