I believe the core of VoltDB (the in memory storage engine) is written in C++. See here: https://github.com/VoltDB/voltdb/blob/master/src/frontend/or...

"VoltDB mainly consists of Java modules for easier development. However, the core EE and its underlying on-memory storage system is implemented in C++. Nowadays Java and C++ have almost same performance for many cases, but C++ still outperforms Java on low-level memory accesses. This is why we have EE and its storage system written in C++."