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java-diagnostics

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.

skills/java-diagnostics/SKILL.md

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# Java Diagnostics

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

## Permission tiers (obey them)

| Tier | Tools | Rule |
|------|-------|------|
| 1. Read-only | `jps -l`, `jcmd <pid> help/VM.version/VM.flags/Thread.print`, `jstat`, `jstack`, reading logs | May 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 / attach | JFR (`JFR.start`, `-XX:StartFlightRecording`), async-profiler, NMT (`-XX:NativeMemoryTracking`), enabling GC logs | Requires 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 / sensitive | Heap 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 production | Requires 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.