Continuous Profiling with JFR

Flags

-XX:StartFlightRecording starts JFR at JVM launch. It can write to disk, limit file size or age, dump on exit, and use different settings profiles.

Flag details

-XX:StartFlightRecording=filename=recordings/app-%p.jfr

Starts JFR immediately and writes the recording to a file. %p expands to the JVM process ID.

dumponexit=true

Dumps the recording when the JVM exits, useful for short-lived jobs or graceful shutdown analysis.

maxsize=256m

Limits how much recording data is kept on disk. Use this to avoid filling local storage during continuous recording.

settings=profile

Uses a more detailed JFR event configuration than the default. It is useful during targeted profiling and may add more overhead than a light continuous recording.

duration=10m

Stops the recording automatically after ten minutes, which is helpful for controlled load tests.

When to use it

Use this for always-available incident context, performance baselines, latency analysis, allocation profiling, lock contention, and CPU investigations without attaching a tool after the process is already sick.

Java version support

Java Flight Recorder became an open feature in Java 11. The -XX:StartFlightRecording launch option is the normal way to start a recording at JVM startup on modern JDKs.

Related JEPs

• JEP 328 - Flight Recorder, delivered in JDK 11.
• JEP 349 - JFR Event Streaming, delivered in JDK 14.

Examples

java -XX:StartFlightRecording=filename=recordings/app-%p.jfr,dumponexit=true,maxsize=256m \
  -jar app.jar

Use this for always-available incident context with bounded disk usage.

java -XX:StartFlightRecording=name=profile,settings=profile,duration=10m,filename=profile.jfr \
  -jar app.jar

Use this during a controlled profiling window, such as a load test.

Verify

Open the resulting .jfr in JDK Mission Control and check CPU, allocation, GC, lock, and thread views.