/*
 * 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.cache;

import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;

import freemarker.template.utility.UndeclaredThrowableException;

Soft cache storage is a cache storage that uses SoftReference objects to hold the objects it was passed, therefore allows the garbage collector to purge the cache when it determines that it wants to free up memory. This class is thread-safe to the extent that its underlying map is. The parameterless constructor uses a thread-safe map since 2.3.24 or Java 5. 
See Also:
Configuration.setCacheStorage(CacheStorage)/**
 * Soft cache storage is a cache storage that uses {@link SoftReference} objects to hold the objects it was passed,
 * therefore allows the garbage collector to purge the cache when it determines that it wants to free up memory. This
 * class is thread-safe to the extent that its underlying map is. The parameterless constructor uses a thread-safe map
 * since 2.3.24 or Java 5.
 *
 * @see freemarker.template.Configuration#setCacheStorage(CacheStorage)
 */
public class SoftCacheStorage implements ConcurrentCacheStorage, CacheStorageWithGetSize {
    private static final Method atomicRemove = getAtomicRemoveMethod();
    
    private final ReferenceQueue queue = new ReferenceQueue();
    private final Map map;
    private final boolean concurrent;
    
    
Creates an instance that uses a ConcurrentMap internally. /**
     * Creates an instance that uses a {@link ConcurrentMap} internally.
     */
    public SoftCacheStorage() {
        this(new ConcurrentHashMap());
    }
    
    
Returns true if the underlying Map is a ConcurrentMap. /**
     * Returns true if the underlying Map is a {@code ConcurrentMap}.
     */
    public boolean isConcurrent() {
        return concurrent;
    }
    
    public SoftCacheStorage(Map backingMap) {
        map = backingMap;
        this.concurrent = map instanceof ConcurrentMap;
    }
    
    public Object get(Object key) {
        processQueue();
        Reference ref = (Reference) map.get(key);
        return ref == null ? null : ref.get();
    }

    public void put(Object key, Object value) {
        processQueue();
        map.put(key, new SoftValueReference(key, value, queue));
    }

    public void remove(Object key) {
        processQueue();
        map.remove(key);
    }

    public void clear() {
        map.clear();
        processQueue();
    }
    
    
Returns a close approximation of the number of cache entries.
Since:
2.3.21/**
     * Returns a close approximation of the number of cache entries.
     * 
     * @since 2.3.21
     */
    public int getSize() {
        processQueue();
        return map.size();
    }

    private void processQueue() {
        for (; ; ) {
            SoftValueReference ref = (SoftValueReference) queue.poll();
            if (ref == null) {
                return;
            }
            Object key = ref.getKey();
            if (concurrent) {
                try {
                    atomicRemove.invoke(map, new Object[] { key, ref });
                } catch (IllegalAccessException e) {
                    throw new UndeclaredThrowableException(e);
                } catch (InvocationTargetException e) {
                    throw new UndeclaredThrowableException(e);
                }
            } else if (map.get(key) == ref) {
                map.remove(key);
            }
        }
    }

    private static final class SoftValueReference extends SoftReference {
        private final Object key;

        SoftValueReference(Object key, Object value, ReferenceQueue queue) {
            super(value, queue);
            this.key = key;
        }

        Object getKey() {
            return key;
        }
    }
    
    private static Method getAtomicRemoveMethod() {
        try {
            return Class.forName("java.util.concurrent.ConcurrentMap").getMethod("remove", new Class[] { Object.class, Object.class });
        } catch (ClassNotFoundException e) {
            return null;
        } catch (NoSuchMethodException e) {
            throw new UndeclaredThrowableException(e);
        }
    }
}