foundation/org.eclipse.persistence.core/src/main/java/org/eclipse/persistence/internal/identitymaps/SoftCacheWeakIdentityMap.java

/*
 * Copyright (c) 1998, 2019 Oracle and/or its affiliates. All rights reserved.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v. 2.0 which is available at
 * http://www.eclipse.org/legal/epl-2.0,
 * or the Eclipse Distribution License v. 1.0 which is available at
 * http://www.eclipse.org/org/documents/edl-v10.php.
 *
 * SPDX-License-Identifier: EPL-2.0 OR BSD-3-Clause
 */

// Contributors:
//     Oracle - initial API and implementation from Oracle TopLink
package org.eclipse.persistence.internal.identitymaps;

import java.lang.ref.SoftReference;

import org.eclipse.persistence.descriptors.ClassDescriptor;
import org.eclipse.persistence.internal.sessions.AbstractSession;

/**
 * <p><b>Purpose</b>: A SoftCacheWeakIdentityMap is identical to the WeakIdentityMap, however the weak reference
 * can be a performance problem for some types of apps because it can cause too much garbage collection
 * of objects read causing them to be re-read and re-built (this defeats the purpose of the cache).
 * The SoftCacheWeakIdentityMap solves this through also holding a fixed number of objects in memory to improve caching.
 * <p><b>Responsibilities</b>:<ul>
 * <li> Guarantees identity
 * <li> Allows garbage collection
 * <li> Increases performance by maintaining a fixed size cache of MRU objects when memory is available.
 * <li> The default size of the reference cache is the max size.
 * </ul>
 * @since TOPLink/Java 1.2
 */
public class SoftCacheWeakIdentityMap extends HardCacheWeakIdentityMap {

    public SoftCacheWeakIdentityMap(int size, ClassDescriptor descriptor, AbstractSession session, boolean isIsolated) {
        super(size, descriptor, session, isIsolated);
    }

    /**
     * Creates a Soft reference to the object.
     * @param object is the domain object to cache.
     */
    @Override
    public Object buildReference(Object object) {
        if (object != null) {
            return new SoftReference(object);
        } else {
            return null;
        }
    }

    /**
     * Checks if the object is null, or reference's object is null.
     * @param reference the object for hard or the reference for soft.
     */
    @Override
    public boolean hasReference(Object reference) {
        return (reference != null) && (((SoftReference)reference).get() != null);
    }
}