JDK Flight Recorder

JDK Flight Recorder (JFR) can work with Native Image.

With the current version of GraalVM, the support of JFR is quite limited. Most VM-internal events and advanced features are still missing. Only custom and system events and disk-based recordings are supported.

Basic Usage

To add JFR support, the native image needs to be built with the -H:+AllowVMInspection option.

native-image -H:+AllowVMInspection Main

When JFR is included, it can be enabled using the -XX:+FlightRecorder option. A recording is started by adding the -XX:StartFlightRecording option. -XX:StartFlightRecording specifies the configurations to start a JFR recording. The value is a comma-separated list of key-value pairs.

Key	Default value	Description
name		Name of the recording
settings		JFR settings file
delay		Delay to start recording
duration	0 (infinite)	Duration of recording in seconds
filename		Name of the recording file
maxage	0 (no limit)	Maximum time to keep the recording
maxsize	0 (no limit)	Maximum amount of bytes of the recording
dumponexit	false	Whether to dump the recording on exit

Start JFR recording

./app -XX:+FlightRecorder \
  -XX:StartFlightRecording="filename=recording.jfr,dumponexit=true,duration=1m"

The recording file can be opened by VisualVM or JDK Mission Control.

The figure below shows a JFR recording in JDK Mission Control. It only has a limited set of events.

Custom Events

An important JFR feature than can be used in the current GraalVM version is custom events. ServiceAccessEvent in the code below is a custom JFR event. This class represents an event that a service is accessed.

Custom JFR event

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

@Label("Service access")
@Description("Access a service")
class ServiceAccessEvent extends Event {
    @Label("Service name")
    String name;
}

The code below shows a JAX-RS resource. In the recordServiceAccess method, a new ServiceAccessEvent is created and committed. When the API endpoint /hello is accessed, a new event will be committed.

Simple REST service

import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;

@Path("/hello")
public class GreetingResource {

    @GET
    @Produces(MediaType.TEXT_PLAIN)
    public String hello() {
        recordServiceAccess("hello");
        return "Hello World!";
    }

    private void recordServiceAccess(String name) {
        ServiceAccessEvent event = new ServiceAccessEvent();
        event.name = name;
        event.commit();
    }
}

In the JFR recording, this custom event will be displayed.