Answer by Tomas Hurka for Using VisualVM with AdoptOpenJDK and OpenJ9

OpenJ9 is a lot different from HotSpot in monitoring area. So if you want to monitor OpenJ9, you have to use JMX. Start your OpenJ9 with JMX enabled. For simple configuration you can use following system properties:

java -Dcom.sun.management.jmxremote.port=9999 \
-Dcom.sun.management.jmxremote.authenticate=false \
-Dcom.sun.management.jmxremote.ssl=false

Start VisualVM on AdoptOpenJDK and add a local JMX connection on the port 9999 - right click on the 'Local' node and select 'Add JMX connection'. Use 'localhost:9999' as a connection string.