Java项目源码学习笔记（一）：IdentityCache

import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Set;

public class IdentityCache<K, V>{
	
	private final HashMap<K, Entry<K,V>> mWeakMap = new HashMap<K, Entry<K, V>>();
	private ReferenceQueue<V> mQueue = new ReferenceQueue<V>();

	public IdentityCache(){}

	private static class Entry<K, V> extends WeakReference<V>{
		K mKey;
		public Entry(K key, V value, ReferenceQueue<V> queue){
			super(value, queue);
			mKey = key;
		}
	}

	private void cleanUpWeakMap(){
		Entry<K, V> entry = (Entry<K,V>)mQueue.poll();
		while(entry != null){
			mWeakMap.remove(entry.mKey);
			entry = (Entry<K,V>)mQueue.poll();
		}
	}

	public synchronized V put(K key, V value){
		cleanUpWeakMap();
		Entry<K, V> entry = mWeakMap.put(key, new Entry<K,V>(key, value, mQueue));
		return entry == null ? null : entry.get();
	}

	public synchronized V get(K key){
		cleanUpWeakMap();
		Entry<K,V> entry = mWeakMap.get(key);
		return entry == null ? null : entry.get();
	}

	public synchronized ArrayList<K> keys(){
		Set<K> set = mWeakMap.keySet();
		ArrayList<K> result = new ArrayList<K>(set);
		return result;
	}
}