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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.ext.beans;

import java.util.HashSet;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;

import freemarker.core._DelayedJQuote;
import freemarker.core._TemplateModelException;
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. 
 */
abstract class ClassBasedModelFactory implements TemplateHashModel {
    private final BeansWrapper wrapper;
    
    private final Map cache = new ConcurrentHashMap<>();
    private final Set classIntrospectionsInProgress = new HashSet<>();
    
    protected ClassBasedModelFactory(BeansWrapper wrapper) {
        this.wrapper = wrapper;
    }

    @Override
    public TemplateModel get(String key) throws TemplateModelException {
        try {
            return getInternal(key);
        } catch (Exception e) {
            if (e instanceof TemplateModelException) {
                throw (TemplateModelException) e;
            } else {
                throw new _TemplateModelException(e,
                        "Failed to get value for key ", new _DelayedJQuote(key), "; see cause exception.");
            }
        }
    }

    private TemplateModel getInternal(String key) throws TemplateModelException, ClassNotFoundException {
        {
            TemplateModel model = cache.get(key);
            if (model != null) return model;
        }

        final ClassIntrospector classIntrospector;
        int classIntrospectorClearingCounter;
        final Object sharedLock = wrapper.getSharedIntrospectionLock();
        synchronized (sharedLock) {
            TemplateModel model = cache.get(key);
            if (model != null) return model;
            
            while (model == null && classIntrospectionsInProgress.contains(key)) {
                // Another thread is already introspecting this class;
                // waiting for its result.
                try {
                    sharedLock.wait();
                    model = cache.get(key);
                } catch (InterruptedException e) {
                    throw new RuntimeException("Class inrospection data lookup aborted: " + e);
                }
            }
            if (model != null) return model;
            
            // This will be the thread that introspects this class.
            classIntrospectionsInProgress.add(key);

            // While the classIntrospector should not be changed from another thread, badly written apps can do that,
            // and it's cheap to get the classIntrospector from inside the lock here:   
            classIntrospector = wrapper.getClassIntrospector();
            classIntrospectorClearingCounter = classIntrospector.getClearingCounter();
        }
        try {
            final Class clazz = ClassUtil.forName(key);
            
            // This is called so that we trigger the
            // class-reloading detector. If clazz is a reloaded class,
            // the wrapper will in turn call our clearCache method.
            // TODO: Why do we check it now and only now?
            classIntrospector.get(clazz);
            
            TemplateModel model = createModel(clazz);
            // Warning: model will be null if the class is not good for the subclass.
            // For example, EnumModels#createModel returns null if clazz is not an enum.
            
            if (model != null) {
                synchronized (sharedLock) {
                    // Save it into the cache, but only if nothing relevant has changed while we were outside the lock: 
                    if (classIntrospector == wrapper.getClassIntrospector()
                            && classIntrospectorClearingCounter == classIntrospector.getClearingCounter()) {  
                        cache.put(key, model);
                    }
                }
            }
            return model;
        } finally {
            synchronized (sharedLock) {
                classIntrospectionsInProgress.remove(key);
                sharedLock.notifyAll();
            }
        }
    }
    
    void clearCache() {
        synchronized (wrapper.getSharedIntrospectionLock()) {
            cache.clear();
        }
    }
    
    void removeFromCache(Class clazz) {
        synchronized (wrapper.getSharedIntrospectionLock()) {
            cache.remove(clazz.getName());
        }
    }

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