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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

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 * 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,

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 *

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package sun.management.snmp.util;

import com.sun.jmx.snmp.SnmpOid;

import java.io.Serializable;

import java.util.Comparator;

import java.util.Arrays;

import java.util.TreeMap;

import java.util.List;

import java.util.Iterator;

import java.lang.ref.WeakReference;

/**

 * This abstract class implements a weak cache that holds table data.

 * <p>The table data is stored in an instance of

 * {@link SnmpCachedData}, which is kept in a {@link WeakReference}.

 * If the WeakReference is null or empty, the cached data is recomputed.</p>

 *

 * <p><b>NOTE: This class is not synchronized, subclasses must implement

 *          the appropriate synchronization when needed.</b></p>

 **/

public abstract class SnmpTableCache implements Serializable {

    /**

     * Interval of time in ms during which the cached table data

     * is considered valid.

     **/

    protected long validity;

    /**

     * A weak refernce holding cached table data.

     **/

    protected transient WeakReference<SnmpCachedData> datas;

    /**

     * true if the given cached table data is obsolete.

     **/

    protected boolean isObsolete(SnmpCachedData cached) {

        if (cached   == null) return true;

        if (validity < 0)     return false;

        return ((System.currentTimeMillis() - cached.lastUpdated) > validity);

    }

    /**

     * Returns the cached table data.

     * Returns null if the cached data is obsolete, or if there is no

     * cached data, or if the cached data was garbage collected.

     * @return a still valid cached data or null.

     **/

    protected SnmpCachedData getCachedDatas() {

        if (datas == null) return null;

        final SnmpCachedData cached = datas.get();

        if ((cached == null) || isObsolete(cached)) return null;

        return cached;

    }

    /**

     * Returns the cached table data, if it is still valid,

     * or recompute it if it is obsolete.

     * <p>

     * When cache data is recomputed, store it in the weak reference,

     * unless {@link #validity} is 0: then the data will not be stored

     * at all.<br>

     * This method calls {@link #isObsolete(SnmpCachedData)} to determine

     * whether the cached data is obsolete, and {

     * {@link #updateCachedDatas(Object)} to recompute it.

     * </p>

     * @param context A context object.

     * @return the valid cached data, or the recomputed table data.

     **/

    protected synchronized SnmpCachedData getTableDatas(Object context) {

        final SnmpCachedData cached   = getCachedDatas();

        if (cached != null) return cached;

        final SnmpCachedData computedDatas = updateCachedDatas(context);

        if (validity != 0) datas = new WeakReference<>(computedDatas);

        return computedDatas;

    }

    /**

     * Recompute cached data.

     * @param context A context object, as passed to

     *        {@link #getTableDatas(Object)}

     **/

    protected abstract SnmpCachedData updateCachedDatas(Object context);

    /**

     * Return a table handler that holds the table data.

     * This method should return the cached table data if it is still

     * valid, recompute it and cache the new value if it's not.

     **/

    public abstract SnmpTableHandler getTableHandler();

}