cannot pull dev versions of JARs, please run "gradlew

Error message

cannot pull dev versions of JARs, please run "gradlew %v" to start your expansion service

What it means

getJar refuses to download JARs whose version string contains ".dev" (dev/snapshot-style versions), telling the user to build and start the expansion service locally with gradlew <target> instead. Dev-version artifacts are not published to the public repositories the downloader queries.

Solutions

Run the suggested Gradle task locally: ./gradlew :sdks:java:extensions:<target-module>:runExpansionService (or the gradleTarget named in the error) so the JAR is cached.
Use a released Beam version (no ".dev") so the JAR can be downloaded from Maven.
Pre-place the built JAR in the expected cache location so jarExists finds it.
Set up a local repository/mirror containing the dev artifact if you must use dev versions.

Example fix

// before: dev Go binary expecting auto-download
version := "2.62.0.dev" // no public artifact exists
// after: build the service once locally
$ ./gradlew :sdks:java:container:expansion-service:shadowJar
// or use a released version
version := "2.62.0"

Defensive patterns

Strategy: fallback

Validate before calling

if strings.Contains(beamVersion, ".dev") {
    jarPath := filepath.Join(cacheDir, expectedJarName(gradleTarget, beamVersion))
    if _, err := os.Stat(jarPath); err != nil {
        // build it locally first: ./gradlew <gradleTarget>
    }
}

Try / catch

jar, err := expansionx.GetBeamJar(gradleTarget, repo, version)
if err != nil && strings.Contains(err.Error(), "dev versions") {
    return startLocalGradleExpansionService(gradleTarget) // fallback path
}

Prevention

Use released Beam versions in production pipelines.
Pre-build dev-version expansion service JARs via gradlew before running Go pipelines from source.
Pre-place the JAR in the expected cache path so download is skipped.

When it happens

Trigger: Using a Beam Go binary built from a dev/snapshot source (version like 2.XX.0.dev) and relying on automatic Java expansion service download (startAutomatedJavaExpansionService / xlang transforms) where the JAR is not already cached.

Common situations: Running a Beam Go pipeline built from source without first building the Java expansion service; CI using a nightly/dev Beam build; mispinned version in a custom repository that lacks dev artifacts.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/019f8437f506958c. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/runtime/xlangx/expansionx/download.go:358

}

func (j *jarGetter) getJar(gradleTarget, version string) (string, error) {
	strippedTarget := dropEndOfGradleTarget(gradleTarget)
	fullURL, jarName := j.getURLForBeamJar(strippedTarget, version)

	err := os.MkdirAll(j.jarCache, 0700)
	if err != nil {
		return "", err
	}

	jarPath := filepath.Join(j.jarCache, jarName)

	if jarExists(jarPath) {
		return jarPath, nil
	}

	if strings.Contains(version, ".dev") {
		return "", fmt.Errorf("cannot pull dev versions of JARs, please run \"gradlew %v\" to start your expansion service",
			gradleTarget)
	}

	// Issue warning when downloading from public repositories
	fullURLStr := string(fullURL)
	if strings.Contains(fullURLStr, "repo.maven.apache.org") ||
		strings.Contains(fullURLStr, "repo1.maven.org") ||
		strings.Contains(fullURLStr, "maven.google.com") ||
		strings.Contains(fullURLStr, "maven-central.storage-download.googleapis.com") {
		log.Printf("WARNING: Downloading JAR file from public repository: %s. "+
			"This may pose security risks or cause instability due to repository availability. Consider pre-staging dependencies or using private mirrors.", fullURLStr)
	}

	resp, err := http.Get(string(fullURL))
	if err != nil {
		return "", err
	}
	defer resp.Body.Close()

View on GitHub (pinned to 12126d8942)