Might be worth considering C# here too. The open-source cross platform tooling is new and still immature, but it's getting serious resources lately, with new language features (including more null safety) coming, cross platform IDE from Jetbrains in development, Mac / Windows / Linux / Android / iOS support, good concurrency features for server side development, JIT and AOT compilers + runtime, development mostly on github etc. They seem to be serious about making a first class cross platform language, I wouldn't be surprised if it displaces Java some in a few years.