OpenJDK

The reference Java implementation — compiler, runtime and tooling.

What it does

OpenJDK provides javac, java, jshell, jar, and the diagnostic tools (jcmd, jstack, jmap) used to inspect a running JVM. It is the upstream all commercial JDK builds derive from.

Where it fits

Mostly as a dependency: Maven needs a JDK, and several self-hosted homelab services — anything Kafka, Elasticsearch, Minecraft or Jenkins-adjacent — ship as JVM applications.

Notes

Homebrew installs OpenJDK keg-only, so it is not linked into PATH automatically. To let macOS’s /usr/libexec/java_home find it:

sudo ln -sfn /opt/homebrew/opt/openjdk/libexec/openjdk.jdk \
  /Library/Java/JavaVirtualMachines/openjdk.jdk

Alternative to

Alternative	Type	Trade-off
Eclipse Temurin	Open source	The most widely used prebuilt OpenJDK, from Adoptium
Amazon Corretto and Azul Zulu	Free	Long-term-supported builds with vendor backing
GraalVM	Open source	Adds native-image ahead-of-time compilation
Oracle JDK	Free	The same code, different licence terms