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With inspiration from other libraries
There is a newer version: 2.12.15
/**
 * Logback: the reliable, generic, fast and flexible logging framework.
 * Copyright (C) 1999-2015, QOS.ch. All rights reserved.
 *
 * This program and the accompanying materials are dual-licensed under
 * either the terms of the Eclipse Public License v1.0 as published by
 * the Eclipse Foundation
 *
 *   or (per the licensee's choosing)
 *
 * under the terms of the GNU Lesser General Public License version 2.1
 * as published by the Free Software Foundation.
 */
package ch.qos.logback.core.spi;

import ch.qos.logback.core.CoreConstants;

import java.util.Collection;
import java.util.Set;

/**
 * Interface for tracking various components by key. Components which have not
 * been accessed for more than a user-specified duration are deemed stale and
 * removed. Components can also be explicitly marked as having reached their
 * {@link #endOfLife(String)} in which case they will linger for a few seconds
 * and then be removed.
 *
 * @author Tommy Becker
 * @author Ceki Gulcu
 * @author David Roussel
 *
 * @since 1.0.12
 */
public interface ComponentTracker {

    /**
     * The default timeout duration is 30 minutes
     */
    public final int DEFAULT_TIMEOUT = (int) (30 * 60 * CoreConstants.MILLIS_IN_ONE_SECOND); // 30 minutes

    /**
     * By default an unlimited number of elements can be tracked.
     */
    int DEFAULT_MAX_COMPONENTS = Integer.MAX_VALUE;

    /**
     * Returns the number of components tracked.
     * @return number of components
     */
    int getComponentCount();

    /**
     * Find the component identified by 'key', without updating the timestamp. Returns null if no
     * corresponding component could be found.
     *
     * @param key
     * @return corresponding component, may be null
     */
    C find(String key);

    /**
     * Get the component identified by 'key', updating its timestamp in the
     * process. If the corresponding component could not be found, it is created.
     *
     * @param key
     * @param timestamp
     * @return
     */
    C getOrCreate(String key, long timestamp);

    /**
     * Remove components which are deemed stale. Components which have not
     * been accessed for more than a user-specified duration are deemed stale.
     *
     * If the number of components exceeds, {@link #getComponentCount()},
     * components in excess will be removed.
     *
     * Depending on the component type, components will be cleared or stopped
     * (as appropriate) right before removal.
     *
     * @param now  current time in milliseconds
     */
    void removeStaleComponents(long now);

    /**
     * Mark component identified by 'key' as having reached its end-of-life. End-of-lifed
     * components will linger for a few more seconds before being removed.
     *
     * @param key
     */
    void endOfLife(String key);

    /**
     * Returns the collection of all components tracked by this instance.
     * @return  collection of components
     */
    Collection allComponents();

    /**
     * Set of all keys in this tracker in no particular order.
     *
     * @return
     */
    Set allKeys();
}