code: @Override
      public byte[] serialize(Object obj) throws IOException {
  		KryoHolder kryoHolder = null;
  		if (obj == null)
  			throw new RuntimeException("obj can not be null");
  		try {
  			kryoHolder = KryoPoolImpl.getInstance().get();
  			kryoHolder.output.clear(); // clear Output -->每次调用的时候 重置
  			kryoHolder.kryo.writeClassAndObject(kryoHolder.output, obj);
  			return kryoHolder.output.toBytes();// 无法避免拷贝 ~~~
  		} catch (RuntimeException e) {
  			throw new RuntimeException(e);
  		} finally {
  			if (kryoHolder != null) {
  				KryoPoolImpl.getInstance().offer(kryoHolder);
  			}
  			// obj = null; //GC
  		}
      }
docs: Serialize object
  @param obj what to serialize
  @return return serialize data