weakreferencecache.java

Java的面向对象数据库系统的源代码

// You can redistribute this software and/or modify it under the terms of
// the Ozone Core License version 1 published by ozone-db.org.
//
// Copyright (C) 2003-@year@, Leo Mekenkamp. All rights reserved.
//
// $Id: WeakReferenceCache.java,v 1.3 2004/03/21 21:05:51 leomekenkamp Exp $

package org.ozoneDB.core.storage;

import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.Properties;

/**
 * <p>Caches objects which may be garbage collected at any time by the Java virtual
 * machine garbage collector. Uses weak references to hold objects in memory.</p>
 *
 * @author <a href="mailto:leoATmekenkampD0Tcom">Leo Mekenkamp (mind the anti sp@m)</a>
 * @version $Id: WeakReferenceCache.java,v 1.3 2004/03/21 21:05:51 leomekenkamp Exp $
 */
public class WeakReferenceCache extends AbstractReferenceCache {
    
    private static class ObjectReference extends WeakReference implements KeyedReference {
        
        /**
         * same key as the key passed in <code>put(key, value)</code>
         */
        private Object key;
        
        ObjectReference(Object key, Object value, ReferenceQueue q) {
            super(value, q);
            this.key = key;
        }
        
        public Object getKey() {
            return key;
        }
        
        public String toString() {
            return key.toString();
        }
        
        public int hashCode() {
            return key.hashCode();
        }
        
        public boolean equals(Object o) {
            if (!(o instanceof ObjectReference)) {
                return false;
            } else {
                ObjectReference otherRef = (ObjectReference) o;
                return otherRef.getKey().equals(getKey());
            }
        }
        
    } // ObjectReference
    
    public WeakReferenceCache(Properties properties, String prefix) {
        super(properties, prefix);
    }
    
    protected KeyedReference createKeyedReference(Object key, Object value, ReferenceQueue referenceQueue) {
        return new ObjectReference(key, value, referenceQueue);
    }
    
// DEBUG AND BUGTEST CODE ONLY FOLLOWING; not needed for normal operation
    

    public WeakReferenceCache() {
    }
    
    private static class Item {
        
        public Object key;
        public byte[] filling = new byte[1000];
        
        Item(Object key) {
            this.key = key;
        }
        
        public String toString() {
            return key.toString();
        }
    }
    
    public static void main(String args[]) {
        WeakReferenceCache cache = new WeakReferenceCache();
        int last = 0;
        for (int i = 0; i < 1000000; i++) {
            Object key = new Integer(i);
            Item item = new Item(key);
            cache.put(key, item);
//            for (int j = 0; j < 20; j++) {
//                cache.get(new Integer(j));
//            }
            if (cache.size() < last) {
                System.out.println(cache.size() + ", last: " + last);
            }
            last = cache.size();
                
        }
        System.out.println("completed successfully");
    }
    
}