I have more than one java application running in separate docker containers. I am trying to collect monitoring data such as GC log, thread dump, heap dump from the java process running inside a container using tools like jstat, jstack, jmap. Is it possible to capture this information from the host(outside containers)?
I am new to the containerized world. I understand that PID namespace of the host and container is different. When I execute jstack <PID> > thread_dump.txt from the host, it shows error message: Unable to open socket file /proc/root/tmp/.java_pid: target process doesn't respond within 10500ms or HotSpot VM not loaded
Where PID is process id from the host PID namespace.
When I execute jstack inside container ( docker exec -it <container_id_or_name>) then it is able to capture thread dump.
Where PID is process id from the container PID namespace.
Any hints on how to solve this?
jattach tool serves exactly for this purpose. The project is free and open source.
Examples:
jattach <pid> threaddump (works like jstack)jattach <pid> inspectheap (works like jmap -histo)jattach <pid> jcmd GC.class_statswhere <pid> is Java process ID in the host namespace.
As a bonus, jattach works even if container runs JRE with no JDK tools (jstack, jcmd, etc.) installed.