Cache.java :  » Aspect-oriented » dynaop » dynaop » util » Java Open Source

package dynaop.util;

import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.SoftReference;
import java.lang.ref.WeakReference;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.WeakHashMap;

/**
 * Cache with weak keys and soft values.
 * 
 * @author Bob Lee (crazybob@crazybob.org)
 */
public abstract class Cache {

  static Object NULL_VALUE = new Object();
  
  Map map;
  ReferenceQueue queue = new ReferenceQueue();
  
  /**
   * Creates cache.
   * 
   * @param weakKeys Use weak references for keys.
   */
  public Cache(boolean weakKeys) {
    this.map = (weakKeys) ? 
      (Map) new WeakHashMap() : new HashMap();
    this.map = Collections.synchronizedMap(this.map);
  }
  
  /**
   * Defaults to weak keys.
   */
  public Cache() {
    this(true);
  }
  
  /**
   * Creates value for key. Called by getter if 
   * value isn't cached.
   */
  protected abstract Object create(Object key);
  
  /**
   * Gets value for key. Creates if necessary.
   */
  public Object get(Object key) {
    Object value = internalGet(key);
    if (value == null) {
      value = create(key);
      if (value == null)
        value = NULL_VALUE;
      this.map.put(key, 
        new ValueReference(key, value));
    }
    return (value == NULL_VALUE) ? null : value;
  }
  
  void cleanUp() {
    Reference reference;
    while ((reference = this.queue.poll()) != null)
      map.remove(
        ((ValueReference) reference).getKey());
  }
  
  Object internalGet(Object key) {
    cleanUp();
    Reference reference = (Reference) map.get(key);
    return (reference == null) ? 
      null : reference.get();
  }
  
  class ValueReference extends SoftReference {
    
    WeakReference keyReference;
    
    ValueReference(Object key, Object value) {
      super(value, queue);
      this.keyReference = new WeakReference(key); 
    }
    
    Object getKey() {
      return this.keyReference.get();
    }
  }
}