Clean Code and Software Principales (KISS, DRY, YAGNI, SOLID ...)
Software Design Patterns
Software Architecture and Patterns (Headless, Microservice, Layered, Event Based ...)
Refactoring and unit testing (TDD)
Performance testing (Jmeter)
Monitoring and analysing memory leaks (JvisualVM, Jprofiler, JMX, Ecliple MAT)
API (open api, swagger, REST ...)
Authorization and authentication
Good Knowledge on Infrastruction (TCP, UDP, HTTP) (LB, Proxies, Cluster, NGINX...)
Database knowledge ( analysing locks, slow queries, debuging .. )
Monitoring (Kibana, New Relic, Dynatrace ...)
Spring: Boot, Cloud, AOP, JPA ...
Great Knowledge on cloud AWS, Azure