code: public static <K, V> ConcurrentMap<K, V> createWeakKeyConcurrentMap(int initialCapacity, int concurrencyLevel) {
  		return new MapMaker().weakKeys().initialCapacity(initialCapacity).concurrencyLevel(concurrencyLevel).makeMap();
  	}
docs: 创建Key为弱引用的ConcurrentMap，Key对象可被回收.
  
  JDK没有WeakHashMap的并发实现, 由Guava提供