Java on Unikraft #

A Java HTTP service on a jlink'd JRE, built with Maven.

Detected by a pom.* (any of Maven's polyglot dialects) or a gradlew wrapper. Gradle and Maven builds of Kotlin, Groovy or Scala come through here too — at this level what matters is the build tool, not the language. (sbt has its own provider; see ../unikraft-scala.)

The runtime is jlinked rather than shipped whole: a full JRE would be far too large for an image that is resident twice at boot. The module set comes from jdeps, unioned with a broad fallback — jdeps sees only what the bytecode references, so anything reached by reflection (a JDBC driver, a logging backend, a crypto provider) is invisible to it, and the failure mode for guessing low is a ClassNotFoundException in a guest that cannot be fixed without a rebuild.

The JVM flags are load-bearing, not tuning. The guest is a single CPU with no cgroup to read limits from and no perf-data file to write; left to itself the JVM sizes a GC thread pool from a CPU count it cannot determine and dies before main().

There is no Dockerfile, no Kraftfile and no build.sh — bsdkrun pack detects the project and generates all three internally.

Build #

bsdkrun pack .

Run #

bsdkrun unikraft . --cmdline "java -- /opt/jre/bin/java -XX:+UseSerialGC -XX:ActiveProcessorCount=1 -XX:-UseContainerSupport -XX:-UsePerfData -XX:TieredStopAtLevel=1 -Xmx256m -jar /usr/src/server.jar"

pack prints that command when it finishes.

Try it #

curl http://<vm-ip>:8080/

Publish it #

bsdkrun pack . --push ghcr.io/you/java:v1
bsdkrun unikraft ghcr.io/you/java:v1

The second command needs no copy of this directory: the kernel is pulled on first use and cached, and the argv comes from the image.