package org.eclipse.aether;
   
   /*
    * Licensed to the Apache Software Foundation (ASF) under one
    * or more contributor license agreements.  See the NOTICE file
    * distributed with this work for additional information
    * regarding copyright ownership.  The ASF licenses this file
    * to you under the Apache License, Version 2.0 (the
    * "License"); you may not use this file except in compliance
   * with the License.  You may obtain a copy of the License at
   * 
   *  http://www.apache.org/licenses/LICENSE-2.0
   * 
   * Unless required by applicable law or agreed to in writing,
   * software distributed under the License is distributed on an
   * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
   * KIND, either express or implied.  See the License for the
   * specific language governing permissions and limitations
   * under the License.
   */
  
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.Map;
  import static java.util.Objects.requireNonNull;
  
  import java.util.Collection;
  
  import org.eclipse.aether.artifact.Artifact;
  import org.eclipse.aether.artifact.ArtifactType;
  import org.eclipse.aether.artifact.ArtifactTypeRegistry;
  import org.eclipse.aether.collection.DependencyGraphTransformer;
  import org.eclipse.aether.collection.DependencyManager;
  import org.eclipse.aether.collection.DependencySelector;
  import org.eclipse.aether.collection.DependencyTraverser;
  import org.eclipse.aether.collection.VersionFilter;
  import org.eclipse.aether.repository.Authentication;
  import org.eclipse.aether.repository.AuthenticationSelector;
  import org.eclipse.aether.repository.LocalRepository;
  import org.eclipse.aether.repository.LocalRepositoryManager;
  import org.eclipse.aether.repository.MirrorSelector;
  import org.eclipse.aether.repository.Proxy;
  import org.eclipse.aether.repository.ProxySelector;
  import org.eclipse.aether.repository.RemoteRepository;
  import org.eclipse.aether.repository.RepositoryPolicy;
  import org.eclipse.aether.repository.WorkspaceReader;
  import org.eclipse.aether.resolution.ArtifactDescriptorPolicy;
  import org.eclipse.aether.resolution.ResolutionErrorPolicy;
  import org.eclipse.aether.transfer.TransferListener;
  import org.eclipse.aether.transform.FileTransformer;
  import org.eclipse.aether.transform.FileTransformerManager;
  
  /**
   * A simple repository system session.
   * <p>
   * <strong>Note:</strong> This class is not thread-safe. It is assumed that the mutators get only called during an
   * initialization phase and that the session itself is not changed once initialized and being used by the repository
   * system. It is recommended to call {@link #setReadOnly()} once the session has been fully initialized to prevent
   * accidental manipulation of it afterwards.
   */
  public final class DefaultRepositorySystemSession
      implements RepositorySystemSession
  {
  
      private boolean readOnly;
  
      private boolean offline;
  
      private boolean ignoreArtifactDescriptorRepositories;
  
      private ResolutionErrorPolicy resolutionErrorPolicy;
  
      private ArtifactDescriptorPolicy artifactDescriptorPolicy;
  
      private String checksumPolicy;
  
      private String updatePolicy;
  
      private LocalRepositoryManager localRepositoryManager;
  
      private FileTransformerManager fileTransformerManager;
  
      private WorkspaceReader workspaceReader;
  
      private RepositoryListener repositoryListener;
  
      private TransferListener transferListener;
  
      private Map<String, String> systemProperties;
  
      private Map<String, String> systemPropertiesView;
  
      private Map<String, String> userProperties;
  
      private Map<String, String> userPropertiesView;
  
      private Map<String, Object> configProperties;
  
      private Map<String, Object> configPropertiesView;
 
     private MirrorSelector mirrorSelector;
 
     private ProxySelector proxySelector;
 
     private AuthenticationSelector authenticationSelector;
 
     private ArtifactTypeRegistry artifactTypeRegistry;
 
     private DependencyTraverser dependencyTraverser;
 
     private DependencyManager dependencyManager;
 
     private DependencySelector dependencySelector;
 
     private VersionFilter versionFilter;
 
     private DependencyGraphTransformer dependencyGraphTransformer;
 
     private SessionData data;
 
     private RepositoryCache cache;
 
     /**
      * Creates an uninitialized session. <em>Note:</em> The new session is not ready to use, as a bare minimum,
      * {@link #setLocalRepositoryManager(LocalRepositoryManager)} needs to be called but usually other settings also
      * need to be customized to achieve meaningful behavior.
      */
     public DefaultRepositorySystemSession()
     {
         systemProperties = new HashMap<>();
         systemPropertiesView = Collections.unmodifiableMap( systemProperties );
         userProperties = new HashMap<>();
         userPropertiesView = Collections.unmodifiableMap( userProperties );
         configProperties = new HashMap<>();
         configPropertiesView = Collections.unmodifiableMap( configProperties );
         mirrorSelector = NullMirrorSelector.INSTANCE;
         proxySelector = NullProxySelector.INSTANCE;
         authenticationSelector = NullAuthenticationSelector.INSTANCE;
         artifactTypeRegistry = NullArtifactTypeRegistry.INSTANCE;
         fileTransformerManager = NullFileTransformerManager.INSTANCE;
         data = new DefaultSessionData();
     }
 
     /**
      * Creates a shallow copy of the specified session. Actually, the copy is not completely shallow, all maps holding
      * system/user/config properties are copied as well. In other words, invoking any mutator on the new session itself
      * has no effect on the original session. Other mutable objects like the session data and cache (if any) are not
      * copied and will be shared with the original session unless reconfigured.
      *
      * @param session The session to copy, must not be {@code null}.
      */
     public DefaultRepositorySystemSession( RepositorySystemSession session )
     {
         requireNonNull( session, "repository system session cannot be null" );
 
         setOffline( session.isOffline() );
         setIgnoreArtifactDescriptorRepositories( session.isIgnoreArtifactDescriptorRepositories() );
         setResolutionErrorPolicy( session.getResolutionErrorPolicy() );
         setArtifactDescriptorPolicy( session.getArtifactDescriptorPolicy() );
         setChecksumPolicy( session.getChecksumPolicy() );
         setUpdatePolicy( session.getUpdatePolicy() );
         setLocalRepositoryManager( session.getLocalRepositoryManager() );
         setWorkspaceReader( session.getWorkspaceReader() );
         setRepositoryListener( session.getRepositoryListener() );
         setTransferListener( session.getTransferListener() );
         setSystemProperties( session.getSystemProperties() );
         setUserProperties( session.getUserProperties() );
         setConfigProperties( session.getConfigProperties() );
         setMirrorSelector( session.getMirrorSelector() );
         setProxySelector( session.getProxySelector() );
         setAuthenticationSelector( session.getAuthenticationSelector() );
         setArtifactTypeRegistry( session.getArtifactTypeRegistry() );
         setDependencyTraverser( session.getDependencyTraverser() );
         setDependencyManager( session.getDependencyManager() );
         setDependencySelector( session.getDependencySelector() );
         setVersionFilter( session.getVersionFilter() );
         setDependencyGraphTransformer( session.getDependencyGraphTransformer() );
         setFileTransformerManager( session.getFileTransformerManager() );
         setData( session.getData() );
         setCache( session.getCache() );
     }
 
     public boolean isOffline()
     {
         return offline;
     }
 
     /**
      * Controls whether the repository system operates in offline mode and avoids/refuses any access to remote
      * repositories.
      * 
      * @param offline {@code true} if the repository system is in offline mode, {@code false} otherwise.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setOffline( boolean offline )
     {
         failIfReadOnly();
         this.offline = offline;
         return this;
     }
 
     public boolean isIgnoreArtifactDescriptorRepositories()
     {
         return ignoreArtifactDescriptorRepositories;
     }
 
     /**
      * Controls whether repositories declared in artifact descriptors should be ignored during transitive dependency
      * collection. If enabled, only the repositories originally provided with the collect request will be considered.
      * 
      * @param ignoreArtifactDescriptorRepositories {@code true} to ignore additional repositories from artifact
      *            descriptors, {@code false} to merge those with the originally specified repositories.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setIgnoreArtifactDescriptorRepositories(
             boolean ignoreArtifactDescriptorRepositories )
     {
         failIfReadOnly();
         this.ignoreArtifactDescriptorRepositories = ignoreArtifactDescriptorRepositories;
         return this;
     }
 
     public ResolutionErrorPolicy getResolutionErrorPolicy()
     {
         return resolutionErrorPolicy;
     }
 
     /**
      * Sets the policy which controls whether resolutions errors from remote repositories should be cached.
      * 
      * @param resolutionErrorPolicy The resolution error policy for this session, may be {@code null} if resolution
      *            errors should generally not be cached.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setResolutionErrorPolicy( ResolutionErrorPolicy resolutionErrorPolicy )
     {
         failIfReadOnly();
         this.resolutionErrorPolicy = resolutionErrorPolicy;
         return this;
     }
 
     public ArtifactDescriptorPolicy getArtifactDescriptorPolicy()
     {
         return artifactDescriptorPolicy;
     }
 
     /**
      * Sets the policy which controls how errors related to reading artifact descriptors should be handled.
      * 
      * @param artifactDescriptorPolicy The descriptor error policy for this session, may be {@code null} if descriptor
      *            errors should generally not be tolerated.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setArtifactDescriptorPolicy(
             ArtifactDescriptorPolicy artifactDescriptorPolicy )
     {
         failIfReadOnly();
         this.artifactDescriptorPolicy = artifactDescriptorPolicy;
         return this;
     }
 
     public String getChecksumPolicy()
     {
         return checksumPolicy;
     }
 
     /**
      * Sets the global checksum policy. If set, the global checksum policy overrides the checksum policies of the remote
      * repositories being used for resolution.
      * 
      * @param checksumPolicy The global checksum policy, may be {@code null}/empty to apply the per-repository policies.
      * @return This session for chaining, never {@code null}.
      * @see RepositoryPolicy#CHECKSUM_POLICY_FAIL
      * @see RepositoryPolicy#CHECKSUM_POLICY_IGNORE
      * @see RepositoryPolicy#CHECKSUM_POLICY_WARN
      */
     public DefaultRepositorySystemSession setChecksumPolicy( String checksumPolicy )
     {
         failIfReadOnly();
         this.checksumPolicy = checksumPolicy;
         return this;
     }
 
     public String getUpdatePolicy()
     {
         return updatePolicy;
     }
 
     /**
      * Sets the global update policy. If set, the global update policy overrides the update policies of the remote
      * repositories being used for resolution.
      * 
      * @param updatePolicy The global update policy, may be {@code null}/empty to apply the per-repository policies.
      * @return This session for chaining, never {@code null}.
      * @see RepositoryPolicy#UPDATE_POLICY_ALWAYS
      * @see RepositoryPolicy#UPDATE_POLICY_DAILY
      * @see RepositoryPolicy#UPDATE_POLICY_NEVER
      */
     public DefaultRepositorySystemSession setUpdatePolicy( String updatePolicy )
     {
         failIfReadOnly();
         this.updatePolicy = updatePolicy;
         return this;
     }
 
     public LocalRepository getLocalRepository()
     {
         LocalRepositoryManager lrm = getLocalRepositoryManager();
         return ( lrm != null ) ? lrm.getRepository() : null;
     }
 
     public LocalRepositoryManager getLocalRepositoryManager()
     {
         return localRepositoryManager;
     }
 
     /**
      * Sets the local repository manager used during this session. <em>Note:</em> Eventually, a valid session must have
      * a local repository manager set.
      * 
      * @param localRepositoryManager The local repository manager used during this session, may be {@code null}.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setLocalRepositoryManager( LocalRepositoryManager localRepositoryManager )
     {
         failIfReadOnly();
         this.localRepositoryManager = localRepositoryManager;
         return this;
     }
 
     @Override
     public FileTransformerManager getFileTransformerManager()
     {
         return fileTransformerManager;
     }
 
     public DefaultRepositorySystemSession setFileTransformerManager( FileTransformerManager fileTransformerManager )
     {
         failIfReadOnly();
         this.fileTransformerManager = fileTransformerManager;
         if ( this.fileTransformerManager == null )
         {
             this.fileTransformerManager = NullFileTransformerManager.INSTANCE;
         }
         return this;
     }
 
     public WorkspaceReader getWorkspaceReader()
     {
         return workspaceReader;
     }
 
     /**
      * Sets the workspace reader used during this session. If set, the workspace reader will usually be consulted first
      * to resolve artifacts.
      * 
      * @param workspaceReader The workspace reader for this session, may be {@code null} if none.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setWorkspaceReader( WorkspaceReader workspaceReader )
     {
         failIfReadOnly();
         this.workspaceReader = workspaceReader;
         return this;
     }
 
     public RepositoryListener getRepositoryListener()
     {
         return repositoryListener;
     }
 
     /**
      * Sets the listener being notified of actions in the repository system.
      * 
      * @param repositoryListener The repository listener, may be {@code null} if none.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setRepositoryListener( RepositoryListener repositoryListener )
     {
         failIfReadOnly();
         this.repositoryListener = repositoryListener;
         return this;
     }
 
     public TransferListener getTransferListener()
     {
         return transferListener;
     }
 
     /**
      * Sets the listener being notified of uploads/downloads by the repository system.
      * 
      * @param transferListener The transfer listener, may be {@code null} if none.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setTransferListener( TransferListener transferListener )
     {
         failIfReadOnly();
         this.transferListener = transferListener;
         return this;
     }
 
     @SuppressWarnings( "checkstyle:magicnumber" )
     private <T> Map<String, T> copySafe( Map<?, ?> table, Class<T> valueType )
     {
         Map<String, T> map;
         if ( table == null || table.isEmpty() )
         {
             map = new HashMap<>();
         }
         else
         {
             map = new HashMap<>( (int) ( table.size() / 0.75f ) + 1 );
             for ( Map.Entry<?, ?> entry : table.entrySet() )
             {
                 Object key = entry.getKey();
                 if ( key instanceof String )
                 {
                     Object value = entry.getValue();
                     if ( valueType.isInstance( value ) )
                     {
                         map.put( key.toString(), valueType.cast( value ) );
                     }
                 }
             }
         }
         return map;
     }
 
     public Map<String, String> getSystemProperties()
     {
         return systemPropertiesView;
     }
 
     /**
      * Sets the system properties to use, e.g. for processing of artifact descriptors. System properties are usually
      * collected from the runtime environment like {@link System#getProperties()} and environment variables.
      * <p>
      * <em>Note:</em> System properties are of type {@code Map<String, String>} and any key-value pair in the input map
      * that doesn't match this type will be silently ignored.
      * 
      * @param systemProperties The system properties, may be {@code null} or empty if none.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setSystemProperties( Map<?, ?> systemProperties )
     {
         failIfReadOnly();
         this.systemProperties = copySafe( systemProperties, String.class );
         systemPropertiesView = Collections.unmodifiableMap( this.systemProperties );
         return this;
     }
 
     /**
      * Sets the specified system property.
      * 
      * @param key The property key, must not be {@code null}.
      * @param value The property value, may be {@code null} to remove/unset the property.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setSystemProperty( String key, String value )
     {
         failIfReadOnly();
         if ( value != null )
         {
             systemProperties.put( key, value );
         }
         else
         {
             systemProperties.remove( key );
         }
         return this;
     }
 
     public Map<String, String> getUserProperties()
     {
         return userPropertiesView;
     }
 
     /**
      * Sets the user properties to use, e.g. for processing of artifact descriptors. User properties are similar to
      * system properties but are set on the discretion of the user and hence are considered of higher priority than
      * system properties in case of conflicts.
      * <p>
      * <em>Note:</em> User properties are of type {@code Map<String, String>} and any key-value pair in the input map
      * that doesn't match this type will be silently ignored.
      * 
      * @param userProperties The user properties, may be {@code null} or empty if none.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setUserProperties( Map<?, ?> userProperties )
     {
         failIfReadOnly();
         this.userProperties = copySafe( userProperties, String.class );
         userPropertiesView = Collections.unmodifiableMap( this.userProperties );
         return this;
     }
 
     /**
      * Sets the specified user property.
      * 
      * @param key The property key, must not be {@code null}.
      * @param value The property value, may be {@code null} to remove/unset the property.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setUserProperty( String key, String value )
     {
         failIfReadOnly();
         if ( value != null )
         {
             userProperties.put( key, value );
         }
         else
         {
             userProperties.remove( key );
         }
         return this;
     }
 
     public Map<String, Object> getConfigProperties()
     {
         return configPropertiesView;
     }
 
     /**
      * Sets the configuration properties used to tweak internal aspects of the repository system (e.g. thread pooling,
      * connector-specific behavior, etc.).
      * <p>
      * <em>Note:</em> Configuration properties are of type {@code Map<String, Object>} and any key-value pair in the
      * input map that doesn't match this type will be silently ignored.
      * 
      * @param configProperties The configuration properties, may be {@code null} or empty if none.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setConfigProperties( Map<?, ?> configProperties )
     {
         failIfReadOnly();
         this.configProperties = copySafe( configProperties, Object.class );
         configPropertiesView = Collections.unmodifiableMap( this.configProperties );
         return this;
     }
 
     /**
      * Sets the specified configuration property.
      * 
      * @param key The property key, must not be {@code null}.
      * @param value The property value, may be {@code null} to remove/unset the property.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setConfigProperty( String key, Object value )
     {
         failIfReadOnly();
         if ( value != null )
         {
             configProperties.put( key, value );
         }
         else
         {
             configProperties.remove( key );
         }
         return this;
     }
 
     public MirrorSelector getMirrorSelector()
     {
         return mirrorSelector;
     }
 
     /**
      * Sets the mirror selector to use for repositories discovered in artifact descriptors. Note that this selector is
      * not used for remote repositories which are passed as request parameters to the repository system, those
      * repositories are supposed to denote the effective repositories.
      * 
      * @param mirrorSelector The mirror selector to use, may be {@code null}.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setMirrorSelector( MirrorSelector mirrorSelector )
     {
         failIfReadOnly();
         this.mirrorSelector = mirrorSelector;
         if ( this.mirrorSelector == null )
         {
             this.mirrorSelector = NullMirrorSelector.INSTANCE;
         }
         return this;
     }
 
     public ProxySelector getProxySelector()
     {
         return proxySelector;
     }
 
     /**
      * Sets the proxy selector to use for repositories discovered in artifact descriptors. Note that this selector is
      * not used for remote repositories which are passed as request parameters to the repository system, those
      * repositories are supposed to have their proxy (if any) already set.
      * 
      * @param proxySelector The proxy selector to use, may be {@code null}.
      * @return This session for chaining, never {@code null}.
      * @see org.eclipse.aether.repository.RemoteRepository#getProxy()
      */
     public DefaultRepositorySystemSession setProxySelector( ProxySelector proxySelector )
     {
         failIfReadOnly();
         this.proxySelector = proxySelector;
         if ( this.proxySelector == null )
         {
             this.proxySelector = NullProxySelector.INSTANCE;
         }
         return this;
     }
 
     public AuthenticationSelector getAuthenticationSelector()
     {
         return authenticationSelector;
     }
 
     /**
      * Sets the authentication selector to use for repositories discovered in artifact descriptors. Note that this
      * selector is not used for remote repositories which are passed as request parameters to the repository system,
      * those repositories are supposed to have their authentication (if any) already set.
      * 
      * @param authenticationSelector The authentication selector to use, may be {@code null}.
      * @return This session for chaining, never {@code null}.
      * @see org.eclipse.aether.repository.RemoteRepository#getAuthentication()
      */
     public DefaultRepositorySystemSession setAuthenticationSelector( AuthenticationSelector authenticationSelector )
     {
         failIfReadOnly();
         this.authenticationSelector = authenticationSelector;
         if ( this.authenticationSelector == null )
         {
             this.authenticationSelector = NullAuthenticationSelector.INSTANCE;
         }
         return this;
     }
 
     public ArtifactTypeRegistry getArtifactTypeRegistry()
     {
         return artifactTypeRegistry;
     }
 
     /**
      * Sets the registry of artifact types recognized by this session.
      * 
      * @param artifactTypeRegistry The artifact type registry, may be {@code null}.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setArtifactTypeRegistry( ArtifactTypeRegistry artifactTypeRegistry )
     {
         failIfReadOnly();
         this.artifactTypeRegistry = artifactTypeRegistry;
         if ( this.artifactTypeRegistry == null )
         {
             this.artifactTypeRegistry = NullArtifactTypeRegistry.INSTANCE;
         }
         return this;
     }
 
     public DependencyTraverser getDependencyTraverser()
     {
         return dependencyTraverser;
     }
 
     /**
      * Sets the dependency traverser to use for building dependency graphs.
      * 
      * @param dependencyTraverser The dependency traverser to use for building dependency graphs, may be {@code null}.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setDependencyTraverser( DependencyTraverser dependencyTraverser )
     {
         failIfReadOnly();
         this.dependencyTraverser = dependencyTraverser;
         return this;
     }
 
     public DependencyManager getDependencyManager()
     {
         return dependencyManager;
     }
 
     /**
      * Sets the dependency manager to use for building dependency graphs.
      * 
      * @param dependencyManager The dependency manager to use for building dependency graphs, may be {@code null}.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setDependencyManager( DependencyManager dependencyManager )
     {
         failIfReadOnly();
         this.dependencyManager = dependencyManager;
         return this;
     }
 
     public DependencySelector getDependencySelector()
     {
         return dependencySelector;
     }
 
     /**
      * Sets the dependency selector to use for building dependency graphs.
      * 
      * @param dependencySelector The dependency selector to use for building dependency graphs, may be {@code null}.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setDependencySelector( DependencySelector dependencySelector )
     {
         failIfReadOnly();
         this.dependencySelector = dependencySelector;
         return this;
     }
 
     public VersionFilter getVersionFilter()
     {
         return versionFilter;
     }
 
     /**
      * Sets the version filter to use for building dependency graphs.
      * 
      * @param versionFilter The version filter to use for building dependency graphs, may be {@code null} to not filter
      *            versions.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setVersionFilter( VersionFilter versionFilter )
     {
         failIfReadOnly();
         this.versionFilter = versionFilter;
         return this;
     }
 
     public DependencyGraphTransformer getDependencyGraphTransformer()
     {
         return dependencyGraphTransformer;
     }
 
     /**
      * Sets the dependency graph transformer to use for building dependency graphs.
      * 
      * @param dependencyGraphTransformer The dependency graph transformer to use for building dependency graphs, may be
      *            {@code null}.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setDependencyGraphTransformer(
             DependencyGraphTransformer dependencyGraphTransformer )
     {
         failIfReadOnly();
         this.dependencyGraphTransformer = dependencyGraphTransformer;
         return this;
     }
 
     public SessionData getData()
     {
         return data;
     }
 
     /**
      * Sets the custom data associated with this session.
      * 
      * @param data The session data, may be {@code null}.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setData( SessionData data )
     {
         failIfReadOnly();
         this.data = data;
         if ( this.data == null )
         {
             this.data = new DefaultSessionData();
         }
         return this;
     }
 
     public RepositoryCache getCache()
     {
         return cache;
     }
 
     /**
      * Sets the cache the repository system may use to save data for future reuse during the session.
      * 
      * @param cache The repository cache, may be {@code null} if none.
      * @return This session for chaining, never {@code null}.
      */
     public DefaultRepositorySystemSession setCache( RepositoryCache cache )
     {
         failIfReadOnly();
         this.cache = cache;
         return this;
     }
 
     /**
      * Marks this session as read-only such that any future attempts to call its mutators will fail with an exception.
      * Marking an already read-only session as read-only has no effect. The session's data and cache remain writable
      * though.
      */
     public void setReadOnly()
     {
         readOnly = true;
     }
 
     private void failIfReadOnly()
     {
         if ( readOnly )
         {
             throw new IllegalStateException( "repository system session is read-only" );
         }
     }
 
     static class NullProxySelector
         implements ProxySelector
     {
 
         public static final ProxySelector INSTANCE = new NullProxySelector();
 
         public Proxy getProxy( RemoteRepository repository )
         {
             requireNonNull( repository, "repository cannot be null" );
             return repository.getProxy();
         }
 
     }
 
     static class NullMirrorSelector
         implements MirrorSelector
     {
 
         public static final MirrorSelector INSTANCE = new NullMirrorSelector();
 
         public RemoteRepository getMirror( RemoteRepository repository )
         {
             requireNonNull( repository, "repository cannot be null" );
             return null;
         }
 
     }
 
     static class NullAuthenticationSelector
         implements AuthenticationSelector
     {
 
         public static final AuthenticationSelector INSTANCE = new NullAuthenticationSelector();
 
         public Authentication getAuthentication( RemoteRepository repository )
         {
             requireNonNull( repository, "repository cannot be null" );
             return repository.getAuthentication();
         }
 
     }
 
     static final class NullArtifactTypeRegistry
         implements ArtifactTypeRegistry
     {
 
         public static final ArtifactTypeRegistry INSTANCE = new NullArtifactTypeRegistry();
 
         public ArtifactType get( String typeId )
         {
             return null;
         }
 
     }
 
     static final class NullFileTransformerManager implements FileTransformerManager
     {
         public static final FileTransformerManager INSTANCE = new NullFileTransformerManager();
 
         @Override
         public Collection<FileTransformer> getTransformersForArtifact( Artifact artifact )
         {
             return Collections.emptyList();
         }
     }
 
 }