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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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/*
 * Copyright (c) 2003 The Visigoth Software Society. All rights
 * reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * 3. The end-user documentation included with the redistribution, if
 *    any, must include the following acknowledgement:
 *       "This product includes software developed by the
 *        Visigoth Software Society (http://www.visigoths.org/)."
 *    Alternately, this acknowledgement may appear in the software itself,
 *    if and wherever such third-party acknowledgements normally appear.
 *
 * 4. Neither the name "FreeMarker", "Visigoth", nor any of the names of the 
 *    project contributors may be used to endorse or promote products derived
 *    from this software without prior written permission. For written
 *    permission, please contact [email protected].
 *
 * 5. Products derived from this software may not be called "FreeMarker" or "Visigoth"
 *    nor may "FreeMarker" or "Visigoth" appear in their names
 *    without prior written permission of the Visigoth Software Society.
 *
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESSED OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED.  IN NO EVENT SHALL THE VISIGOTH SOFTWARE SOCIETY OR
 * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
 * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 * ====================================================================
 *
 * This software consists of voluntary contributions made by many
 * individuals on behalf of the Visigoth Software Society. For more
 * information on the Visigoth Software Society, please see
 * http://www.visigoths.org/
 */

package freemarker.ext.beans;

import java.util.HashMap;
import java.util.Map;

import freemarker.template.TemplateHashModel;
import freemarker.template.TemplateModel;
import freemarker.template.TemplateModelException;
import freemarker.template.utility.ClassUtil;

/**
 * Base class for hash models keyed by Java class names. 
 * @author Attila Szegedi
 * @version $Id: ClassBasedModelFactory.java,v 1.1.2.2 2006/11/12 10:23:02 szegedia Exp $
 */
abstract class ClassBasedModelFactory implements TemplateHashModel {
    private final BeansWrapper wrapper;
    private final Map cache = new HashMap();
    
    protected ClassBasedModelFactory(BeansWrapper wrapper) {
        this.wrapper = wrapper;
    }

    public TemplateModel get(String key) throws TemplateModelException {
        synchronized(cache) {
            TemplateModel model = (TemplateModel)cache.get(key);
            if(model == null) {
                try {
                    Class clazz = ClassUtil.forName(key);
                    model = createModel(clazz);
                    // This is called so that we trigger the
                    // class-reloading detector. If there was a class reload,
                    // the wrapper will in turn call our clearCache method.
                    wrapper.introspectClass(clazz);
                } catch(Exception e) {
                    throw new TemplateModelException(e);
                }
                cache.put(key, model);
            }
            return model;
        }
    }
    
    void clearCache() {
        synchronized(cache) {
            cache.clear();
        }
    }
    
    void removeIntrospectionInfo(Class clazz) {
        synchronized(cache) {
            cache.remove(clazz.getName());
        }
    }

    public boolean isEmpty() {
        return false;
    }
    
    protected abstract TemplateModel createModel(Class clazz) 
    throws TemplateModelException;
    
    protected BeansWrapper getWrapper() {
        return wrapper;
    }
}