[java] Why am I getting a "401 Unauthorized" error in Maven?

Also had 401's from Nexus. Having tried all the suggestions above and more without success I eventually found that it was a Jenkins setting that was in error.

In the Jenkins configuration for the failing project, we have a section in the 'Post Build' actions entitled 'Deploy Artifacts To Maven Repository'. This has a 'Repository ID' field which was set to the wrong value. It has to be the same as the repository ID in settings.xml for Jenkins to read the user and password fields:

Jenkins Project Configuration

 <servers>
    <server>
      <id>snapshot-repository</id>  <!-- must match this -->
      <username>deployment</username>
      <password>password</password>
    </server>
  </servers>