QuickJas is an open source and multi-process (not multi-thread) Java application server. QuickJas is well designed for OLTP applications with many large size objects in a JVM of a huge heap.
You might find that QuickJas could be the simplest Java application server. QuickJas utilizes non-invasive POJO classes as services and the users are not required to learn any special API to build a client/server system.
QuickJas is written in Java, however you can also use many native clients to call QuickJas services.