articlecache.java

java高级使用教程 全书一共分六章

// ArticleCache - a cache for netnews database articles
//
// Copyright (C) 1996 by Jef Poskanzer <[e-mail]>.  All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions
// are met:
// 1. Redistributions of source code must retain the above copyright
//    notice, this list of conditions and the following disclaimer.
// 2. Redistributions in binary form must reproduce the above copyright
//    notice, this list of conditions and the following disclaimer in the
//    documentation and/or other materials provided with the distribution.
//
// THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
// ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
// ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
// FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
// DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
// OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
// HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
// LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
// OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
// SUCH DAMAGE.
//
// Visit the ACME Labs Java page for up-to-date versions of this and other
// fine Java utilities: http://www.acme.com/java/

package Acme.Nnrpd;

import java.util.*;
import Acme.LruHashtable;

/// A cache for netnews database articles.
// <P>
// <A HREF="/resources/classes/Acme/Nnrpd/ArticleCache.java">Fetch the software.</A><BR>
// <A HREF="/resources/classes/Acme.tar.gz">Fetch the entire Acme package.</A>

public class ArticleCache
    {

    private int aCacheSize;
    private int maxArtSize;
    private Hashtable articles = new Hashtable();
    private LruHashtable groupArtNumTable;
    private LruHashtable messageIdTable;

    /// Constructor.
    // @param aCacheSize maximum size of the cache
    // @param maxArtSize maximum-size article to cache
    public ArticleCache( int aCacheSize, int maxArtSize )
	{
	this.aCacheSize = aCacheSize;
	this.maxArtSize = maxArtSize;
	int numArtsEstimate = aCacheSize / 2000;
	groupArtNumTable = new LruHashtable( numArtsEstimate );
	messageIdTable = new LruHashtable( numArtsEstimate );
	}


    /// Get an article by group and number.
    public NewsDbArticle getArticle( NewsDbGroup group, int artNum )
	{
	Integer n = (Integer) groupArtNumTable.get(
	    groupArtNumKey( group, artNum ) );
	if ( n == null )
	    return null;
	return (NewsDbArticle) articles.get( n );
	}

    /// Get an article by message-id.
    public NewsDbArticle getArticle( String messageId )
	{
	Integer n = (Integer) messageIdTable.get( messageId );
	if ( n == null )
	    return null;
	return (NewsDbArticle) articles.get( n );
	}


    /// Add an article by group and number.
    public void addArticle( NewsDbArticle article, NewsDbGroup group, int artNum )
	{
	Integer n = addArticle( article );
	if ( n == null )
	    return;
	groupArtNumTable.put( groupArtNumKey( group, artNum ), n );
	}

    /// Add an article by message-id.
    public void addArticle( NewsDbArticle article, String messageId )
	{
	Integer n = addArticle( article );
	if ( n == null )
	    return;
	messageIdTable.put( messageId, n );
	}


    private int usage = 0;
    private int firstNumber = 0;
    private int lastNumber = 0;

    /// Internal routine to add an article to the queue and assign it a
    // unique immutable number for putting into the search tables.
    // This two-layer approach provides weak-referencing, so when the cache
    // gets too large we can throw away articles here without bothering to
    // clean up the references to them in the search tables, which do
    // their own LRU expiry.
    private Integer addArticle( NewsDbArticle article )
	{
	int len = article.getText().length();
	if ( len > maxArtSize )
	    return null;
	Integer l = new Integer( lastNumber++ );
	articles.put( l, article );
	usage += len;
	while ( usage > aCacheSize )
	    {
	    Integer f = new Integer( firstNumber++ );
	    if ( article != null )
		{
		article = (NewsDbArticle) articles.get( f );
		len = article.getText().length();
		usage -= len;
		articles.remove( f );
		}
	    }
	return l;
	}


    /// Generate a single hashtable key for the (group,artNum) pair.
    private Integer groupArtNumKey( NewsDbGroup group, int artNum )
	{
	return new Integer( group.getName().hashCode() ^ artNum );
	}

    }