Running Java applications with jgo

This example shows how to run an arbitrary package available from a Maven repository using jgo.

import cjdk
import jgo
import os
from contextlib import contextmanager

jgo requires Apache Maven, so we’ll install that first:

# From https://maven.apache.org/download.html
maven_url = "tgz+https://dlcdn.apache.org/maven/maven-3/3.9.12/binaries/apache-maven-3.9.12-bin.tar.gz"
maven_sha512 = "0a1be79f02466533fc1a80abbef8796e4f737c46c6574ede5658b110899942a94db634477dfd3745501c80aef9aac0d4f841d38574373f7e2d24cce89d694f70"

maven_dir = cjdk.cache_package("Maven", maven_url, sha512=maven_sha512)

cjdk: Installing Maven to /home/runner/.cache/cjdk

The Maven .tar.gz file has been extracted into maven_dir; find the bin directory within it:

maven_bin = list(maven_dir.glob("apache-maven-*"))[0] / "bin"
assert (maven_bin / "mvn").is_file()

Let’s write a context manager that we can use to temporarily put the Maven bin directory on PATH.

@contextmanager
def path_prepended(path):
    """
    Context manager to temporarily prepend the given path to PATH.
    """
    save_path = os.environ.get("PATH", "")
    new_path = str(path) + os.pathsep + save_path
    os.environ["PATH"] = new_path
    try:
        yield
    finally:
        os.environ["PATH"] = save_path

Now for the magic: run a program by specifying its Maven coordinates.

The JRE and all required Jars are downloaded (and cached) automatically.

with cjdk.java_env(vendor="zulu-jre", version="8"):
    with path_prepended(maven_bin):
        jgo.main_from_endpoint(
            "com.puppycrawl.tools:checkstyle:9.3",
            primary_endpoint_main_class="com.puppycrawl.tools.checkstyle.Main",
            argv=["--version"],
        )

cjdk: Installing JDK zulu-jre:8.0.472 to /home/runner/.cache/cjdk

INFO 2026-01-20 14:53:15,526: First time start-up may be slow. Downloaded dependencies will be cached for shorter start-up times in subsequent executions.

Checkstyle version: 9.3