Reddit mentions of Effective Java

Learning AWS, machine learning, javascript frameworks - these are worth playing with to see if you like them, and maybe give you a direction to take your career in the future. Since you're a junior right now though, that might be thinking too far ahead. Right now, unless you are looking to escape Java development, your goal should be to just get that "junior" off your title, and become the coveted Mid Level Engineer who can Hit The Ground Running and Get Things Done Right Away, because that's who people want to hire. To that end:

• Unit/Automated Testing. Writing tests is a big part of your job (if not, it should be, and it will be in your next job) and there is skill in writing good ones.
  
• Effective Java by Joshua Bloch is a great book to read if you want to stick with Java for any length of time.
  
• Since you're in a Spring shop, you have the opportunity to get familiar with Spring itself and that can be useful. Understanding the guts of Spring can help you design solutions and become more self-sufficient as a developer in your current shop. You can also look into alternative frameworks like Dropwizard (a Spring Boot competitor) and other DI frameworks like Guice and Dagger, just to see some alternative design decisions.
  
• SQL. A lot of java work is "backend" which means you'll be accessing relational databases, and knowing how to write good SQL (and design relational data structures) can help your code perform well and avoid errors. I've also used SQL to help debug and make design decisions.
  
• Git. Helping your peers get out of Git hell is a great way to look senior; understanding how it works is part of your day to day. Git is popular enough that even if your current job doesn't use it, your next job will.
  
• Kubernetes. I just threw this in because everyone seems to love it.