Maven: How to check if an artifact exists?

How do I check from within a Mojo if an artifact already exists in the local repository?

I'm installing large binaries into the local Maven repository and I need to know if they already exist before attempting to download them.

Solved with the help of http://docs.codehaus.org/display/MAVENUSER/Mojo+Developer+Cookbook

/**
 * The local maven repository.
 *
 * @parameter expression="${localRepository}"
 * @required
 * @readonly
 */
@SuppressWarnings("UWF_UNWRITTEN_FIELD")
private ArtifactRepository localRepository;
/**
 * @parameter default-value="${project.remoteArtifactRepositories}"
 * @required
 * @readonly
 */
private List<?> remoteRepositories;
/**
 * Resolves Artifacts in the local repository.
 * 
 * @component
 */
private ArtifactResolver artifactResolver;
/**
 * @component
 */
private ArtifactFactory artifactFactory;
[...]
Artifact artifact = artifactFactory.createArtifactWithClassifier(groupId, artifactId, version, packagingType, classifier);
boolean artifactExists;
try
{
  // Downloads the remote artifact, if necessary
  artifactResolver.resolve(artifact, remoteRepositories, localRepository);
  artifactExists = true;
}
catch (ArtifactResolutionException e)
{
  throw new MojoExecutionException("", e);
}
catch (ArtifactNotFoundException e)
{
  artifactExists = false;
}
if (artifactExists)
  System.out.println("Artifact found at: " + artifact.getFile());

If you want to check if a remote artifact exists without downloading it, you can use the Aether library to do the following (based on http://dev.eclipse.org/mhonarc/lists/aether-users/msg00127.html):

MavenDefaultLayout defaultLayout = new MavenDefaultLayout();
RemoteRepository centralRepository = new RemoteRepository.Builder("central", "default", "http://repo1.maven.org/maven2/").build();
URI centralUri = URI.create(centralRepository.getUrl());
URI artifactUri = centralUri.resolve(defaultLayout.getPath(artifact));
HttpURLConnection connection = (HttpURLConnection) artifactUri.toURL().openConnection();
connection.setRequestMethod("HEAD");
connection.connect();
boolean artifactExists = connection.getResponseCode() != 404;

With following dependency: org.eclipse.aether:aether-util:0.9.0.M2 and following imports:

import java.net.HttpURLConnection;
import java.net.URI;

import org.eclipse.aether.artifact.Artifact;
import org.eclipse.aether.artifact.DefaultArtifact;
import org.eclipse.aether.repository.RemoteRepository;
import org.eclipse.aether.util.repository.layout.MavenDefaultLayout;