in flink-connector-gcp-pubsub/src/main/java/org/apache/flink/streaming/connectors/gcp/pubsub/PubSubSource.java [293:297]
public PubSubSourceBuilder<OUT> withPubSubSubscriberFactory(
PubSubSubscriberFactory pubSubSubscriberFactory) {
this.pubSubSubscriberFactory = pubSubSubscriberFactory;
return this;
}