Kafka consumer api failed to subscribe to topic

问题

I am using simple Kafka client API. As far as I know there are two ways to consumer messages, subscribe to a topic and assign partition to consumer.

However the first method does not work. Consumer poll() would hang forever. It only works with assign.

    // common config for consumer
    Map<String, Object> config = new HashMap<>();
    config.put("bootstrap.servers", bootstrap);

    config.put("group.id", KafkaTestConstants.KAFKA_GROUP);
    config.put("enable.auto.commit", "true");
    config.put("auto.offset.reset", "earliest");
    config.put("key.deserializer", StringDeserializer.class.getName());
    config.put("value.deserializer", StringDeserializer.class.getName());
    StringDeserializer deserializer = new StringDeserializer();
    KafkaConsumer<String, String> consumer = new KafkaConsumer<>(config, deserializer, deserializer);

    // subscribe does not work, poll() hangs
    consumer.subscribe(Arrays.asList(KafkaTestConstants.KAFKA_TOPIC));

Here is the code that works by assigning the partition.

    // assign works
    TopicPartition tp = new TopicPartition(KafkaTestConstants.KAFKA_TOPIC, 0);
    List<TopicPartition> tps = Arrays.asList(tp);
    consumer.assign(tps);

Since I'd like to utilize the auto commit feature which is supposed to only work with consumer group management according to this post. Why does not subscribe() work?

来源：https://stackoverflow.com/questions/42707131/kafka-consumer-api-failed-to-subscribe-to-topic