A Maven mojo that shows how to resolve dependencies locations, be it in remote or in the local repository, or even in the reactor.

ReactorDependenciesResolverMojoTest.java

package whatever;

import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.HashSet;

import org.apache.maven.artifact.Artifact;
import org.apache.maven.artifact.DefaultArtifact;
import org.apache.maven.artifact.handler.DefaultArtifactHandler;
import org.apache.maven.plugin.AbstractMojo;
import org.apache.maven.plugin.MojoExecutionException;
import org.apache.maven.project.MavenProject;
import org.apache.maven.repository.internal.MavenRepositorySystemUtils;
import org.eclipse.aether.DefaultRepositorySystemSession;
import org.eclipse.aether.RepositorySystem;
import org.eclipse.aether.RepositorySystemSession;
import org.eclipse.aether.impl.DefaultServiceLocator;
import org.eclipse.aether.repository.LocalRepository;
import org.eclipse.aether.repository.LocalRepositoryManager;
import org.eclipse.aether.repository.RemoteRepository;
import org.junit.Assert;
import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.TemporaryFolder;

public class CopyOfResolveMojoTest extends AbstractTest {

	@Rule
	public TemporaryFolder folder = new TemporaryFolder();


	@Test( expected = MojoExecutionException.class )
	public void testWithInvalidRoboconfDependencies() throws Exception {

		// Prepare the project
		final String projectName = "project--valid";

		File baseDir = this.resources.getBasedir( projectName );
		Assert.assertNotNull( baseDir );
		Assert.assertTrue( baseDir.isDirectory());

		AbstractMojo mojo = ...;
		this.rule.setVariableValueToObject( mojo, "repoSession", newRepositorySession());
		this.rule.setVariableValueToObject( mojo, "repositories", new ArrayList<RemoteRepository>( 0 ));

		mojo.execute();
	}


	private RepositorySystemSession newRepositorySession() throws IOException {

		DefaultServiceLocator locator = MavenRepositorySystemUtils.newServiceLocator();
		RepositorySystem repositorySystem = locator.getService( RepositorySystem.class );

		LocalRepository localRepo = new LocalRepository( this.folder.newFolder());

		DefaultRepositorySystemSession session = MavenRepositorySystemUtils.newSession();
		LocalRepositoryManager lrm = repositorySystem.newLocalRepositoryManager( session, localRepo );
		session.setLocalRepositoryManager( lrm );

		return session;
	}
}

ReactorDependenciesResolverMojo.java

package whatever;

import java.io.File;
import java.util.List;

import org.apache.maven.artifact.Artifact;
import org.apache.maven.plugin.AbstractMojo;
import org.apache.maven.plugin.MojoExecutionException;
import org.apache.maven.plugin.MojoFailureException;
import org.apache.maven.plugins.annotations.Component;
import org.apache.maven.plugins.annotations.Mojo;
import org.apache.maven.plugins.annotations.Parameter;
import org.apache.maven.project.MavenProject;
import org.eclipse.aether.RepositorySystem;
import org.eclipse.aether.RepositorySystemSession;
import org.eclipse.aether.artifact.DefaultArtifact;
import org.eclipse.aether.repository.RemoteRepository;
import org.eclipse.aether.resolution.ArtifactRequest;
import org.eclipse.aether.resolution.ArtifactResolutionException;
import org.eclipse.aether.resolution.ArtifactResult;

@Mojo( name="mojoName" )
public class ReactorDependenciesResolverMojo extends AbstractMojo {

	@Parameter( defaultValue = "${project}", readonly = true )
	private MavenProject project;

	@Component
	private RepositorySystem repoSystem;

	@Parameter( defaultValue = "${repositorySystemSession}", readonly = true, required = true )
	private RepositorySystemSession repoSession;

	@Parameter( defaultValue = "${project.remoteProjectRepositories}", readonly = true, required = true )
	private List<RemoteRepository> repositories;



	@Override
	public void execute() throws MojoExecutionException, MojoFailureException {

		for( Artifact unresolvedArtifact : this.project.getDependencyArtifacts()) {

			// Here, it becomes messy. We ask Maven to resolve the artifact's location.
			// It may imply downloading it from a remote repository,
			// searching the local repository or looking into the reactor's cache.

			// To achieve this, we must use Aether
			// (the dependency mechanism behind Maven).
			String artifactId = unresolvedArtifact.getArtifactId();
			org.eclipse.aether.artifact.Artifact aetherArtifact = new DefaultArtifact(
					unresolvedArtifact.getGroupId(),
					unresolvedArtifact.getArtifactId(),
					unresolvedArtifact.getClassifier(),
					unresolvedArtifact.getType(),
					unresolvedArtifact.getVersion());

			ArtifactRequest req = new ArtifactRequest().setRepositories( this.repositories ).setArtifact( aetherArtifact );
			ArtifactResult resolutionResult;
			try {
				resolutionResult = this.repoSystem.resolveArtifact( this.repoSession, req );

			} catch( ArtifactResolutionException e ) {
				throw new MojoExecutionException( "Artifact " + artifactId + " could not be resolved.", e );
			}

			// The file should exists, but we never know.
			File file = resolutionResult.getArtifact().getFile();
			if( file == null || ! file.exists()) {
				getLog().warn( "Artifact " + artifactId + " has no attached file. Its content will not be copied in the target model directory." );
				continue;
			}

			// Do whatever you want with the file...
		}
	}
}

Notice this is intended for Maven 3.1 and higher versions.

The interest of unit tests to verify such a mojo is very limited.
People will generally opt to do integration tests with the Maven invoker plugin.

Many thanks!