스레드 덤프와 프로파일러로 장애 원인 찾기

public final class DiscardedExceptionCauseBug {
    static int parseViewCount(String value) {
        try {
            return Integer.parseInt(value);
        } catch (NumberFormatException exception) {
            throw new IllegalArgumentException("invalid view count: " + value);
        }
    }

    public static void main(String[] args) {
        try {
            parseViewCount("forty");
        } catch (IllegalArgumentException exception) {
            System.out.println("message=" + exception.getMessage());
            System.out.println("wrong-cause-null=" + (exception.getCause() == null));
        }
    }
}

message=invalid view count: forty
wrong-cause-null=true

public final class DiagnosticExceptionBoundary {
    static int parseViewCount(String value) {
        try {
            int viewCount = Integer.parseInt(value);
            if (viewCount <= 0) {
                throw new IllegalArgumentException("not positive");
            }
            return viewCount;
        } catch (RuntimeException exception) {
            throw new IllegalArgumentException("parse board view count failed", exception);
        }
    }

    public static void main(String[] args) {
        try {
            parseViewCount("forty");
        } catch (IllegalArgumentException exception) {
            exception.printStackTrace(System.out);
        }
        try {
            parseViewCount("0");
        } catch (IllegalArgumentException exception) {
            System.out.println("0 -> " + exception.getMessage() + ", cause=" + exception.getCause());
        }
    }
}

java.lang.IllegalArgumentException: parse board view count failed
	at DiagnosticExceptionBoundary.parseViewCount(DiagnosticExceptionBoundary.java:10)
	at DiagnosticExceptionBoundary.main(DiagnosticExceptionBoundary.java:16)
Caused by: java.lang.NumberFormatException: For input string: "forty"
	at java.base/java.lang.NumberFormatException.forInputString(NumberFormatException.java:67)
	at java.base/java.lang.Integer.parseInt(Integer.java:662)
	at java.base/java.lang.Integer.parseInt(Integer.java:778)
	at DiagnosticExceptionBoundary.parseViewCount(DiagnosticExceptionBoundary.java:4)
	... 1 more
0 -> parse board view count failed, cause=java.lang.IllegalArgumentException: not positive

import java.time.Instant;
import java.util.List;

public final class DiagnosticHypothesisModel {
    private enum Status {
        OPEN,
        CONFIRMED,
        REJECTED
    }

    private record Hypothesis(
            String symptom,
            String claim,
            List<String> evidenceNeeded,
            String falsifier,
            Instant startedAt,
            Status status) {}

    public static void main(String[] args) {
        Hypothesis lock =
                new Hypothesis(
                        "save p99 > 2s",
                        "threads wait on PostRepository monitor",
                        List.of("three thread dumps", "JFR JavaMonitorEnter", "request timestamps"),
                        "no blocked threads and monitor wait below 10ms",
                        Instant.parse("2026-07-14T10:00:00Z"),
                        Status.OPEN);
        System.out.println(lock);
    }
}

Hypothesis[symptom=save p99 > 2s, claim=threads wait on PostRepository monitor, evidenceNeeded=[three thread dumps, JFR JavaMonitorEnter, request timestamps], falsifier=no blocked threads and monitor wait below 10ms, startedAt=2026-07-14T10:00:00Z, status=OPEN]

import java.lang.management.ManagementFactory;
import java.lang.management.ThreadInfo;
import java.lang.management.ThreadMXBean;
import java.util.concurrent.CountDownLatch;

public final class ThreadStateEvidence {
    public static void main(String[] args) throws Exception {
        CountDownLatch waiting = new CountDownLatch(1);
        CountDownLatch release = new CountDownLatch(1);
        Thread worker =
                Thread.ofPlatform()
                        .name("board-waiter")
                        .start(
                                () -> {
                                    waiting.countDown();
                                    try {
                                        release.await();
                                    } catch (InterruptedException exception) {
                                        Thread.currentThread().interrupt();
                                    }
                                });

        waiting.await();
        try {
            ThreadMXBean threads = ManagementFactory.getThreadMXBean();
            ThreadInfo info = threads.getThreadInfo(worker.threadId(), 8);
            if (info == null) {
                System.out.println("no thread info");
            } else {
                StackTraceElement[] stack = info.getStackTrace();
                System.out.println("name=" + info.getThreadName());
                System.out.println("state=" + info.getThreadState());
                System.out.println("top=" + (stack.length > 0 ? stack[0] : "(empty stack)"));
            }
        } finally {
            release.countDown();
        }
        worker.join();
    }
}

name=board-waiter
state=WAITING
[email protected]/jdk.internal.misc.Unsafe.park(Native Method)

import java.lang.management.ManagementFactory;
import java.lang.management.ThreadMXBean;
import java.time.Duration;

public final class CpuAndWallTimeEvidence {
    public static void main(String[] args) throws Exception {
        ThreadMXBean threads = ManagementFactory.getThreadMXBean();
        if (!threads.isCurrentThreadCpuTimeSupported()) {
            System.out.println("cpu-time-unsupported");
            return;
        }
        if (!threads.isThreadCpuTimeEnabled()) {
            threads.setThreadCpuTimeEnabled(true);
        }

        long wallStart = System.nanoTime();
        long cpuStart = threads.getCurrentThreadCpuTime();
        Thread.sleep(Duration.ofMillis(20));
        long checksum = 0;
        for (int value = 0; value < 100_000; value++) {
            checksum += (long) value * value;
        }
        long cpu = threads.getCurrentThreadCpuTime() - cpuStart;
        long wall = System.nanoTime() - wallStart;
        System.out.println("wall-ns=" + wall);
        System.out.println("cpu-ns=" + cpu);
        System.out.println("checksum=" + checksum);
    }
}

wall-ns=22462200
cpu-ns=2282093
checksum=333328333350000

jcmd -l
jcmd <pid> Thread.print -l
jcmd <pid> JFR.start name=incident settings=profile duration=60s filename=/tmp/incident.jfr
jfr summary /tmp/incident.jfr