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Google App Engine compliant variation of FreeMarker. FreeMarker is a "template engine"; a generic tool to generate text output based on templates.

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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 limitations
 * under the License.
 */

package freemarker.template;

/**
 * "node" template language data type: an object that is a node in a tree.
 * A tree of nodes can be recursively visited using the <#visit...> and <#recurse...>
 * directives. This API is largely based on the W3C Document Object Model
 * (DOM) API. However, it's meant to be generally useful for describing
 * any tree of objects that you wish to navigate using a recursive visitor
 * design pattern (or simply through being able to get the parent
 * and child nodes).
 * 
 * See the XML
 * Processing Guide for a concrete application.
 *
 * @since FreeMarker 2.3
 */
public interface TemplateNodeModel extends TemplateModel {
    
    /**
     * @return the parent of this node or null, in which case
     * this node is the root of the tree.
     */
    TemplateNodeModel getParentNode() throws TemplateModelException;
    
    /**
     * @return a sequence containing this node's children.
     * If the returned value is null or empty, this is essentially 
     * a leaf node.
     */
    TemplateSequenceModel getChildNodes() throws TemplateModelException;

    /**
     * @return a String that is used to determine the processing
     * routine to use. In the XML implementation, if the node 
     * is an element, it returns the element's tag name.  If it
     * is an attribute, it returns the attribute's name. It 
     * returns "@text" for text nodes, "@pi" for processing instructions,
     * and so on.
     */    
    String getNodeName() throws TemplateModelException;
    
    /**
     * @return a String describing the type of node this is.
     * In the W3C DOM, this should be "element", "text", "attribute", etc.
     * A TemplateNodeModel implementation that models other kinds of
     * trees could return whatever it appropriate for that application. It
     * can be null, if you don't want to use node-types.
     */
    String getNodeType() throws TemplateModelException;
    
    
    /**
     * @return the XML namespace URI with which this node is 
     * associated. If this TemplateNodeModel implementation is 
     * not XML-related, it will almost certainly be null. Even 
     * for XML nodes, this will often be null.
     */
    String getNodeNamespace() throws TemplateModelException;
}