cache.java

Java Communicating Agents是一个用于开发网络反应式信息agent的框架

/*
 * $Source: /home/data/cvsroot/src/jacomma/util/Cache.java,v $
 * $Revision: 1.8 $
 * $Date: 2000/10/28 20:09:08 $
 *
 * This file is part of the jacomma framework
 * Copyright (c) 2000 	Dimitrios Vyzovitis
 *			mailto:[e-mail]
 *			
 *
 *			
 *			
 *			
 *
 *	This library is free software; you can redistribute it and/or modify
 *	it under the terms of the GNU Library General Public License as published by
 *	the Free Software Foundation; either version 2 of the License, or
 *	(at your option) any later version.
 *
 *	This library 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 Library General Public License for more details.
 *
 *	You should have received a copy of the GNU Library General Public License
 *	along with this library; if not, write to the Free Software
 *	Foundation, Inc., 59 Temple Place, Suite 330, 
 *	Boston, MA  02111-1307  USA
 */

package jacomma.util;

/**
 * TBA
 **/
public class Cache {
	
	RdWrMap ht_;
	java.lang.ref.ReferenceQueue que_;
	
	_Recent rc_; // recent cache hits (very simple, allows duplicate entries)
	static final int DEFAULT_RLENGTH = 10;

	public Cache() {
		this( DEFAULT_RLENGTH );
	}

	public Cache( int rlength ) {
		ht_ = new RdWrMap();
		que_ = new java.lang.ref.ReferenceQueue();
		rc_ = new _Recent( rlength );
	}
	
	public Object put( Object key, Object entry ) {
		rc_.add( entry );
		return ht_.put( key, new java.lang.ref.SoftReference( entry, que_ ) );
	}

	public Object try_put( Object key, Object entry ) {
		ht_.lock.wrlock();
		try {
			if ( !ht_.containsKey( key ) ) {
				rc_.add( entry );
				ht_.put( key, new java.lang.ref.SoftReference( entry, que_ ) );
				return entry;
			} else {
				Object obj = ((java.lang.ref.Reference)ht_.get( key )).get();
				rc_.add( obj );
				return obj;
			}
		} finally {
			ht_.lock.unlock();
		}

	}
	
	public Object get( Object key ) {
		try {
			Object obj = ((java.lang.ref.Reference)ht_.get( key )).get();
			rc_.add( obj );

			return obj;
		} catch ( NullPointerException exc ) {
			return null;
		} finally {
			// check the ref.que
			for ( Object dead = que_.poll();
				  dead != null;
				  dead = que_.poll() )
				ht_.removeValue( dead );
		}
	}
	
	private class _Recent {
		Object[] arr_;
		int index_;

		_Recent( int length ) {
			arr_ = new Object[length];
		}
		
		synchronized void add( Object obj ) {
			if ( obj == null )
				throw new NullPointerException();
			arr_[index_++] = obj;
			index_ %= arr_.length;
		}
	}
}