`jstat` - monitor JVM statistics

jstat -<option> [-t] [-h<lines>] <vmid> [<interval> [<count>]]

Options you will use

-gcutil: Percent used of S0 S1 Eden Old Metaspace CCS, plus GC counts and times.
-gc: Capacities and usage in KB.
-gccause: Like -gcutil plus the last and current GC cause.
-t: Timestamp column (seconds since JVM start).
-gcnew: Young generation statistics.
-gcold: Old generation and Metaspace statistics.
-class: Class loader statistics.
-h N: Repeat the column header every N samples.

Examples

$ sudo -u appuser jstat -gcutil 1210 1000 10

Ten samples one second apart.

$ jstat -gccause 1210 1s

Until Ctrl+C.

Gotchas

Columns: YGC/YGCT young collections and total seconds, FGC/FGCT full, CGC/CGCT concurrent (G1), GCT all. If FGC climbs every sample and O stays near 100, the JVM is thrashing: that is your "high CPU".
jstat reads /tmp/hsperfdata_<user>/<pid>; for another user it just says "<pid> not found".

Taught in

Try jstat in a real terminal Free, in your browser - a real Ubuntu terminal to try it in, with missions that check your work.