apollo-client/src/main/java/com/ctrip/framework/apollo/spring/config/CachedCompositePropertySource.java/CachedCompositePropertySource/getPropertyNames
class CachedCompositePropertySource:  @Override
  public String[] getPropertyNames() {
    String[] propertyNames = this.names;
    if (propertyNames == null) {
      this.names = propertyNames = super.getPropertyNames();
    }
    return propertyNames;
  }