JDK Flight Recorder (JFR) with Native Image

JDK Flight Recorder (JFR) is an event recorder for capturing information about a JVM, and an application running on the JVM. GraalVM Native Image supports building a native executable with JFR events, and users can use jdk.jfr.Event API with a similar experience to using JFR in the Java HotSpot VM.

To record JFR events when running a native executable, JFR support and JFR recording must be enabled.

Add JFR Support and Record Events at Run Time #

To build a native executable with the JFR events support, you first need to include JFR at build time, then enable the system, start a recording, and configure logging at native executable run time.

To build a native executable with JFR, use the -H:+AllowVMInspection flag:

native-image -H:+AllowVMInspection JavaApplication

To enable the system, start a recording, and configure logging at run time, the following options are supported:

  • -XX:+FlightRecorder: use to enable JFR
  • -XX:StartFlightRecording: use to start a recording on application’s startup
  • -XX:FlightRecorderLogging: use to configure the log output for the JFR system

To enable JFR and start a recording, use -XX:+FlightRecorder and -XX:StartFlightRecording together. For example:

./javaapplication -XX:+FlightRecorder -XX:StartFlightRecording="filename=recording.jfr"

Run a Demo #

Transform this very simple demo application into a native image and see how to use JFR events from it. Save the following code to the Example.java file.

import jdk.jfr.Event;
import jdk.jfr.Description;
import jdk.jfr.Label;

public class Example {

  @Label("Hello World")
  @Description("Helps programmer getting started")
  static class HelloWorldEvent extends Event {
      @Label("Message")
      String message;
  }

  public static void main(String... args) {
      HelloWorldEvent event = new HelloWorldEvent();
      event.message = "hello, world!";
      event.commit();
  }
}

You can further configure the JFR recording or enable logging.

Configure JFR Recording #

You can configure the JFR recording by passing a comma-separated list of key-value pairs to the -XX:StartFlightRecording option. For example:

-XX:StartFlightRecording="filename=recording.jfr,dumponexit=true,duration=10s"

The following key-value pairs are supported:

Name Default Value Description
name none Name that can be used to identify the recording, e.g., “name=MyRecording”
settings none Settings file (profile.jfc, default.jfc, etc.), e.g., “settings=myprofile.jfc”
delay none Delay recording start with (s)econds, (m)inutes), (h)ours), or (d)ays, e.g., “delay=5h”
duration infinite (0) Duration of recording in (s)econds, (m)inutes, (h)ours, or (d)ays, e.g., “duration=300s”
filename none Resulting recording filename, e.g., “filename=recording1.jfr”
maxage no limit (0) Maximum time to keep the recorded data on disk in (s)econds, (m)inutes, (h)ours, or (d)ays, e.g., 60m, or 0 for no limit. For example, “maxage=1d”
maxsize no limit (0) Maximum amount of bytes to keep on disk in (k)B, (M)B or (G)B, e.g., 500M, or 0 for no limit. For example, “maxsize=1G”
dumponexit false Whether to dump a running recording when the JVM shuts down, e.g., “dumponexit=true”

Configure JFR System Logging #

You can configure the logging for the JFR system with a separate flag -XX:FlightRecorderLogging. The usage is: -XX:FlightRecorderLogging=[tag1[+tag2...][*][=level][,...]]. For example:

-XX:FlightRecorderLogging=jfr,system=debug
-XX:FlightRecorderLogging=all=trace
-XX:FlightRecorderLogging=jfr*=error
  • When this option is not set, logging is enabled at a level of WARNING.
  • When this option is set to an empty string, logging is enabled at a level of INFO.
  • When this option is set to “disable”, logging is disabled entirely.

Available log levels are: trace, debug, info, warning, error, off.

Available log tags are: all, jfr, system, event, setting, bytecode, parser, metadata, dcmd.

Otherwise, this option expects a comma separated list of tag combinations, each with an optional wildcard (*) and level.

  • A tag combination without a level is given a default level of INFO.
  • Messages with tags that match a given tag combination will be logged if they meet the tag combination’s level.
  • If a tag combination does not have a wildcard, then only messages with exactly the same tags are matched. Otherwise, messages whose tags are a subset of the tag combination are matched.
  • If more than one tag combination matches a message’s tags, the rightmost one will apply.
  • Messages with tags that do not have any matching tag combinations are set to log at a default level of WARNING.
  • This option is case insensitive.

Current Limitations #

The JFR support is still limited, for example, most VM-internal events and advanced features such as stack traces or memory leak detection are still missing. A subset of JFR features are currently available: custom and system events and disk-based recordings. Note that:

  • JFR events recording is not supported on GraalVM distribution for Windows.
  • JFR is only supported with native executables built on GraalVM JDK 11.

Further Reading #

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