Useful Java tools to try

java mission control - to monitor jvm

lincheck - testing concurrent data structures

jmh - it is tool for microbenchmarking your code

debezium - tool to capture database changes (mongo, oracle...)

Spring Cloud Contract

Cool tool to test interaction between you apps without running all of them. Need just describe contract of you service and inject in via wiremock were it is needed.  Here is a tutorial

Spring reactive

Spring introduced reactive support with WebFlux it implements basic Reactive concepts. It works with Netty, non blocking server.

Also Spring added support of the RSocket protocol it is not based on HTTP and provides nice features as back-pressure. You can establish direct connection from the web-browser to rsocket server. Spring provides API to implement listeners, smth similar like rest controllers.