JFR

JFR, or Java Flight Recorder, is a built-in performance profiling and event-logging framework in the Java Virtual Machine. It records low-overhead, high-fidelity data about runtime events, enabling diagnostics and performance tuning for Java applications in production as well as in development.

The feature originated in Oracle's JDK and was later integrated into OpenJDK. It was introduced to the platform as a programmable flight recorder via JEP 328 in Java 9 and has since become a standard component in modern JDK releases. Java Mission Control (JMC) provides a graphical environment for analyzing JFR recordings, and third-party tools also exist.

JFR collects events from the JVM and the running application. Built-in event types cover VM lifecycle (VMStart, VMStop), class loading and bytecode instrumentation, method sampling, JIT compilation, garbage collection, memory pools, thread activity, I/O, and more. Applications can emit custom events by extending framework-provided base classes.

Configuration and usage: recordings can be started at JVM launch or runtime. A typical command-line approach is to start a recording with -XX:StartFlightRecording=duration=60s,filename=app.jfr. The data is stored in a .jfr file and can be opened by Java Mission Control or other JFR-aware tools. The API also allows programmatic control through the jdk.jfr package, enabling developers to annotate code with custom events.

Applications: JFR is used for performance profiling, latency analysis, GC tuning, thread contention investigations, and reliability work in production environments with low overhead.

Limitations: while designed to be low overhead, recording incurs some performance impact and can generate large files. Proper filtering and sampling are recommended, and sensitive data should be considered when recording in production.