JFR and jcmd

Learn to profile with Java Flight Recorder and jcmd.

Profiling with JFR

JFR (Java Flight Recorder) collects low-overhead event records from a running application. Method sampling, allocation, garbage collection and lock contention events are recorded. It can run continuously in production.

Recording starts with jcmd PID JFR.start duration=60s filename=rec.jfr. The resulting .jfr file is inspected with jfr print or JDK Mission Control.

The jcmd tool

jcmd is a versatile tool that sends commands to a running JVM. Besides JFR recording it supports heap dumps, thread dumps, GC control and diagnostic commands. jcmd PID help lists the available commands.

After the JFR recording finishes jcmd PID JFR.dump filename=rec.jfr dumps the data. JFR is the first tool to reach for in production issue analysis.

Exercise Write the JfrRecorder class

Define class JfrRecorder with a static String start(int pid, int seconds) method that returns "jcmd " + pid + " JFR.start duration=" + seconds + "s". In main print System.out.println(start(1234, 60));.

Show example solution

class JfrRecorder {
    static String start(int pid, int seconds) {
        return "jcmd " + pid + " JFR.start duration=" + seconds + "s";
    }
    public static void main(String[] args) {
        System.out.println(start(1234, 60));
    }
}

Test yourself

Complete the JFR start command

jcmd ____ duration=60s filename=rec.jfr

Show answer

jcmd JFR.start duration=60s filename=rec.jfr

What is the distinctive advantage of JFR?
- Low overhead
- It compiles classes
- It starts threads
- It deletes files
Show answer

Low overhead — JFR can run in production with low overhead.
Match the JFR tools

JFR.start

Starts recording
jcmd

Control tool
JMC

Analysis tool
duration

Recording length

Next lesson: GC tuning

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