maven: what does ` -U,--update-snapshots` really do?
maven
Solution
If you do not use `-U`, maven might cache results - even if a dependency could not be found previously (e.g. because your nexus [or alike] was unavailable, misconfigured, didn't contain the dependency [yet] or whatever). SNAPSHOT versioned jars are cached similarly.
If that's the case. Maven follows the repository's `updatePolicy`, which tells it how often (if ever) maven checks if a dependency has been updated (in the case of SNAPSHOT), or has become available, in the case of a released version. Default is `daily` therefore if a temp error causes maven to not download a dependency, it might take one day before maven tries again. `-U` overwrites that and tells it to check now.
`-U` does not re-download a SNAPSHOT dependency if it has already been downloaded and if the checksum is the same! It only checks for the checksum.
Update: as @Stas pointed out, if the checksum differs, it will re-download and override you local JARs with the ones from the remote repository.
** `-U` also checks for "updated" release versions if you specify a "version" range etc.
BTW: Maven uses a timestamp file that has the same name as the dependency + `".lastUpdated"` to know when a dependency has been last checked on which server. E.g. `~/.m2/repository/org/springframework/spring-webmvc/3.1.2.RELEASE/spring-webmvc-3.1.2.RELEASE.jar.lastUpdated`
Example for updatePolicy:
<repositories>
<repository>
<releases>
<enabled>false</enabled>
<updatePolicy>always</updatePolicy>
</releases>
<snapshots>
<enabled>true</enabled>
<updatePolicy>never</updatePolicy>
</snapshots>
<!-- ... -->
</repository>
<!-- ... -->
</repositories>
See http://maven.apache.org/pom.html#Repositories for further information about the updatePolicy.
Problem
In the command line help, I see that maven "checks" for updates: ``` -U,--update-snapshots Forces a check for updated releases and snapshots on remote repositories ``` However, most questions on Stack Overflow imply that this option forces Maven to update. Does this mean it forces a re-download of the dependencies? e.g. https://stackoverflow.com/a/9697970/1119779