JFR로 실행 기록 남기고 분석하기

import java.util.ArrayList;
import java.util.List;

public final class BoardWorkload {
    public static void main(String[] args) {
        long end = System.nanoTime() + 3_000_000_000L; // 3초 동안 반복한다
        int rounds = 0;
        while (System.nanoTime() < end) {
            List<String> titles = new ArrayList<>();
            for (int i = 0; i < 10_000; i++) {
                titles.add("post-" + (i * 7_919 % 10_000));
            }
            titles.sort(null);
            rounds++;
        }
        System.out.println("rounds=" + rounds);
    }
}

javac BoardWorkload.java
java -XX:StartFlightRecording=filename=board.jfr,settings=profile,duration=30s,dumponexit=true BoardWorkload

[0.530s][info][jfr,startup] Started recording 1. The result will be written to:
[0.530s][info][jfr,startup] 
[0.530s][info][jfr,startup] .../board.jfr
rounds=1036

jfr summary board.jfr

 Version: 2.1
 Chunks: 1
 Start: 2026-10-27 00:38:30 (UTC)
 Duration: 3 s

 Event Type                              Count  Size (bytes)
=============================================================
 jdk.ObjectAllocationSample                910         12760
 jdk.InitialEnvironmentVariable            231         29684
 jdk.ExecutionSample                       131          1369
 jdk.GarbageCollection                      11           253
 jdk.CPULoad                                 3            57
 jdk.JavaMonitorEnter                        0             0

jfr view hot-methods board.jfr

                                Java Methods that Executes the Most

Method                                                                              Samples Percent
----------------------------------------------------------------------------------- ------- -------
java.util.ComparableTimSort.mergeHi(int, int, int, int)                                  48  36.64%
java.util.ComparableTimSort.mergeLo(int, int, int, int)                                  40  30.53%
java.util.ComparableTimSort.binarySort(Object[], int, int, int)                          21  16.03%
java.lang.Integer.getChars(int, int, byte[])                                              9   6.87%
BoardWorkload.main(String[])                                                              5   3.82%

                                             Garbage Collections

Start    GC ID Type                                                Heap Before GC Heap After GC Longest Pause
-------- ----- --------------------------------------------------- -------------- ------------- -------------
09:38:31     2 Young Garbage Collection                                   19.4 MB        4.6 MB       5.92 ms
09:38:31     3 Young Garbage Collection                                   57.6 MB        4.4 MB       5.93 ms
09:38:31     4 Young Garbage Collection                                   58.4 MB        3.9 MB       2.80 ms
09:38:31     5 Young Garbage Collection                                   70.9 MB        4.1 MB       3.46 ms
09:38:32     6 Young Garbage Collection                                   71.1 MB        4.0 MB       3.69 ms

jfr print --events jdk.GarbageCollection board.jfr

jdk.GarbageCollection {
  startTime = 09:38:31.079 (2026-10-27)
  duration = 5.92 ms
  gcId = 2
  name = "G1New"
  cause = "G1 Evacuation Pause"
  sumOfPauses = 5.92 ms
  longestPause = 5.92 ms
  eventThread = "VM Thread" (osThreadId = 20993)
}

import java.nio.file.Files;
import java.nio.file.Path;
import java.time.Duration;
import jdk.jfr.FlightRecorder;
import jdk.jfr.Recording;

public final class ProgrammaticJfrRecording {
    private static long workload() {
        long checksum = 0;
        for (int value = 0; value < 200_000; value++) {
            checksum += (long) value * value;
        }
        return checksum;
    }

    public static void main(String[] args) throws Exception {
        System.out.println("available=" + FlightRecorder.isAvailable());
        Path file = Files.createTempFile("board-recording", ".jfr");
        try (Recording recording = new Recording()) {
            recording.enable("jdk.CPULoad").withPeriod(Duration.ofMillis(20));
            recording.enable("jdk.ExecutionSample").withPeriod(Duration.ofMillis(10));
            recording.start();
            long checksum = workload();
            recording.stop();
            recording.dump(file);
            System.out.println("checksum=" + checksum);
        }
        System.out.println("file=" + file);
        System.out.println("bytes=" + Files.size(file));
    }
}

available=true
checksum=2666646666700000
file=/tmp/board-recording16647477040254527460.jfr
bytes=112334

import jdk.jfr.Recording;

public final class ClosedJfrRecordingFailure {
    public static void main(String[] args) {
        Recording recording = new Recording();
        recording.close();
        recording.start();
    }
}

Exception in thread "main" java.lang.IllegalStateException: Recording can only be started once.
	at jdk.jfr/jdk.jfr.internal.PlatformRecording.start(PlatformRecording.java:116)
	at jdk.jfr/jdk.jfr.Recording.start(Recording.java:177)
	at ClosedJfrRecordingFailure.main(ClosedJfrRecordingFailure.java:7)

 Event Type                              Count  Size (bytes)
=============================================================
 jdk.Checkpoint                              5          9024
 jdk.Metadata                                1        103223
 jdk.CPULoad                                 1            19

import jdk.jfr.Category;
import jdk.jfr.Event;
import jdk.jfr.Label;
import jdk.jfr.Name;
import jdk.jfr.StackTrace;

public final class CustomBoardOperationEvent {
    @Name("com.example.BoardOperation")
    @Label("Board operation")
    @Category({"Application", "Board"})
    @StackTrace(false)
    private static final class BoardOperation extends Event {
        @Label("Operation")
        String operation;

        @Label("User id")
        long userId;

        @Label("Success")
        boolean success;
    }

    static void save(long userId) {
        BoardOperation event = new BoardOperation();
        event.operation = "save-summary";
        event.userId = userId;
        event.begin();
        try {
            long checksum = 0;
            for (int value = 0; value < 10_000; value++) {
                checksum += value;
            }
            event.success = checksum > 0;
        } finally {
            event.commit();
        }
    }

    public static void main(String[] args) {
        save(7);
        System.out.println("event-committed");
    }
}

javac CustomBoardOperationEvent.java
java -XX:StartFlightRecording=filename=board-op.jfr CustomBoardOperationEvent
jfr print --events com.example.BoardOperation board-op.jfr

com.example.BoardOperation {
  startTime = 09:50:25.042 (2026-10-27)
  duration = 0.140 ms
  operation = "save-summary"
  userId = 7
  success = true
  eventThread = "main" (javaThreadId = 1)
}

import java.nio.file.Files;
import java.nio.file.Path;
import jdk.jfr.Event;
import jdk.jfr.Name;
import jdk.jfr.Recording;
import jdk.jfr.consumer.RecordedEvent;
import jdk.jfr.consumer.RecordingFile;

public final class JfrCustomEventRoundTrip {
    @Name("com.example.BoardScore")
    private static final class BoardScore extends Event {
        long userId;
        int score;
    }

    public static void main(String[] args) throws Exception {
        Path file = Files.createTempFile("board-score", ".jfr");
        try (Recording recording = new Recording()) {
            recording.enable("com.example.BoardScore");
            recording.start();
            BoardScore event = new BoardScore();
            event.userId = 7;
            event.score = 95;
            event.commit();
            recording.stop();
            recording.dump(file);
        }

        int count = 0;
        try (RecordingFile events = new RecordingFile(file)) {
            while (events.hasMoreEvents()) {
                RecordedEvent event = events.readEvent();
                if (event.getEventType().getName().equals("com.example.BoardScore")) {
                    count++;
                    System.out.println("user=" + event.getLong("userId"));
                    System.out.println("score=" + event.getInt("score"));
                }
            }
        }
        System.out.println("count=" + count);
    }
}

user=7
score=95
count=1

jcmd <pid> JFR.start name=incident settings=profile duration=60s filename=/tmp/incident.jfr
jcmd <pid> JFR.dump name=incident filename=/tmp/incident-now.jfr
jcmd <pid> JFR.stop name=incident