For collections, we used to use trove but migrated to fastutil a few years ago.

For JSON parsing, we are processing lots of very small messages, so use LazyJson. The biggest downside to LazyJson is it doesn't have cheap iteration of keys; the framework could easily provide it. For larger documents, say over a few MB, libraries like Jackson are faster.

Yeah, perhaps Java isn't the right tool for our job. And yeah, more recent benchmarking and testing might suggest newer, better libraries than those I have just listed.

Its horrific the lengths you have to go to to get good performance Java for the workloads we have; python prototypes run much faster with pypy, and I think that is really about heap management more than code generation.

For those of us who know C/C++, its kinda uncomfortable when staring at code and thinking "that temporary string there? 40+ bytes just for the object header!" and things. But, of course, there are advantages to working in memory-safe languages.

C# and its structs, yummy.