Utility to fetch a reference for an instance mapped by a particular key.

InstanceProvider.java

/*
 * Copyright (c) 2012 Michael Nischt
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * Redistributions of source code must retain the above copyright notice,
 * this list of conditions and the following disclaimer.
 *
 * 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.
 *
 * Neither the name of the project's author nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS 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 COPYRIGHT
 * HOLDER 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.
 *
 */
package net.monoid.util.ref;

/**
 * Provides an instance mapped to a particular key
 * @author Michael Nischt
 */
public interface InstanceProvider<K, V> {
    
    /**
     * Provides the instance to the mapped key
     * @param key mapped to the reference
     * @return a reference to the mapped key
     */
    V get(K key);
}

ReferenceProvider.java

/*
 * Copyright (c) 2012 Michael Nischt
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * Redistributions of source code must retain the above copyright notice,
 * this list of conditions and the following disclaimer.
 *
 * 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.
 *
 * Neither the name of the project's author nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS 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 COPYRIGHT
 * HOLDER 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.
 *
 */
package net.monoid.util.ref;

import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.HashMap;
import java.util.Map;

/**
 * Provides a reference to an instance. If no instance  mapped by the specified key exists, a new one is created.
 * 
 * @author Michael Nischt
 * 
 */
public final class ReferenceProvider<K, V> implements InstanceProvider<K, V> {
        
    private static class Reference<K, V> extends WeakReference<V> {
        private final K key;

        public Reference(K key, V referent, ReferenceQueue<? super V> q) {
            super(referent, q);
            this.key = key;
        }

        public K getKey() {
            return key;
        }
    }
    
    private final InstanceProvider<K, V> instanceProvider;
    private final Map<K, Reference<K, V>> map = new HashMap<>();
    private final ReferenceQueue<V> queue = new ReferenceQueue<> ();

    /**
     * Creates a new ReferenceProvider
     * @param instanceProvider used the create a new instance if no reference exists
     */
    public ReferenceProvider(InstanceProvider<K, V> instanceProvider) {
        if(instanceProvider == null) {
            throw new NullPointerException("InstanceProvider cannut be null");
        }
        this.instanceProvider = instanceProvider;
    }    

    /**
     * Retrieves a new reference for the specified key. If no instance exists yet, a new is created using the wrapped @link{InstanceProvider}
     * @param key mapped to the reference
     * @return a reference to the mapped key
     */
    @Override
    synchronized public V get(K key) {
        V value; {
            Reference<K, V> reference = map.get(key);
            value = reference != null ? reference.get() : null;
        }
        if(value == null) {
            value = instanceProvider.get(key); 
            map.put(key, new Reference<>(key, value, queue));
        }
        return value;
    }
    
    /**
     * Cleans dead references.
     * @return the number dead references removedS
     */
    synchronized public int clean() {
        int n=0;
        Reference<K, V> reference;
        do {
            reference = (Reference<K, V>) queue.poll();
            if(reference == null) {
                continue;
            }
            map.remove(reference.getKey());
            n++; 
        } while(reference != null);
        return n;
    }
}