apollo-client/src/main/java/com/ctrip/framework/apollo/spring/config/ConfigPropertySource.java/ConfigPropertySource/getPropertyNames
class ConfigPropertySource:  @Override
  public String[] getPropertyNames() {
    Set<String> propertyNames = this.source.getPropertyNames();
    if (propertyNames.isEmpty()) {
      return EMPTY_ARRAY;
    }
    return propertyNames.toArray(new String[propertyNames.size()]);
  }