why is the first member of the tuple received from kafka using spark direct stream is null

问题

when reading messages from kafka using KafkaUtils.createDirectStream, the v1._1 member of the Tuple2 is null:

KafkaUtils.createDirectStream(
                streamingContext,
                String.class,
                String.class,
                StringDecoder.class,
                StringDecoder.class,
                kafkaParams,
                topicsSet
                ).map(new Function<Tuple2<String,String>, String>() {
                    @Override
                    public String call(Tuple2<String, String> v1)
                            throws Exception {
                        System.out.println(v1._1);
                        return null;
                    }
                });

while the _2 member contains the message itself that was passed to kafka.

I have two questions:

1) why is v1._1 null?

2) is there a way to pass the topic name in kafka (the same way the message is put into kafka) so that v1._1 will contain the topic name?

来源：https://stackoverflow.com/questions/36778977/why-is-the-first-member-of-the-tuple-received-from-kafka-using-spark-direct-stre