src/main/java/freemarker/ext/util/ModelCache.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
 * 
 *   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.util;

import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.util.Map;

import freemarker.template.TemplateModel;
import freemarker.template.TemplateModelAdapter;

/**
 * Internally used by various wrapper implementations to implement model
 * caching.
 */
public abstract class ModelCache {
    private boolean useCache = false;
    private Map modelCache = null;
    private ReferenceQueue refQueue = null;
    
    protected ModelCache() {
    }
    
    /**
     * Sets whether this wrapper caches model instances. Default is false.
     * When set to true, calling {@link #getInstance(Object)} 
     * multiple times for the same object will return the same model.
     */
    public synchronized void setUseCache(boolean useCache) {
        this.useCache = useCache;
        if (useCache) {
            modelCache = new IdentityHashMap();
            refQueue = new ReferenceQueue();
        } else {
            modelCache = null;
            refQueue = null;
        }
    }

    /**
     * @since 2.3.21
     */
    public synchronized boolean getUseCache() {
        return useCache;
    }
    
    public TemplateModel getInstance(Object object) {
        if (object instanceof TemplateModel) {
            return (TemplateModel) object;
        }
        if (object instanceof TemplateModelAdapter) {
            return ((TemplateModelAdapter) object).getTemplateModel();
        }
        if (useCache && isCacheable(object)) {
            TemplateModel model = lookup(object);
            if (model == null) {
                model = create(object);
                register(model, object);
            }
            return model;
        } else {
            return create(object);
        }
    }
    
    protected abstract TemplateModel create(Object object);
    protected abstract boolean isCacheable(Object object);
    
    public void clearCache() {
        if (modelCache != null) {
            synchronized (modelCache) {
                modelCache.clear();
            }
        }
    }

    private final TemplateModel lookup(Object object) {
        ModelReference ref = null;
        // NOTE: we're doing minimal synchronizations -- which can lead to
        // duplicate wrapper creation. However, this has no harmful side-effects and
        // is a lesser performance hit.
        synchronized (modelCache) {
            ref = (ModelReference) modelCache.get(object);
        }

        if (ref != null)
            return ref.getModel();

        return null;
    }

    private final void register(TemplateModel model, Object object) {
        synchronized (modelCache) {
            // Remove cleared references
            for (; ; ) {
                ModelReference queuedRef = (ModelReference) refQueue.poll();
                if (queuedRef == null)
                    break;
                modelCache.remove(queuedRef.object);
            }
            // Register new reference
            modelCache.put(object, new ModelReference(model, object, refQueue));
        }
    }

    /**
     * A special soft reference that is registered in the modelCache.
     * When it gets cleared (that is, the model became unreachable)
     * it will remove itself from the model cache.
     */
    private static final class ModelReference extends SoftReference {
        Object object;

        ModelReference(TemplateModel ref, Object object, ReferenceQueue refQueue) {
            super(ref, refQueue);
            this.object = object;
        }

        TemplateModel getModel() {
            return (TemplateModel) this.get();
        }
    }

}