jetbrick.collection.SoftHashMap Maven / Gradle / Ivy

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/**
 * Copyright 2013-2016 Guoqiang Chen, Shanghai, China. All rights reserved.
 *
 *   Author: Guoqiang Chen
 *    Email: [email protected]
 *   WebURL: https://github.com/subchen
 *
 * Licensed 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 jetbrick.collection;

import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.util.AbstractMap;
import java.util.HashMap;
import java.util.LinkedHashSet;
import java.util.Map;
import java.util.Set;

public class SoftHashMap extends AbstractMap {
    private final Map> map;
    private final ReferenceQueue queue;

    public SoftHashMap() {
        this.map = new HashMap>();
        this.queue = new ReferenceQueue();
    }

    public SoftHashMap(int initialCapacity) {
        this.map = new HashMap>(initialCapacity);
        this.queue = new ReferenceQueue();
    }

    @SuppressWarnings("unchecked")
    private void processQueue() {
        SoftValue o;
        while ((o = (SoftValue) queue.poll()) != null) {
            map.remove(o.key);
        }
    }

    @Override
    public V get(Object key) {
        processQueue();
        SoftValue ref = map.get(key);
        return (ref != null) ? ref.get() : null;
    }

    @Override
    public V put(K key, V softValue) {
        processQueue();
        SoftValue ref = map.put(key, new SoftValue(softValue, key, queue));
        return (ref != null) ? ref.get() : null;
    }

    @Override
    public V remove(Object key) {
        processQueue();
        SoftValue ref = map.remove(key);
        return (ref != null) ? ref.get() : null;
    }

    @Override
    public void clear() {
        processQueue();
        map.clear();
    }

    @Override
    public int size() {
        processQueue();
        return map.size();
    }

    /**
     * Returns a copy of the key/values in the map at the point of
     * calling.  However, setValue still sets the value in the
     * actual SoftHashMap.
     */
    @Override
    public Set> entrySet() {
        processQueue();
        Set> result = new LinkedHashSet>();
        for (final Entry> entry : map.entrySet()) {
            final V value = entry.getValue().get();
            if (value != null) {
                result.add(new Entry() {
                    @Override
                    public K getKey() {
                        return entry.getKey();
                    }

                    @Override
                    public V getValue() {
                        return value;
                    }

                    @Override
                    public V setValue(V v) {
                        entry.setValue(new SoftValue(v, entry.getKey(), queue));
                        return value;
                    }
                });
            }
        }
        return result;
    }

    /**
     * A soft reference that has a hard reference to the key.
     */
    private static class SoftValue extends SoftReference {
        private final Object key;

        private SoftValue(T ref, Object key, ReferenceQueue queue) {
            super(ref, queue);
            this.key = key;
        }
    }
}