mvn -U (update snapshots): Usage & Common CI Errors

Force a fresh check for updated SNAPSHOT and missing release artifacts.

The -U (or --update-snapshots) flag forces Maven to check remote repositories for newer SNAPSHOT versions and to retry artifacts previously marked as missing. It is the fix for a build stuck on a stale SNAPSHOT or a cached resolution failure.

What it does

By default Maven updates SNAPSHOTs once per day. -U overrides that, re-resolving SNAPSHOTs immediately and clearing the cached "not found" markers (the _remote.repositories / resolution-cache entries) that otherwise suppress retries.

Common usage

Terminal

mvn -U package
mvn --update-snapshots verify
mvn -U -o package   # invalid: -U needs network; -o forbids it

Common error in CI (and the fix)

Symptom: CI keeps resolving an old SNAPSHOT even after a new one was published, or a transient "could not find artifact" persists after the registry recovered. Cause: Maven cached the prior resolution for the day. Fix: add -U to force a fresh remote check. Note -U requires network access, so it cannot be combined with -o (offline) - pick one.

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