# 5. jlink: 133 MB smaller, 15 metrics quieter

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[`Dockerfile.jlink-distroless`](../docker/Dockerfile.jlink-distroless) asks `jdeps` which JDK modules
the application needs and builds a runtime with only those:

```bash
jdeps --ignore-missing-deps -q --recursive --multi-release 25 --print-module-deps \
      --class-path 'extracted/dependencies/lib/*' extracted/application/application.jar
```

It chose 18 modules ([`jlink-metrics.txt`](output/jlink-metrics.txt)) - including `java.desktop`,
because Spring uses `java.beans`. On `distroless/java-base` the image is **119 MB** on disk against
252 MB for distroless with the full JRE.

## It runs. It is also missing something.

The jdeps-only image starts, serves requests and passes its health check. Its startup log has two
warnings:

```
i.m.c.i.binder.jvm.JvmGcMetrics          : GC notifications will not be available because com.sun.management.GarbageCollectionNotificationInfo is not present
i.m.c.i.binder.jvm.JvmGcMetrics          : GC notifications will not be available because no GarbageCollectorMXBean of the JVM provides any. GCs=[G1 Young Generation, G1 Concurrent GC, G1 Old Generation]
```

and `/actuator/prometheus` exports 46 metric names instead of 61. Gone: `jvm_gc_pause_seconds_*`,
`jvm_gc_memory_allocated_bytes_total`, `jvm_gc_live_data_size_bytes`, `process_cpu_usage`,
`system_cpu_usage`, `process_files_open_files` and more - 15 in total.

`com.sun.management.*` lives in the `jdk.management` module, and Micrometer touches it
reflectively, which `jdeps` cannot see. Nothing fails; the GC and CPU panels of your dashboard just
go flat after the image switch. The Dockerfile now adds it by default:

```dockerfile
ARG EXTRA_MODULES="jdk.management"
```

With it, the only difference left is `jvm_gc_concurrent_phase_time_*`, which Micrometer registers
lazily inside its GC notification listener - it appears after the first G1 concurrent cycle, which
may or may not have happened a few seconds after startup, in either image.

The general rule: diff the metric names, not just the health check, before shipping a jlink image.
Other modules commonly needed only reflectively are `jdk.crypto.cryptoki` (PKCS#11),
`jdk.localedata` (non-English locale data) and `jdk.naming.dns` (DNS lookups through JNDI).