JVM introspection

Install

emisar pack install fetches this pack, re-validates it, and verifies its content hash against the --hash below — the exact bytes this page was rendered against, so a tampered copy is rejected — before copying it into the runner's packs dir. The command reloads a running daemon itself; no manual restart.

sudo emisar pack install java-jvm --hash sha256:2060fc8e75684d5d0ae0bd6456fb56284eb72e77c93f116ac3e867865724c787

• jvm.classloader_stats exec low

  jcmd VM.classloader_stats

  Show per-classloader statistics — instance count, loaded classes, and chunk/block bytes. Use to spot a class-loader leak. Uses the attach API, so it works on JVMs running -XX:+PerfDisableSharedMem (the Cassandra default) where jvm.jstat_class fails.

  View source on GitHub
• jvm.heap_dump exec critical

  jmap -dump:live (heap dump)

  Triggers a full GC, then dumps the live heap to /tmp/<pid>-heap.hprof. Can be many GB. Causes a long STW pause (seconds). Use only when you have analyst capacity to load the .hprof and disk space to hold it.

  View source on GitHub
• jvm.heap_summary exec low

  jcmd GC.heap_info

  Show heap geometry — used/committed per generation (Eden, Survivor, Old, Metaspace) via the attach API. Works on JDK 9+ where `jmap -heap` was removed, and on JVMs running -XX:+PerfDisableSharedMem (e.g. Cassandra).

  View source on GitHub
• jvm.jcmd_gc_run exec medium

  jcmd <pid> GC.run

  Trigger a full GC. Use to compare heap usage before/after, or to force collection ahead of a heap dump for cleaner output. Causes a pause whose length depends on heap size + collector tuning.

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• jvm.jfr_dump exec medium

  Dump active JFR recording

  Dumps the active "emisar" recording to its current file. Use mid-flight to grab a snapshot without stopping the recording.

  View source on GitHub
• jvm.jfr_start exec medium

  Start a Java Flight Recording

  Starts a 60-second Java Flight Recording named "emisar". JFR is low-overhead (~1%). Pair with `jvm.jfr_dump` to retrieve the recording, then `jvm.jfr_stop` to end it.

  View source on GitHub
• jvm.jfr_stop exec medium

  Stop active JFR recording

  Stops the "emisar" recording and finalizes the .jfr file.

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• jvm.jmap_histo_live exec medium

  jmap -histo:live (top 50)

  List top classes by retained-heap size after a forced full GC. Use to find the leak candidate.

  View source on GitHub
• jvm.jstack exec low

  jstack -l (thread dump)

  Dump full thread state with lock info. The canonical "what are all my threads doing?" diagnostic.

  View source on GitHub
• jvm.jstack_blocked exec low

  jstack filtered to BLOCKED threads

  Runs jstack and filters output to threads in BLOCKED state plus their lock owners. Use to find deadlocks fast.

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• jvm.jstat_class exec low

  jstat -class

  Show class-loader stats — loaded class count and bytes. Use to spot a class-loading leak. Needs jvmstat perf data: fails with "<pid> not found" on JVMs running -XX:+PerfDisableSharedMem (the Cassandra default) — there use jvm.jmap_histo_live (live-object histogram) or JFR instead.

  View source on GitHub
• jvm.jstat_gc exec low

  jstat -gc (10 samples)

  Show ten 1-second samples of GC stats. Surfaces young + old GC time, survivor sizes, metaspace usage. Use to spot a GC storm. Needs jvmstat perf data: fails with "<pid> not found" on JVMs running -XX:+PerfDisableSharedMem (the Cassandra default) — there use jvm.heap_summary for heap state, or JFR (jvm.jfr_start / jvm.jfr_dump) for GC behavior over time.

  View source on GitHub
• jvm.perfdata exec low

  jcmd PerfCounter.print

  Show JVM performance counters — class loading, JIT, GC. Cheaper than jstat for one-shot. Reads the same jvmstat perf data as jstat: fails on JVMs running -XX:+PerfDisableSharedMem (the Cassandra default) — there use the attach-based reads (jvm.heap_summary, jvm.vm_flags, jvm.jstack) or JFR.

  View source on GitHub
• jvm.thread_count exec low

  OS thread count for a JVM

  Counts /proc/<pid>/task entries — fastest way to spot "do we have an unbounded thread pool?"

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• jvm.vm_flags exec low

  jcmd VM.flags

  Returns the JVM command-line and tuning flags. Use to confirm "is -Xmx set to what we expect?"

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• jvm.vm_system_properties exec low

  jcmd VM.system_properties

  Dump every -Dkey=value system property the JVM was started with (jcmd VM.system_properties = System.getProperties()). Use to confirm config overrides took effect — but note this deliberately surfaces every launch -D property, which routinely carries secrets (spring.datasource.password, javax.net.ssl.keyStorePassword, JDBC / cloud credentials). Scope it by policy. The runner's redaction is a fail-closed backstop, not a guarantee — it is pattern-bound and can miss a bespoke secret whose name and value match no rule.

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• jvm.vm_uptime exec low

  jcmd VM.uptime

  Show how long this JVM has been running.

  View source on GitHub