project
- nice little sample projects that could be run in the core as well as integration tests

All Possible Errors
- invalid lifecycle phase (maybe same as bad CLI param, though you were talking about embedder too)
- <module> specified is not found
- malformed settings
- malformed POM
- local repository not writable
- remote repositories not available
- artifact metadata missing
- extension metadata missing

                    field.setAccessible(true);
                    field.set(v4src, ModelProblemUtils.toId(model));
                } catch (Throwable t) {
                    // TODO: use a lazy source ?
                    throw new IllegalStateException("Unable to set modelId on InputSource", t);
                }
            }
        } catch (XmlReaderException e) {
            problems.add(

     * @param test the test output directory, or {@code null} if none
     * @throws IOException if an error occurred while reading module information
     *
     * TODO: this is currently not called
     */
    void addOutputDirectory(Path main, Path test) throws IOException {
        if (outputModules != null) {

import org.apache.maven.lifecycle.mapping.LifecycleMojo;
import org.apache.maven.lifecycle.mapping.LifecyclePhase;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * TODO: this is session scoped as SPI can contribute.
 */
@Named
@Singleton
public class DefaultPackagingRegistry
        extends ExtensibleEnumRegistries.DefaultExtensibleEnumRegistry<Packaging, PackagingProvider>

                    Dependency present = dependencies.putIfAbsent(key, dependency);
                    if (present != null && !equals(dependency, present) && !directDependencies.contains(key)) {
                        // TODO: https://issues.apache.org/jira/browse/MNG-8004
                        problems.add(new ModelProblemCollectorRequest(
                                        ModelProblem.Severity.WARNING, ModelProblem.Version.V40)

            } catch (Exception e) {
                throw new RuntimeException("Unable to create repository", e);
            }
        } else {
            // TODO
            throw new UnsupportedOperationException("Not yet implemented");
        }
    }

                    flattenResult.addNode(node);
                }
                result = flattenResult;
            } else {
                PathModularizationCache cache = new PathModularizationCache(); // TODO: should be project-wide cache.
                DefaultDependencyResolverResult resolverResult = new DefaultDependencyResolverResult(

 * For more information about the usage tag, have a look to:
 * <a href="https://maven.apache.org/developers/mojo-api-specification.html">
 * https://maven.apache.org/developers/mojo-api-specification.html</a>
 *
 * TODO is there a need for the delegation of MavenMojoDescriptor to this?
 * Why not just extend ComponentDescriptor here?
 */
public class MojoDescriptor extends ComponentDescriptor<Mojo> implements Cloneable {

                    selected = type;
                } else if (unknown) {
                    // More than one filtered value, and we don't know how to handle at least one of them.
                    // TODO: add a plugin mechanism for allowing plugin to specify their selection algorithm.
                    return Optional.empty();
                }
            }
        }
        /*

import static java.util.Arrays.asList;
import static java.util.Collections.singleton;
import static org.apache.maven.internal.impl.Lifecycles.phase;
import static org.apache.maven.internal.impl.Lifecycles.plugin;

/**
 * TODO: this is session scoped as SPI can contribute.
 */
@Named
@Singleton
public class DefaultLifecycleRegistry implements LifecycleRegistry {

    private final List<LifecycleProvider> providers;