/*
 * Copyright (c) 2003, 2012, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */
package sun.rmi.server;

import java.lang.ref.Reference;
import java.lang.ref.SoftReference;
import java.util.Map;
import java.util.WeakHashMap;

Abstract class that maps Class objects to lazily-computed values of
type V.  A concrete subclass must implement the computeValue method
to determine how the values are computed.
The keys are only weakly reachable through this map, so this map
does not prevent a class (along with its class loader, etc.) from
being garbage collected if it is not otherwise strongly reachable.
The values are only softly reachable through this map, so that the
computed values generally persist while not otherwise strongly
reachable, but their storage may be reclaimed if necessary.  Also,
note that if a key is strongly reachable from a value, then the key
is effectively softly reachable through this map, which may delay
garbage collection of classes (see 4429536).
/**
 * Abstract class that maps Class objects to lazily-computed values of
 * type V.  A concrete subclass must implement the computeValue method
 * to determine how the values are computed.
 *
 * The keys are only weakly reachable through this map, so this map
 * does not prevent a class (along with its class loader, etc.) from
 * being garbage collected if it is not otherwise strongly reachable.
 * The values are only softly reachable through this map, so that the
 * computed values generally persist while not otherwise strongly
 * reachable, but their storage may be reclaimed if necessary.  Also,
 * note that if a key is strongly reachable from a value, then the key
 * is effectively softly reachable through this map, which may delay
 * garbage collection of classes (see 4429536).
 **/
public abstract class WeakClassHashMap<V> {

    private Map<Class<?>,ValueCell<V>> internalMap = new WeakHashMap<>();

    protected WeakClassHashMap() { }

    public V get(Class<?> remoteClass) {
        /*
         * Use a mutable cell (a one-element list) to hold the soft
         * reference to a value, to allow the lazy value computation
         * to be synchronized with entry-level granularity instead of
         * by locking the whole table.
         */
        ValueCell<V> valueCell;
        synchronized (internalMap) {
            valueCell = internalMap.get(remoteClass);
            if (valueCell == null) {
                valueCell = new ValueCell<V>();
                internalMap.put(remoteClass, valueCell);
            }
        }
        synchronized (valueCell) {
            V value = null;
            if (valueCell.ref != null) {
                value = valueCell.ref.get();
            }
            if (value == null) {
                value = computeValue(remoteClass);
                valueCell.ref = new SoftReference<V>(value);
            }
            return value;
        }
    }

    protected abstract V computeValue(Class<?> remoteClass);

    private static class ValueCell<T> {
        Reference<T> ref = null;
        ValueCell() { }
    }
}