For someone who used to be very proficient in Java, what would be a good way to get back into the language again?

I used to work professionally with it until about 2011/Java 7 - then pursued a university career (machine learning, data science, etc.), and now will likely get a job which again requires me to use Java. What resources would you recommend looking into for getting back on track? I'm interested especially in

- new language features introduced since back then

- popular frameworks everybody uses nowadays

- software projects that use modern Java (and may need contribution)

- any recent books about modern Java development

Looking forward to your suggestions!