apollo-client/src/main/java/com/ctrip/framework/apollo/util/OrderedProperties.java/OrderedProperties/keys
class OrderedProperties:  @Override
  public synchronized Enumeration<Object> keys() {
    return new Enumeration<Object>() {
      private final Iterator<String> i = propertyNames.iterator();

      @Override
      public boolean hasMoreElements() {
        return i.hasNext();
      }

      @Override
      public Object nextElement() {
        return i.next();
      }
    };
  }