fprime-jre

A Java runtime you can pip install. fprime-jre bundles a jlink-trimmed Eclipse Temurin Java 25 runtime and puts a java command in your Python environment, so F Prime tools that need Java — fpp and fprime-yamcs — run on machines with no Java installed, with no changes to those tools.

pip install fprime-jre
java -version
# openjdk version "25.0.4.1" 2026-08-18 LTS
# OpenJDK Runtime Environment Temurin-25.0.4.1+1 (build 25.0.4.1+1-LTS)

This is a runtime only: it runs JARs. It does not include javac, jar, jshell, sbt, or anything else needed to build Java or Scala software.

How it works

The wheel installs a java console script next to pip and python in your environment's bin/ (Scripts\ on Windows). Inside a virtual environment that directory precedes the system PATH, so tools that look up java on the PATH — fpp-check and friends, yamcsd — find this one. The script execs the bundled runtime with your arguments, stdio, exit code and signals unchanged.

Java 25 is the version required by fpp's published JARs and supported by YAMCS.

Which Java wins?

java from fprime-jre is unconditional: if it is invoked, the bundled runtime runs. It never looks at JAVA_HOME. Choosing between a Java you already have and this one is the caller's job, in the conventional JAVA_HOME then PATH order:

• yamcsd uses $JAVA_HOME/bin/java when JAVA_HOME is set, otherwise java on the PATH.
• fpp's wrapper currently uses the PATH only (nasa/fpp#1107 requests JAVA_HOME be honoured first).

So: set JAVA_HOME and tools use your Java; leave it unset inside an environment with fprime-jre and they use this one. To point other tooling at the bundled runtime:

export JAVA_HOME="$(fprime-jre)"

From Python

import subprocess
from fprime_jre import JAVA, JAVA_HOME, JAVA_VERSION  # Path, Path, (25, 0, 4, 1)

subprocess.run([str(JAVA), "-jar", "tool.jar"])

Platforms

Wheels are published for Linux x86_64 and aarch64 (glibc 2.17+), macOS x86_64 and arm64 (11.0+), and Windows x64. Alpine/musl is not supported. Wheels are platform-specific but Python-version independent (py3-none-<platform>), for Python 3.8+.

Versioning

The package version is the Temurin JAVA_VERSION followed by a repackaging number: 25.0.4.1.0 bundles Temurin jdk-25.0.4.1+1. Consumers wanting Java 25 can depend on fprime-jre>=25,<26.

Building

Wheels are built by CI, one job per platform, since the runtime must be linked on its target OS. To build locally (Python 3.11+ for the build script):

python build_runtime.py               # downloads Temurin from Adoptium, verifies SHA-256, runs jlink
python -m build --wheel               # dist/fprime_jre-<version>-py3-none-<platform>.whl
pip install dist/*.whl pytest && pytest tests

FPRIME_JRE_PLATFORM_TAG overrides the wheel's platform tag (CI sets the manylinux/macosx tags). Optional integration tests run when FPRIME_JRE_TEST_FPP_JAR (path to an fpp.jar) or FPRIME_JRE_TEST_YAMCS_DIR (an unpacked YAMCS bundle) are set.

Updating Java: change TEMURIN_RELEASE in build_runtime.py and the version in pyproject.toml together; the build fails if they disagree. Publishing is by GitHub release through publish.yml using PyPI trusted publishing.

License

The Python code in this repository and the bundled OpenJDK runtime are distributed under the GNU General Public License, version 2, with the Classpath Exception (LICENSE, ASSEMBLY_EXCEPTION, ADDITIONAL_LICENSE_INFO). Per-module third-party notices ship inside the runtime under legal/; Temurin's notices are in NOTICE.

Java and OpenJDK are trademarks or registered trademarks of Oracle and/or its affiliates. Eclipse Temurin is a trademark of the Eclipse Foundation.