It's your fault if you trade equities, or you do anything else in which milliseconds matter, with a software platform that is aggressively optimized for average performance and for server stability at the expense of hard real time guarantees and with a language that privileges enforcing constraints (both to ensure correctness and allow optimizations) over allowing the user explicit control. ("Your fault" at the organizational level, not personally, of course.)

Can you explain how Java was chosen for your trading application? Also, why were you unable to test it properly?

The org chose Scala for the trading application in the hope that developers and researchers could both use it, when previously they had been using very different languages. I thought this went mostly well. Scala enables you to use some shenanigans to pretend that a byte-array is an array of structs, and the 31 other threads of the application ran acceptably on the JVM. As an aside, my impression is that Java is somewhat common in the industry.

The application was tested in an environment that's a lot like production, except that we had substantially fewer cores than an exchange running the same number of matching engines, so the incoming ticks were a bit less bursty. That burstiness was important in this instance. A big reason for the differences between the production environment and the test environment was the cost of hardware.

I'm not sure I've ever heard of the JVM being used in trading level software before. I've gotten a broad but vague impression that custom applications are the norm.

No criticism, as I've only read others' accounts, but that sounds out of character.

Java or Clojure applications are "custom" too: custom code running in some kind of application server is more or less the same as custom code relying on libraries and frameworks in C or C++ or other leaner languages.

What's "out of character" (nice euphemism!) is adopting state of the art automatic garbage collection technology, writing in a language that makes actual garbage collection practically unavoidable, and then expecting that garbage collection doesn't happen or happens accordingly to arbitrary expectations. The JVM is clearly inappropriate technology if latency is important: it can perform well in common cases and with a reasonable level of tuning effort, but other options simply do not have the threat of GC pauses.