KafkaConsumer08.java 文件源码

@Override
public MessageAndOffset read() throws StageException {
  try {
    //has next blocks indefinitely if consumer.timeout.ms is set to -1
    //But if consumer.timeout.ms is set to a value, like 6000, a ConsumerTimeoutException is thrown
    //if no message is written to kafka topic in that time.
    if(consumerIterator.hasNext()) {
      MessageAndMetadata<byte[], byte[]> messageAndMetadata = consumerIterator.next();
      byte[] message = messageAndMetadata.message();
      long offset = messageAndMetadata.offset();
      int partition = messageAndMetadata.partition();
      return new MessageAndOffset(message, offset, partition);
    }
    return null;
  } catch (ConsumerTimeoutException e) {
    /*For high level consumer the fetching logic is handled by a background
      fetcher thread and is hidden from user, for either case of
      1) broker down or
      2) no message is available
      the fetcher thread will keep retrying while the user thread will wait on the fetcher thread to put some
      data into the buffer until timeout. So in a sentence the high-level consumer design is to
      not let users worry about connect / reconnect issues.*/
    return null;
  }
}