I do both C# and Java professionally - more of the latter right now because libraries/portability.
Java: The sad fact is that there is a huge surface area here, and which bit you focus on is going to depend on what kind of work you want to be doing (and where you're doing it).
As a bare minimum I'd want to look at the following standards: JPA (JSR-338); Dependency Injection (JSR-330); JAX-RS (JSR-339).
A solid understanding of Spring Framework would help, as that's pretty common. If you're deploying onto an application server, Wildfly would be the direction I'd look at.
From there it's religion - your best bet is to look at the jobs your interested and bone up on the toolchains that keep coming up.
Java: The sad fact is that there is a huge surface area here, and which bit you focus on is going to depend on what kind of work you want to be doing (and where you're doing it).
As a bare minimum I'd want to look at the following standards: JPA (JSR-338); Dependency Injection (JSR-330); JAX-RS (JSR-339).
A solid understanding of Spring Framework would help, as that's pretty common. If you're deploying onto an application server, Wildfly would be the direction I'd look at.
From there it's religion - your best bet is to look at the jobs your interested and bone up on the toolchains that keep coming up.
Hope that helps :)