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java-diagnostics

Skill41k starsChanged 6 days agoMITView on GitHub

Symptom-first runtime diagnostics for JVM applications using JDK tools: jcmd, jstack, jmap, jstat, JFR, GC logs, async-profiler, Native Memory Tracking, and Eclipse MAT. Use when a Java app has high CPU, memory leaks or OutOfMemoryError, slow startup, deadlocks or thread starvation, long GC pauses, native memory growth, or slow requests. Includes permission tiers for safe use.

We split this skill into the rules it gives an agent

6 rules. About 0 are common, 6 are distinctive, and 0 are very specific to this project.

  • ≈0

    Common · estimate

    Many files like this have a rule of this kind.

  • ≈6

    Distinctive · estimate

    This author's own choices. Worth a look.

  • 0

    Very specific

    Tied to their project's names, files or tools. Probably skip.

  • Common
  • Distinctive
  • Very specific
  1. Distinctive:

    Never run Tier 2 or Tier 3 commands against production unless the user explicitly instructs it.

    line 18
  2. Distinctive:

    Start with jps/jcmd basics to identify the target JVM and confirm which JDK version is running.

    line 33
  3. Distinctive:

    Ask the user for consent before adding any dependency or installing the skill.

    line 52
  4. Distinctive:

    Use it in a local/dev profile only.

    line 53
  5. Distinctive:

    Check the capability and version first (read its README for the supported Spring Boot / Quarkus versions) instead of assuming features.

    line 54
  6. Distinctive:

    Follow the project's own documentation for usage.

    line 56

≈ “Common” is an estimate: at least 1 in 5 of the 308,232 skill files we read have a rule of the same kind, not the same words. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

Keep what fits and drop the rest. Your version, ready for your agents.
Files
SKILL.md

Java Diagnostics

Pick the lightest tool that answers the question, starting from the symptom.

Permission tiers (obey them)

TierToolsRule
1. Read-onlyjps -l, jcmd <pid> help/VM.version/VM.flags/Thread.print, jstat, jstack, reading logsMay run after stating the exact command. Class histograms (jcmd <pid> GC.class_histogram, jmap -histo, jmap -histo:live) are NOT Tier 1: they run heap inspection at a safepoint and can pause application threads on a large heap, so treat them as Tier 2 (consent, stated overhead). The default forms and -histo:live additionally force a full GC; -all=true / jmap -histo skip the full GC but still pause during the scan (and count unreachable objects).
2. Recording / attachJFR (JFR.start, -XX:StartFlightRecording), async-profiler, NMT (-XX:NativeMemoryTracking), enabling GC logsRequires user consent. State duration and expected overhead (JFR default profile is about 1-2%; async-profiler depends on mode; NMT summary roughly 5-10%, detail more) before running.
3. Intrusive / sensitiveHeap dumps (GC.heap_dump, jmap -dump), jhsdb, commands that print JVM arguments or system properties (jps -v/-lv/-m, bare jcmd, VM.system_properties, VM.command_line, jinfo, ps aux on a Java process; they can expose secrets, redact output), JDWP with suspend, anything on productionRequires explicit consent, a stated target path, free-disk check (a dump is about the size of the used heap), and a stated pause. Heap dumps contain secrets (passwords, tokens, personal data). Never share or upload them; redaction is NOT a reliable safeguard. Analyze locally and delete afterwards.

Never run Tier 2 or Tier 3 commands against production unless the user explicitly instructs it. Run diagnostic tools as the same OS user as the target JVM. Prefer reproducing on dev or staging.

Decision table

SymptomFirst stepThenReference
High CPUThread dumps x3 (Tier 1) + top -H to map hot native thread ids to nidJFR or async-profiler CPU flame graph (Tier 2)threads, jfr, async-profiler
Memory leak / OOMjstat -gcutil, GC.class_histogram -all=true twice, minutes apart (Tier 2: safepoint pause even with -all=true; without it a full GC too; get consent on production)Heap dump + Eclipse MAT (Tier 3) or JFR OldObjectSample (Tier 2)heap, gc
Slow startup-Xlog:class+load, framework startup/condition reports, timestamped logsJFR from start, async-profiler wall-clockjfr, async-profiler
Deadlock / thread starvationjcmd <pid> Thread.print (reports deadlocks), compare dumps over timeVirtual threads: Thread.dump_to_filethreads
GC pausesGC log (-Xlog:gc*) and jstat -gcJFR GC events; tune collector/heapgc
Native memory growth (RSS grows, heap does not)Compare RSS vs heap; enable NMT, jcmd VM.native_memory baseline / summary.diff (Tier 2)async-profiler nativemem or allocator toolsnmt, containers
Slow requestsTracing/metrics and logs first; Actuator or Dev UI (see below)JFR (socket/file/lock events), async-profiler wall-clockjfr, async-profiler
JVM crash / hang, no tools respondhs_err_pid*.log, core dumpjhsdb (Tier 3)basics

Start with jps/jcmd basics to identify the target JVM and confirm which JDK version is running.

Workflow

  1. Capture the symptom precisely (when, how often, since which change, JDK version, container limits).
  2. Choose the tier needed; state the command, its overhead, and output location, and get consent for Tier 2/3.
  3. Collect data to a dedicated directory, with timestamps. Prefer several samples over one.
  4. Form a hypothesis from the data, then confirm with a second source (for example a thread dump plus a flame graph).
  5. Fix, re-measure with the same method, and clean up artifacts (dumps, recordings).

Application-level insights (optional)

JDK tools see the JVM; some questions (which endpoint, which SQL query, which bean) are easier at the application level. Optional options, in order of preference:

  • Spring Boot Actuator (metrics, health, loggers, httpexchanges) and Quarkus Dev UI / Micrometer metrics are built in and need no extra tool. Expose Actuator endpoints minimally; For heapdump and threaddump, follow the policy in the spring-observability skill: never expose them over public HTTP; the only exception is a temporary, user-consented window with authentication, TLS and network restriction, removed afterwards.
  • BootUI (https://github.com/jdubois/boot-ui , https://www.julien-dubois.com/boot-ui/): a local-only developer console with an opt-in MCP server for Spring Boot and Quarkus. Optional. Its skill can be installed with gh skill install jdubois/boot-ui skills/bootui.

If considering BootUI:

  • Ask the user for consent before adding any dependency or installing the skill.
  • Use it in a local/dev profile only; never expose it on a network, and never enable its heap dump, thread dump endpoints or MCP server in production.
  • Check the capability and version first (read its README for the supported Spring Boot / Quarkus versions) instead of assuming features.

Follow the project's own documentation for usage; this skill does not reproduce it.

Containers

See containers for attaching tools inside containers, -XX:MaxRAMPercentage, and cgroup memory limits.

References

  • jps / jcmd basics
  • Thread dumps and deadlocks
  • Heap, histograms, Eclipse MAT
  • GC logs and jstat
  • JFR and JDK Mission Control
  • async-profiler
  • Native Memory Tracking
  • Containers
  • Debugger-based investigation: the java-debugging skill.

Official docs (prefer them over this skill for the JDK in use; change 21 to your version): jcmd https://docs.oracle.com/en/java/javase/21/docs/specs/man/jcmd.html, jfr https://docs.oracle.com/en/java/javase/21/docs/specs/man/jfr.html, JFR guide https://docs.oracle.com/en/java/javase/21/jfr/introduction-jdk-flight-recorder.html, troubleshooting guide https://docs.oracle.com/en/java/javase/21/troubleshoot/, async-profiler releases https://github.com/async-profiler/async-profiler/releases.

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The biggest 5 of 86 mentions linking to the repository that our crawl found. Each line is their own headline.

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Talked about on Bluesky, GitHub, YouTube, Hacker News and the web (86 times).

On Bluesky · 40

35 more on Bluesky

On GitHub · 34

29 more on GitHub

On YouTube · 5

On the web · 2

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