JFR EventWriter does not use first string from StringPool with id 0

src/jdk.jfr/share/classes/jdk/jfr/internal/StringPool.java indexes from 0 for string pool entries with:

private final AtomicLong sidIdx = new AtomicLong();

and entries obtained via:

long sid = this.sidIdx.getAndIncrement();

src/jdk.jfr/share/classes/jdk/jfr/internal/EventWriter.java writes string pool entries with:


            long l = StringPool.addString(s);
            if (l > 0) {
                 ...
            }

The first entry to the StringPool will have id 0 but will never be used for events using the EventWriter. This can be seen with a test application that repeatedly emits a custom event with string field. The string will be in the constant pool, but the actual event data will have written the entire string out.

Something like:

import java.lang.annotation.Annotation;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.nio.file.Files;
import java.util.Arrays;

import jdk.jfr.Configuration;
import jdk.jfr.FlightRecorder;
import jdk.jfr.Recording;
import jdk.jfr.Description;
import jdk.jfr.Enabled;
import jdk.jfr.Event;
import jdk.jfr.Label;

public class EventCommit {

    @Label("Basic Event")
    @Description("An event with just a message as payload")
    static class BasicEvent extends Event {
        @Label("Message")
        public String message;
    }

    public static void main(String[] args) {
        try {
            Configuration c = Configuration.getConfiguration("default");
            Recording r = new Recording(c);
            r.start();
            for (int i = 0; i < 10; i++) {
                BasicEvent event = new BasicEvent();
                event.message = "123456789010111213141516";
                event.commit();
            }
            r.stop();
            r.dump(Files.createTempFile("my-recording", ".jfr"));
            r.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}