Diagnostic tools (jcmd)

Definition

jcmd talks to a running Java process by its process id. It can print thread stacks (to find deadlocks and stuck threads), show heap and class statistics, trigger a heap dump and start Flight Recorder, all without restarting the application.

Example code

Looking inside a running JVM

jcmd                                   # list running Java processes and their ids

jcmd <pid> Thread.print                # all thread stacks; deadlocks are reported at the end
jcmd <pid> GC.heap_info                # heap sizes and usage
jcmd <pid> GC.class_histogram          # which classes use the most memory
jcmd <pid> GC.heap_dump /tmp/heap.hprof   # a heap dump to open in a tool such as Eclipse MAT
jcmd <pid> JFR.start duration=60s filename=/tmp/rec.jfr   # Flight Recorder: low-overhead profiling