http://commons.apache.org/configuration/: Tools to assist in the reading of configuration/preferences files in various formats. (The Apache Software Foundation)
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/*
 * 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
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 */
package org.apache.commons.configuration.tree;

import java.util.List;



Definition of an interface for evaluating keys for hierarchical
configurations.




An expression engine knows how to map a key for a configuration's property to a single or a set of configuration nodes. Thus it defines the way how properties are addressed in this configuration. Methods of a configuration that have to handle property key (e.g. getProperty() or addProperty() do not interpret the passed in keys on their own, but delegate this task to an associated expression engine. This expression engine will then find out, which configuration nodes are addressed by the key. 



Separating the task of evaluating property keys from the configuration object
has the advantage that many different expression languages (i.e. ways for
querying or setting properties) can be supported. Just set a suitable
implementation of this interface as the configuration's expression engine,
and you can use the syntax provided by this implementation.



Author:Commons
Configuration team
Since:1.3
Version:$Id: ExpressionEngine.java 1206474 2011-11-26 16:14:09Z oheger $
/**
 * <p>
 * Definition of an interface for evaluating keys for hierarchical
 * configurations.
 * </p>
 * <p>
 * An <em>expression engine</em> knows how to map a key for a configuration's
 * property to a single or a set of configuration nodes. Thus it defines the way
 * how properties are addressed in this configuration. Methods of a
 * configuration that have to handle property key (e.g.
 * {@code getProperty()} or {@code addProperty()} do not interpret
 * the passed in keys on their own, but delegate this task to an associated
 * expression engine. This expression engine will then find out, which
 * configuration nodes are addressed by the key.
 * </p>
 * <p>
 * Separating the task of evaluating property keys from the configuration object
 * has the advantage that many different expression languages (i.e. ways for
 * querying or setting properties) can be supported. Just set a suitable
 * implementation of this interface as the configuration's expression engine,
 * and you can use the syntax provided by this implementation.
 * </p>
 *
 * @since 1.3
 * @author <a
 * href="http://commons.apache.org/configuration/team-list.html">Commons
 * Configuration team</a>
 * @version $Id: ExpressionEngine.java 1206474 2011-11-26 16:14:09Z oheger $
 */
public interface ExpressionEngine
{
    
Finds the node(s) that is (are) matched by the specified key. This is the
main method for interpreting property keys. An implementation must
traverse the given root node and its children to find all nodes that are
matched by the given key. If the key is not correct in the syntax
provided by that implementation, it is free to throw a (runtime)
exception indicating this error condition.

Params:
  • root – the root node of a hierarchy of configuration nodes
  • key – the key to be evaluated
Returns:a list with the nodes that are matched by the key (should never
be null)
/**
     * Finds the node(s) that is (are) matched by the specified key. This is the
     * main method for interpreting property keys. An implementation must
     * traverse the given root node and its children to find all nodes that are
     * matched by the given key. If the key is not correct in the syntax
     * provided by that implementation, it is free to throw a (runtime)
     * exception indicating this error condition.
     *
     * @param root the root node of a hierarchy of configuration nodes
     * @param key the key to be evaluated
     * @return a list with the nodes that are matched by the key (should never
     * be <b>null</b>)
     */
    List<ConfigurationNode> query(ConfigurationNode root, String key);

    
Returns the key for the specified node in the expression language supported by an implementation. This method is called whenever a property key for a node has to be constructed, e.g. by the getKeys() method. 
Params:
  • node – the node, for which the key must be constructed
  • parentKey – the key of this node's parent (can be null for
the root node)
Returns:this node's key
/**
     * Returns the key for the specified node in the expression language
     * supported by an implementation. This method is called whenever a property
     * key for a node has to be constructed, e.g. by the
     * {@link org.apache.commons.configuration.Configuration#getKeys() getKeys()}
     * method.
     *
     * @param node the node, for which the key must be constructed
     * @param parentKey the key of this node's parent (can be <b>null</b> for
     * the root node)
     * @return this node's key
     */
    String nodeKey(ConfigurationNode node, String parentKey);

    
Returns information needed for an add operation. This method gets called
when new properties are to be added to a configuration. An implementation
has to interpret the specified key, find the parent node for the new
elements, and provide all information about new nodes to be added.

Params:
  • root – the root node
  • key – the key for the new property
Returns:an object with all information needed for the add operation
/**
     * Returns information needed for an add operation. This method gets called
     * when new properties are to be added to a configuration. An implementation
     * has to interpret the specified key, find the parent node for the new
     * elements, and provide all information about new nodes to be added.
     *
     * @param root the root node
     * @param key the key for the new property
     * @return an object with all information needed for the add operation
     */
    NodeAddData prepareAdd(ConfigurationNode root, String key);
}