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 * DO NOT REMOVE OR ALTER!

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/**

 * 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

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package com.sun.org.apache.xml.internal.security.utils;

import java.lang.ref.WeakReference;

import java.util.Collections;

import java.util.Map;

import java.util.WeakHashMap;

import java.util.concurrent.BlockingQueue;

import java.util.concurrent.LinkedBlockingDeque;

/**

 * Abstract base class for pooling objects.  The two public methods are

 * {@link #getObject()} and ({@link #repool(Object)}.  Objects are held through

 * weak references so even objects that are not repooled are subject to garbage collection.

 *

 * Subclasses must implement the abstract {@link #createObject()}.

 * <p>

 *

 * Internally, the pool is stored in a java.util.concurrent.LinkedBlockingDeque

 * instance.

 */

public abstract class WeakObjectPool<T, E extends Throwable> {

    private static final Integer MARKER_VALUE = Integer.MAX_VALUE;//once here rather than auto-box it?

    /** created, available objects to be checked out to clients */

    private final BlockingQueue<WeakReference<T>> available;

    /**

     * Synchronized, identity map of loaned out objects (WeakHashMap);

     * use to ensure we repool only object originating from here

     * and do it once.

     */

    private final Map<T, Integer> onLoan;

    /**

     * The lone constructor.

     */

    protected WeakObjectPool() {

        //alternative implementations: ArrayBlockingQueue has a fixed size

        //  PriorityBlockingQueue: requires a dummy comparator; less memory but more overhead

        available = new LinkedBlockingDeque<WeakReference<T>>();

        this.onLoan = Collections.synchronizedMap(new WeakHashMap<T, Integer>());

    }

    /**

     * Called whenever a new pool object is desired; subclasses must implement.

     *

     * @return object of the type desired by the subclass

     * @throws E Throwable's subclass

     */

    protected abstract T createObject() throws E;

    /**

     * Subclasses can subclass to return a more specific type.

     *

     * @return an object from the pool; will block until an object is available

     * @throws E

     */

    public T getObject() throws E {

        WeakReference<T> ref;

        T retValue = null;

        do {

            //remove any stale entries as well

            ref = available.poll();

        } while (ref != null && (retValue = ref.get()) == null);

        if (retValue == null) {

            //empty pool; create & add new one

            retValue = createObject();

        }

        onLoan.put(retValue, MARKER_VALUE);

        return retValue;

    }

    /**

     * Adds the given object to the pool, provided that the object

     * was created by this pool.

     *

     * @param obj the object to return to the pool

     * @return whether the object was successfully added as available

     */

    public boolean repool(T obj) {

        if (obj != null && onLoan.containsKey(obj)) {

            //synchronize to protect against a caller returning the same object again...

            synchronized (obj) {

                //...and check to see that it was removed

                if (onLoan.remove(obj) != null) {

                    return available.offer(new WeakReference<T>(obj));

                }

            }

        }

        return false;

    }

}