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