Java Flight Recorder

JVM · lesson 16 of 34 · 3 min read

A profiler built into the JVM itself, cheap enough to leave switched on in production.

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Key points

  • JFR records events the VM already knows about: allocations, GC pauses, lock contention, exceptions, IO and cpu samples.
  • Overhead on the default profile is around one percent, because the VM emits the events rather than a tool polling from outside.
  • Start it at launch with -XX:StartFlightRecording, or attach to a running process later with jcmd.
  • Open the .jfr file in JDK Mission Control. Its automated analysis ranks the likely problems before you look at anything.
  • The real win: a 60 second recording from the box that is actually misbehaving beats hours of guessing locally.

Example

# record 60 seconds from a process that is already running
jcmd <pid> JFR.start name=hot settings=profile duration=60s filename=/tmp/hot.jfr
jcmd <pid> JFR.check
jcmd <pid> JFR.dump name=hot filename=/tmp/hot.jfr

# or record continuously from startup, capped in size
java -XX:StartFlightRecording=settings=profile,filename=/var/log/app.jfr,maxsize=200M -jar app.jar

# then open the file in JDK Mission Control
jmc

JFR is the profiler you can afford to leave running.

Heap dumps and thread dumps Startup, shutdown and exit codes

This is a reading copy. The full lesson — with the visual explainer, the interactive lab and a Run button for the code — lives in the JVM course, and every lesson in it is listed on the JVM contents page.