How do I get Maven to use the correct repositories?

I have just checked out some projects and need to build them, however I installed Maven quite some time ago (6 months maybe?) and really haven't used it since - the pom.xml for the project I have doesn't have this "http://repo1.maven.org/myurlhere" anywhere in it - it has the absolute url where the Maven repo is for the project, but Maven is still trying to download from the general Maven repo:

Macintosh:trunk$ mvn clean install
[INFO] Scanning for projects...
Downloading: http://repo1.maven.org/url/project/project/x.x/project-x.x.pom
[INFO] Unable to find resource 'url.project:project:pom:x.x' in repository central (http://repo1.maven.org/)
[INFO] ------------------------------------------------------------------------
[ERROR] FATAL ERROR
[INFO] ------------------------------------------------------------------------
[INFO] Failed to resolve artifact.

GroupId: url.project
ArtifactId: project
Version: x.x

Reason: Unable to download the artifact from any repository

  url.project:project:pom:x.x

from the specified remote repositories:
  central (http://repo1.maven.org/)

Can anyone help me with what I'm not doing right?
Basically, I have just checked the projects out from the command line, cd-ed into the directory and ran mvn clean install - nothing else.
Any help is greatly appreciated.

the pom.xml for the project I have doesn't have this "http://repo1.maven.org/myurlhere" anywhere in it

All projects have http://repo1.maven.org/ declared as <repository> (and <pluginRepository>) by default. This repository, which is called the central repository, is inherited like others default settings from the "Super POM" (all projects inherit from the Super POM). So a POM is actually a combination of the Super POM, any parent POMs and the current POM. This combination is called the "effective POM" and can be printed using the effective-pom goal of the Maven Help plugin (useful for debugging).

And indeed, if you run:

mvn help:effective-pom

You'll see at least the following:

  <repositories>
    <repository>
      <snapshots>
        <enabled>false</enabled>
      </snapshots>
      <id>central</id>
      <name>Maven Repository Switchboard</name>
      <url>http://repo1.maven.org/maven2</url>
    </repository>
  </repositories>
  <pluginRepositories>
    <pluginRepository>
      <releases>
        <updatePolicy>never</updatePolicy>
      </releases>
      <snapshots>
        <enabled>false</enabled>
      </snapshots>
      <id>central</id>
      <name>Maven Plugin Repository</name>
      <url>http://repo1.maven.org/maven2</url>
    </pluginRepository>
  </pluginRepositories>

it has the absolute url where the maven repo is for the project but maven is still trying to download from the general maven repo

Maven will try to find dependencies in all repositories declared, including in the central one which is there by default as we saw. But, according to the trace you are showing, you only have one repository defined (the central repository) or maven would print something like this:

Reason: Unable to download the artifact from any repository

  url.project:project:pom:x.x

from the specified remote repositories:
  central (http://repo1.maven.org/),
  another-repository (http://another/repository)

So, basically, maven is unable to find the url.project:project:pom:x.x because it is not available in central.

But without knowing which project you've checked out (it has maybe specific instructions) or which dependency is missing (it can maybe be found in another repository), it's impossible to help you further.

By default, Maven will always look in the official Maven repository, which is http://repo1.maven.org.

When Maven tries to build a project, it will look in your local repository (by default ~/.m2/repository but you can configure it by changing the <localRepository> value in your ~/.m2/settings.xml) to find any dependency, plugin or report defined in your pom.xml. If the adequate artifact is not found in your local repository, it will look in all external repositories configured, starting with the default one, http://repo1.maven.org.

You can configure Maven to avoid this default repository by setting a mirror in your settings.xml file:

<mirrors>
    <mirror>
        <id>repoMirror</id>
        <name>Our mirror for Maven repository</name>
        <url>http://the/server/</url>
        <mirrorOf>*</mirrorOf>
    </mirror>
</mirrors>

This way, instead of contacting http://repo1.maven.org, Maven will contact your entreprise repository (http://the/server in this example).

If you want to add another repository, you can define a new one in your settings.xml file:

<profiles>
    <profile>
        <activation>
            <activeByDefault>true</activeByDefault>
        </activation>
        <repositories>
            <repository>
                <id>foo.bar</id>
                <releases>
                    <enabled>true</enabled>
                </releases>
                <snapshots>
                    <enabled>true</enabled>
                </snapshots>
                <url>http://new/repository/server</url>
            </repository>
        </repositories>

You can see the complete settings.xml model here.

Concerning the clean process, you can ask Maven to run it offline. In this case, Maven will not try to reach any external repositories:

mvn -o clean 

tl;dr

All maven POMs inherit from a base Super POM.
The snippet below is part of the Super POM for Maven 3.5.4.

  <repositories>
    <repository>
      <id>central</id>
      <name>Central Repository</name>
      <url>https://repo.maven.apache.org/maven2</url>
      <layout>default</layout>
      <snapshots>
        <enabled>false</enabled>
      </snapshots>
    </repository>
  </repositories>

I think what you have missed here is this:

https://maven.apache.org/settings.html#Servers

The repositories for download and deployment are defined by the repositories and distributionManagement elements of the POM. However, certain settings such as username and password should not be distributed along with the pom.xml. This type of information should exist on the build server in the settings.xml.

This is the prefered way of using custom repos. So probably what is happening is that the url of this repo is in settings.xml of the build server.

Once you get hold of the url and credentials, you can put them in your machine here: ~/.m2/settings.xml like this:

<settings ...> 

        .
        .
        .
        <servers>
            <server>
              <id>internal-repository-group</id>
              <username>YOUR-USERNAME-HERE</username>
              <password>YOUR-PASSWORD-HERE</password>
            </server>
        </servers>
</settings>

EDIT:

You then need to refer this repository into project POM. The id internal-repository-group can be used in every project. You can setup multiple repos and credentials setting using different IDs in settings xml.

The advantage of this approach is that project can be shared without worrying about the credentials and don't have to mention the credentials in every project.

Following is a sample pom of a project using "internal-repository-group"

<repositories>
    <repository>
        <id>internal-repository-group</id>
        <name>repo-name</name>
        <url>http://project.com/yourrepourl/</url>
        <layout>default</layout>
        <releases>
            <enabled>true</enabled>
            <updatePolicy>never</updatePolicy>
        </releases>
        <snapshots>
            <enabled>true</enabled>
            <updatePolicy>never</updatePolicy>
        </snapshots>
    </repository>
</repositories>