Java's concurrency primitives and libraries are really overlooked in these discussions. Is the syntax too off-putting, or is it merely that Java is unfashionable?
Very much unfashionable for the HN crowd. Most of the new research in concurrency is happening in Java and funded by the high performance trading industry - an industry which is very far from the HN crowd. The new Java8 stampedlock is a good example. It's possible to implement it in C++ as well, but because of the guarantees required by the lock it is a very difficult lock to integrate into C++ code. On the other hand, the JRE guarantees the correct constraints for Java code making a stampedlock very easy to use [1]. The performance of a stampedlock also seems to be the best case for any multi-reader environment. [2]
I find your claim that most new research in concurrency happens in Java strange. Perhaps you are unfamiliar with academic research in concurrency and parallelism? A way to get a small taste is to look at recent papers from the conference Practice and Principles of Parallel Programming (PPoPP).
If Java is involved, it's more likely they're interested in cutting developer costs than doing any "new research." That isn't to say they aren't doing any "new research," or that the research quality is poor, or anything negative at all, really. It's just a restatement of the observation that the labor market for Java developers is quite different from C developers or C++ developers (there are a lot more of the former than of the latter--a lot more).
I don't think there is anything wrong with Java as a language, but I have a question for you. Does Java's concurrency engine allow for many threads like Go or is it similar to C# in how your threads are not lightweight and are therefore limited to ixN where N is the number of CPU cores and i is a small integer < 10.
The Java language specification does not define how threads are implemented.
The first set of JVMs did implement green threads, which are what goroutines are. Shortly thereafter most of them switched to red threads, aka real threads.
You can still find a few JVMs that use green threads, like Squawk.
Java doesn't have a "concurrency engine", but a very large set of concurrency primitives: schedulers, locking and lock-free data structures, atomics etc.
To answer your question, yes: my own library, Quasar[1], provides lightweight threads for Java.
