What causes "Unknown error during artifact resolution, {}, {}" in apache/maven?

Read the exception list embedded in the message and re-run with -e -X for full stack traces - the actionable cause is in result.getExceptions()

apache/maven · error · ArtifactResolutionException

Unknown error during artifact resolution, {}, {}

Error message

Unknown error during artifact resolution, {}, {}

What it means

DefaultResolutionErrorHandler.onErrors converts an ArtifactResult into legacy exceptions; after handling missing artifacts it checks result.hasExceptions() again and, if exceptions remain, wraps them in ArtifactResolutionException ('Unknown error during artifact resolution') - the code itself comments this 'should never happen'. It is a defensive catch-all for resolver failures that are neither not-found nor already-handled, and the real causes are printed in the message via result.getExceptions().

Source

Thrown at compat/maven-compat/src/main/java/org/apache/maven/artifact/resolver/DefaultResolutionErrorHandler.java:74

        }

        // Transfer Error

        if (result.hasErrorArtifactExceptions()) {
            throw result.getErrorArtifactExceptions().get(0);
        }

        if (result.hasMissingArtifacts()) {
            throw new MultipleArtifactsNotFoundException(
                    request.getArtifact(),
                    toList(result.getArtifacts()),
                    result.getMissingArtifacts(),
                    request.getRemoteRepositories());
        }

        // this should never happen since we checked all possible error sources before but better be sure
        if (result.hasExceptions()) {
            throw new ArtifactResolutionException(
                    "Unknown error during artifact resolution, " + request + ", " + result.getExceptions(),
                    request.getArtifact(),
                    request.getRemoteRepositories());
        }
    }

    private static <T> List<T> toList(Collection<T> items) {
        return (items != null) ? new ArrayList<>(items) : null;
    }
}

View on GitHub (pinned to e4093d4e12)

Solutions

Read the exception list embedded in the message and re-run with -e -X for full stack traces - the actionable cause is in result.getExceptions()
Fix the dominant underlying exception (usually a TransferFailedException, SSL, or disk error)
If reproducible, audit and remove conflicting resolver/wagon extensions declared in the build or settings
For intermittent network cases, retry the build and route repositories through one stable internal mirror

Defensive patterns

Strategy: try-catch

Try / catch

Catch ArtifactResolutionException and iterate the exception chain (getCause() and getExceptions() where available); find the first TransferFailed/IO exception and handle that root cause. Treat the 'Unknown error during artifact resolution' message as a wrapper signal, log the children verbatim.

Prevention

Run CI builds with -e and archive the debug log for post-mortem.
Route all repositories through a single internal mirror to reduce multi-repository fan-out failures.
Keep resolver/wagon extensions minimal and current; remove ones you no longer need.

When it happens

Trigger: Artifact resolution completing with exception(s) that are not ArtifactNotFoundException and with no missing-artifact list: simultaneous multi-repository transport errors, a custom Wagon or resolver extension throwing, checksum-policy failures after local file cleanup, or disk-full while spooling artifacts.

Common situations: Flaky networks producing concurrent failures across several repositories; custom resolver extensions in the build misbehaving; repository managers resetting long-lived connections; disk exhaustion during download.

Related errors

AI-assisted analysis of apache/maven@e4093d4e12 (2026-08-21). Data as JSON: /api/errors/1dc2a4ae50285bc8. Report an issue: GitHub.