If you are looking to accelerate your career and become a better Java programmer, these are skillset that will distinguish you from other programmers.
1. Learn software design and architecture
See the big picture and think through the problem at hand, and decide on the right architecture and technology stack.
2. Learn containers and DevOps tools (Jenkins, Docker, and Kubernetes)
Be at least familiar with CI and CD and how Jenkins helps to achieve that.
3. Learn Spring framework (Spring Boot)
Spring MVC, Spring Boot, and Spring Cloud.
4. Learn unit testing (JUnit and Mockito)
It separates a good Java developer from an average Java developer.
5. Learn APIs and libraries
Jackson and Gson, JAXB and Xerces, Mockito and JUnit etc.
6. Learn JVM internals
Find performance bottlenecks e.g. (memories, CPUs.)
7. Learn design patterns
It makes your application more flexible and easier to change.
8. Learn Kotlin
Be a Polyglot programmer & ease your Android development onboarding.
9. Learn microservices
Take advantage of this recent monolithic to Microservices wave.
10. Learn your ide better
Spend some time to learn Eclipse, NetBeans, and IntelliJ IDEA better.