The JVM is one of the best virtual machines out there and is pretty solid - most architects and CTOs will recommend Java (or other JVM languages) for any seriously large projects, otherwise .NET if the team has more experience with the microsoft side and with Microsoft open-sourcing .NET, demand might increase for .NET in the future, but so will demand for Java developers.
Ruby is another safe bet and if run on the JVM via Jruby, you get the benefits of the JVM, but get to develop in Ruby syntax (Java syntax is loaded with boilerplate compared to more modern languages like Kotlin and Scala)
If you do a search on job databases, let's take indeed as an example, you'll see Java is still very much in demand (obviously depending on region).
- Java: 69,543
- .NET: 51,176
- C#: 29,164
- Ruby: 19,915
- PHP: 19,279
- Scala: 4,895
- Kotlin: 52
Either way, if you are strong in multiple languages, you should be able to pickup any language in the future and just go with it. Most companies hire for the ability to solve problems and are often happy to give you a bit of time to pickup the language they're using.
PS, it sounds like it's been a while since you've worked with the Java ecosystem, lots has changed, here's a good introduction:
blog.paralleluniverse.co/2014/05/01/modern-java
blog.paralleluniverse.co/2014/05/08/modern-java-p…
blog.paralleluniverse.co/2014/05/15/modern-java-p…
Also, if you're looking to use it in your side projects, Kotlin is a good choice as well, it's a simpler Scala, but a better Java and you'll be using a lot of Java in your code, just with Kotlin syntax.
Not many job openings for Kotlin yet, but it is growing slowly.