Why do the Sonatype docs suggest redefining the central repository with a bogus URL in settings.xml when using mirrorOf?

According to the Maven documentation:

You can force Maven to use a single repository by having it mirror all repository requests. The repository must contain all of the desired artifacts, or be able to proxy the requests to other repositories. This setting is most useful when using an internal company repository with the Maven Repository Manager to proxy external requests.

To ac开发者_如何学Chieve this, set mirrorOf to *.

This StackOverflow question also suggests that setting mirrorOf is sufficient to block an external repository, so why does the Sonatype documentation suggest overloading central with an unreachable URL?

The bogus URL is really irrelevant - you can set it to the original one if you need to, or the URL of your repository manager - as long as the mirrorOf is applicable, it won't be used.

The reason these examples redefine central is to set policies on artifact requests to the default repositories. By default, Maven does not enable snapshot requests to central, and uses default update and checksum policies. Redeclaring central allows these to be overridden - in this case, to enable snapshot artifacts and plugins, and the mirror then redirects all of these to the repository manager. This avoids the need to declare the repositories in your POM (as long as all users have their settings correct).

I wrote that so I can tell you what I was thinking ;-)

The central repository definition needs to be updated to enable snapshot retrieval for at least one repo, otherwise Maven won't even ask the repository manager (pointed to by the mirrorOf) for any snapshots.

While not required, I like to change the definition of the url to be an invalid one also so if there is a misconfiguration somewhere else in the system, it becomes immediately obvious what is happening. Otherwise Maven may still reach out to Central and mask the problems. It's essentially a fail-fast setup.

There's more information on this topic in an old blog I wrote

maven needs project dependencies to be available locally for it to run. It does not care about how it is made available - whether manually installed (using mvn install:install-file), through a mirror or by from central repository. It will fail to run if it is unable to find dependencies.

The sonatype documentation that you are referring to is on using nexus to mirror/proxy repositories. The url specified should be a valid nexus url and cannot be unreachable.

The same is suggested in the SO question as well.