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Constrains the number of concurrently mounted archive file systems in order to save some heap space. This module provides a JMX interface for monitoring and management. Add the JAR artifact of this module to the run time class path to make its services available for service location in the client API modules.

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/*
 * Copyright © 2005 - 2021 Schlichtherle IT Services.
 * All rights reserved. Use is subject to license terms.
 */
package net.java.truevfs.ext.pacemaker;

import net.java.truevfs.comp.inst.InstrumentingCompositeDriver;
import net.java.truevfs.comp.inst.InstrumentingManager;
import net.java.truevfs.comp.jmx.JmxMediator;
import net.java.truevfs.kernel.spec.*;

import java.util.Set;

import static java.lang.Math.max;

/**
 * A mediator for the instrumentation of the TrueVFS Kernel with a {@link PaceManager} and a {@link PaceController}.
 *
 * @author Christian Schlichtherle
 */
final class PaceMediator extends JmxMediator {

    /**
     * The name of the system property which determines the initial maximum number of mounted archive file systems.
     */
    private static final String maximumFileSystemsMountedPropertyKey =
            PaceMediator.class.getPackage().getName() + ".maximumFileSystemsMounted";

    /**
     * The minimum value for the maximum number of mounted archive file systems.
     * This value must not be less than two or otherwise you couldn't even copy entries from one archive file to
     * another.
     * Well, actually you could because the pace manager doesn't unmount archive file systems with open streams or
     * channels, but let's play it safe and pretend it would.
     */
    private static final int maximumFileSystemsMountedMinimumValue = 2;

    /**
     * The default value of the system property which determines the initial maximum number of mounted archive file
     * systems.
     * The value of this constant will be set to {@link #maximumFileSystemsMountedMinimumValue} unless a system
     * property with the key string {@link #maximumFileSystemsMountedPropertyKey} is set to a value which is greater
     * than {@link #maximumFileSystemsMountedMinimumValue}.
     */
    private static final int maximumFileSystemsMountedDefaultValue = max(
            maximumFileSystemsMountedMinimumValue,
            Integer.getInteger(maximumFileSystemsMountedPropertyKey, maximumFileSystemsMountedMinimumValue)
    );

    final LruCache cachedMountPoints = new LruCache<>(maximumFileSystemsMountedDefaultValue);
    final Set evictedMountPoints = cachedMountPoints.getEvictedView();

    int getMaximumSize() {
        return cachedMountPoints.getMaximumSize();
    }

    void setMaximumSize(final int maximumSize) {
        if (maximumFileSystemsMountedMinimumValue > maximumSize) {
            throw new IllegalArgumentException();
        }
        cachedMountPoints.setMaximumSize(maximumSize);
    }

    @Override
    public FsManager instrument(FsManager subject) {
        return activate(new PaceManager(this, subject));
    }

    @Override
    public FsCompositeDriver instrument(InstrumentingManager context, FsCompositeDriver subject) {
        return new InstrumentingCompositeDriver<>(this, subject);
    }

    // FIXME: Shouldn't the first parameter be the InstrumentingCompositeDriver?
    // So then the given controller should get decorated and memoized only once instead of decorating it time and time
    // again when there is a controller lookup.
    @Override
    public FsController instrument(InstrumentingManager context, FsController subject) {
        return new PaceController((PaceManager) context, subject);
    }

    @Override
    public FsModel instrument(InstrumentingCompositeDriver context, FsModel subject) {
        return new PaceModel(this, subject);
    }
}