Quadcap Embeddable Database

com/quadcap/sql/file/Cacheable.java

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package com.quadcap.sql.file;

/* Copyright 1997 - 2003 Quadcap Software.  All rights reserved.
 *
 * This software is distributed under the Quadcap Free Software License.
 * This software may be used or modified for any purpose, personal or
 * commercial.  Open Source redistributions are permitted.  Commercial
 * redistribution of larger works derived from, or works which bundle
 * this software requires a "Commercial Redistribution License"; see
 * http://www.quadcap.com/purchase.
 *
 * Redistributions qualify as "Open Source" under  one of the following terms:
 *   
 *    Redistributions are made at no charge beyond the reasonable cost of
 *    materials and delivery.
 *
 *    Redistributions are accompanied by a copy of the Source Code or by an
 *    irrevocable offer to provide a copy of the Source Code for up to three
 *    years at the cost of materials and delivery.  Such redistributions
 *    must allow further use, modification, and redistribution of the Source
 *    Code under substantially the same terms as this license.
 *
 * Redistributions of source code must retain the copyright notices as they
 * appear in each source code file, these license terms, and the
 * disclaimer/limitation of liability set forth as paragraph 6 below.
 *
 * Redistributions in binary form must reproduce this Copyright Notice,
 * these license terms, and the disclaimer/limitation of liability set
 * forth as paragraph 6 below, in the documentation and/or other materials
 * provided with the distribution.
 *
 * The Software is provided on an "AS IS" basis.  No warranty is
 * provided that the Software is free of defects, or fit for a
 * particular purpose.  
 *
 * Limitation of Liability. Quadcap Software shall not be liable
 * for any damages suffered by the Licensee or any third party resulting
 * from use of the Software.
 */

import java.io.IOException;

import com.quadcap.util.DListItem;

/**
 * The construction of these objects needs to be managed by the
 * cache object.  They are initialized using the 
 *
 * @author Stan Bailes
 */
public abstract class Cacheable extends Object {
    /** has object been modified while in the cache? */
    boolean             dirty           = false;

    /** Is this cache item read only? */
    boolean             readOnly        = false;

    /**
     * We reference-count the cache items to know when it's ok to flush
     * older items to make room for new ones.  We should probably keep
     * some statistics which keep track of the percentage of cache items
     * currently "in-use"; i.e., with <code>refCount &gt; 0</code>
     */
    int                 refCount        = 0;

    /** Back pointer to my place in the LRU list. */
    DListItem           me;

    /**
     * The key used to locate this item in the underlying store, as well
     * as in the cache itself.
     */
    protected long              key;

    protected Object            store;

    /**
     * Initialization and (recycling)
     */
    public void init(Object store, long key) throws IOException {
        this.store = store;
        this.key = key;
        dirty = false;
    }

    /**
     * Return the cache to which this cacheable belongs.
     */
    public Object getStore() { return store; }

    /**
     * Am I read only?
     */
    public boolean isReadOnly() { return readOnly; }

    /**
     * Set the 'read only' flag
     */
    public void setReadOnly(boolean v) { this.readOnly = v; }

    /**
     * Read the dirty bit.
     */
    public final boolean isDirty() { return dirty; }

    /**
     * Set the dirty bit.
     */
    public void setDirty(boolean d) { 
        if (readOnly) {
            throw new RuntimeException("Attempt to modify read-only item");
        }
        dirty = d; 
    }

    /**
     * Read the reference count
     */
    public final int getRefCount() { return refCount; }

    /**
     * Set the reference count
     */
    public /* synchronized */ final void setRefCount(int x) { refCount = x; }

    /**
     * Increment the reference count
     */
    public /* synchronized */ final void incrRefCount() {
        refCount++;
    }

    /**
     * Decrement the reference count
     */
    public /* synchronized */ final void decrRefCount() { refCount--; }

    /**
     * Set the LRU back pointer.
     */
    public final void setDListItem(DListItem d) { me = d; }

    /**
     * Get the LRU back pointer.
     */
    public final DListItem getDListItem() { return me; }

    /**
     * Get the cache item's key.
     */
    public long getKey() { return key; }

    /**
     * Set the cache item's key.
     */
    public void setKey(long key) { this.key = key; }

    /**
     * Get the cache item's data.
     */
    abstract public Object getData();

    /**
     * Set the cache item's data.
     */
    abstract public void setData(Object data);
    
    /**
     * Flush this item and clear the dirty bit.
     */
    public void flush() throws IOException {
        dirty = false;
    }
}