src/main/java/org/apache/commons/configuration2/SubnodeConfiguration.java

/*
 * 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
 *
 *     https://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.
 */
package org.apache.commons.configuration2;

import org.apache.commons.configuration2.tree.ImmutableNode;
import org.apache.commons.configuration2.tree.InMemoryNodeModel;
import org.apache.commons.configuration2.tree.InMemoryNodeModelSupport;
import org.apache.commons.configuration2.tree.NodeModel;
import org.apache.commons.configuration2.tree.NodeSelector;
import org.apache.commons.configuration2.tree.TrackedNodeModel;

/**
 * <p>
 * A specialized hierarchical configuration class with a node model that uses a tracked node of another node model as
 * its root node.
 * </p>
 * <p>
 * Configurations of this type are initialized with a special {@link NodeModel} operating on a specific tracked node of
 * the parent configuration and the corresponding {@link NodeSelector}. All property accessor methods are evaluated
 * relative to this root node. A good use case for a {@code SubnodeConfiguration} is when multiple properties from a
 * specific sub tree of the whole configuration need to be accessed. Then a {@code SubnodeConfiguration} can be created
 * with the parent node of the affected sub tree as root node. This allows for simpler property keys and is also more
 * efficient.
 * </p>
 * <p>
 * By making use of a tracked node as root node, a {@code SubnodeConfiguration} and its parent configuration initially
 * operate on the same hierarchy of configuration nodes. So if modifications are performed at the subnode configuration,
 * these changes are immediately visible in the parent configuration. Analogously will updates of the parent
 * configuration affect the {@code SubnodeConfiguration} if the sub tree spanned by the {@code SubnodeConfiguration}'s
 * root node is involved.
 * </p>
 * <p>
 * Note that by making use of a {@code NodeSelector} the {@code SubnodeConfiguration} is not associated with a physical
 * node instance, but the selection criteria stored in the selector are evaluated after each change of the nodes
 * structure. As an example consider that the selector uses a key with an index into a list element, say index 2. Now if
 * an update occurs on the underlying nodes structure which removes the first element in this list structure, the
 * {@code SubnodeConfiguration} still references the element with index 2 which is now another one.
 * </p>
 * <p>
 * There are also possible changes of the underlying nodes structure which completely detach the
 * {@code SubnodeConfiguration} from its parent configuration. For instance, the key referenced by the
 * {@code SubnodeConfiguration} could be removed in the parent configuration. If this happens, the
 * {@code SubnodeConfiguration} stays functional; however, it now operates on a separate node model than its parent
 * configuration. Changes made by one configuration are no longer visible for the other one (as the node models have no
 * longer overlapping nodes, there is no way to have a synchronization here).
 * </p>
 * <p>
 * When a subnode configuration is created, it inherits the settings of its parent configuration, for example some flags like
 * the {@code throwExceptionOnMissing} flag or the settings for handling list delimiters) or the expression engine. If
 * these settings are changed later in either the subnode or the parent configuration, the changes are not visible for
 * each other. So you could create a subnode configuration, and change its expression engine without affecting the
 * parent configuration.
 * </p>
 * <p>
 * Because the {@code SubnodeConfiguration} operates on the same nodes structure as its parent it uses the same
 * {@code Synchronizer} instance per default. This means that locks held on one {@code SubnodeConfiguration} also impact
 * the parent configuration and all of its other {@code SubnodeConfiguration} objects. You should not change this
 * without a good reason! Otherwise, there is the risk of data corruption when multiple threads access these
 * configuration concurrently.
 * </p>
 * <p>
 * From its purpose this class is quite similar to {@link SubsetConfiguration}. The difference is that a subset
 * configuration of a hierarchical configuration may combine multiple configuration nodes from different sub trees of
 * the configuration, while all nodes in a subnode configuration belong to the same sub tree. If an application can live
 * with this limitation, it is recommended to use this class instead of {@code SubsetConfiguration} because creating a
 * subset configuration is more expensive than creating a subnode configuration.
 * </p>
 * <p>
 * It is strongly recommended to create {@code SubnodeConfiguration} instances only through the
 * {@code configurationAt()} methods of a hierarchical configuration. These methods ensure that all necessary
 * initializations are done. Creating instances manually without doing proper initialization may break some of the
 * functionality provided by this class.
 * </p>
 *
 * @since 1.3
 */
public class SubnodeConfiguration extends BaseHierarchicalConfiguration {

    /** Stores the parent configuration. */
    private final BaseHierarchicalConfiguration parent;

    /** The node selector selecting the root node of this configuration. */
    private final NodeSelector rootSelector;

    /**
     * Creates a new instance of {@code SubnodeConfiguration} and initializes it with all relevant properties.
     *
     * @param parent the parent configuration
     * @param model the {@code TrackedNodeModel} to be used for this configuration
     * @throws IllegalArgumentException if a required argument is missing
     */
    public SubnodeConfiguration(final BaseHierarchicalConfiguration parent, final TrackedNodeModel model) {
        super(model);
        if (parent == null) {
            throw new IllegalArgumentException("Parent configuration must not be null!");
        }
        if (model == null) {
            throw new IllegalArgumentException("Node model must not be null!");
        }

        this.parent = parent;
        rootSelector = model.getSelector();
    }

    /**
     * {@inheritDoc} This implementation returns a copy of the current node model with the same settings. However, it has to
     * be ensured that the track count for the node selector is increased.
     *
     * @return the node model for the clone
     */
    @Override
    protected NodeModel<ImmutableNode> cloneNodeModel() {
        final InMemoryNodeModel parentModel = (InMemoryNodeModel) getParent().getModel();
        parentModel.trackNode(getRootSelector(), getParent());
        return new TrackedNodeModel(getParent(), getRootSelector(), true);
    }

    /**
     * Closes this sub configuration. This method closes the underlying {@link TrackedNodeModel}, thus causing the tracked
     * node acting as root node to be released. Per default, this happens automatically when the model is claimed by the
     * garbage collector. By calling this method explicitly, it can be indicated that this configuration is no longer used
     * and that resources used by it can be freed immediately.
     */
    public void close() {
        getTrackedModel().close();
    }

    /**
     * {@inheritDoc} This implementation makes sure that the correct node model (the one of the parent) is used for the new
     * sub configuration.
     */
    @Override
    protected SubnodeConfiguration createSubConfigurationForTrackedNode(final NodeSelector selector, final InMemoryNodeModelSupport parentModelSupport) {
        return super.createSubConfigurationForTrackedNode(selector, getParent());
    }

    /**
     * {@inheritDoc} This implementation returns a newly created node model with the correct root node set. Note that this
     * model is not used for property access, but only made available to clients that need to operate on the node structure
     * of this {@code SubnodeConfiguration}. Be aware that the implementation of this method is not very efficient.
     */
    @Override
    public InMemoryNodeModel getNodeModel() {
        final ImmutableNode root = getParent().getNodeModel().getTrackedNode(getRootSelector());
        return new InMemoryNodeModel(root);
    }

    /**
     * Gets the parent configuration of this subnode configuration.
     *
     * @return the parent configuration
     */
    public BaseHierarchicalConfiguration getParent() {
        return parent;
    }

    /**
     * Gets the node model of the root configuration. {@code SubnodeConfiguration} instances created from a hierarchical
     * configuration operate on the same node model, using different nodes as their local root nodes. With this method the
     * top-level node model can be obtained. It works even in constellations where a {@code SubnodeConfiguration} has been
     * created from another {@code SubnodeConfiguration}.
     *
     * @return the root node model
     * @since 2.2
     */
    public InMemoryNodeModel getRootNodeModel() {
        if (getParent() instanceof SubnodeConfiguration) {
            return ((SubnodeConfiguration) getParent()).getRootNodeModel();
        }
        return getParent().getNodeModel();
    }

    /**
     * Gets the selector to the root node of this configuration.
     *
     * @return the {@code NodeSelector} to the root node
     */
    public NodeSelector getRootSelector() {
        return rootSelector;
    }

    /**
     * {@inheritDoc} This implementation returns a sub selector of the selector of this configuration.
     */
    @Override
    protected NodeSelector getSubConfigurationNodeSelector(final String key) {
        return getRootSelector().subSelector(key);
    }

    /**
     * {@inheritDoc} This implementation returns the parent model of the {@link TrackedNodeModel} used by this
     * configuration.
     */
    @Override
    protected InMemoryNodeModel getSubConfigurationParentModel() {
        return getTrackedModel().getParentModel();
    }

    /**
     * Convenience method that returns the tracked model used by this sub configuration.
     *
     * @return the {@code TrackedNodeModel}
     */
    private TrackedNodeModel getTrackedModel() {
        return (TrackedNodeModel) getModel();
    }
}