How to find Java Heap Size and Memory Used (Linux)?
How can I check Heap Size (and Used Memory) of a Java Application on Linux through the command line?

Each Java process has a pid, which you first need to find with the jps command. 
Once you have the pid, you can use jstat -gc [insert-pid-here] to find statistics of the behavior of the garbage collected heap.

jstat -gccapacity [insert-pid-here] will present information about memory pool generation and space capabilities.
jstat -gcutil [insert-pid-here] will present the utilization of each generation as a percentage of its capacity. Useful to get an at a glance view of usage.

See jstat docs on Oracle's site.
