Given this code, that tries to listen to the event jdk.VirtualThreadPinned when a virtual thread is pinned:

import static org.junit.Assert.assertTrue;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;

import jdk.jfr.Event;
import jdk.jfr.Name;
import jdk.jfr.consumer.RecordingStream;
import org.junit.Test;

public class JFRTest {

    @Test
    public void pinnedVirtualThreads() throws InterruptedException {
        CountDownLatch stopListening = new CountDownLatch(1);
        CountDownLatch startListening = new CountDownLatch(1);
        try (RecordingStream recordingStream = new RecordingStream()) {
            // Create and start the recording stream
            startRecordingStream(recordingStream, startListening, stopListening);
    
            // Make sure JFR listener is running before continue
            while (true) {
                new StartEvent().commit();
                if (startListening.await(1, TimeUnit.SECONDS)) {
                    break;
                }
            }
    
            // Simulate blocking operation
            Thread vt = Thread.ofVirtual().start(() -> {
                synchronized (this) {
                    System.out.println("Virtual Thread is pinned");
                }
            });
            vt.join();
        }
        // Wait till the jdk.VirtualThreadPinned triggers
        assertTrue("jdk.VirtualThreadPinned was not sent", stopListening.await(5, TimeUnit.SECONDS));
    }
    
    private void startRecordingStream(RecordingStream recordingStream, CountDownLatch startListening, CountDownLatch stopListening) {
        // Enable the jdk.VirtualThreadPinned event
        recordingStream.enable("jdk.VirtualThreadPinned").withStackTrace();

        // Notify listener is running after receiving the StartEvent
        recordingStream.onEvent("StartEvent", event -> {
            System.out.println("Received " + event);
            startListening.countDown();
        });

        // Set an event handler
        recordingStream.onEvent("jdk.VirtualThreadPinned", event -> {
            System.out.println("VirtualThreadPinned event detected!");
            System.out.println("Timestamp: " + event);
            stopListening.countDown();
        });

        // Start the recording stream
        recordingStream.startAsync();
    }

    @Name("StartEvent")
    private static class StartEvent extends Event {
    }
}

This is the output:

Received StartEvent {
  startTime = 10:15:32.723 (2024-06-12)
  eventThread = "main" (javaThreadId = 1)
  stackTrace = [
    com.example.general.JFRTest.pinnedVirtualThreads() line: 25
    jdk.internal.reflect.DirectMethodHandleAccessor.invoke(Object, Object[]) line: 103
    java.lang.reflect.Method.invoke(Object, Object[]) line: 580
    org.junit.runners.model.FrameworkMethod$1.runReflectiveCall() line: 59
    org.junit.internal.runners.model.ReflectiveCallable.run() line: 12
  ]
}
Virtual Thread is pinned
[ERROR] Tests run: 1, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 6.211 s <<< FAILURE! - in com.example.general.JFRTest

As you can see, this part should trigger the event jdk.VirtualThreadPinned, but it is not happening. On the other hand, the other event StartEvent is caught:

Thread vt = Thread.ofVirtual().start(() -> {
    synchronized (this) {
        System.out.println("Virtual Thread is pinned");
    }
});

I don’t know why.

This is my JDK version is: 21.0.3+7-LTS-152