目前Kafka並沒有提供直接的工具來幫助我們獲取某個topic的當前總消息數，需要我們自行寫程序來實現。下列代碼可以實現這一功能，特此記錄一下：

/**
     * 獲取某個topic的當前消息數
     * Java 8+ only
     *
     * @param topic
     * @param brokerList
     * @return
     */
    public static long totalMessageCount(String topic, String brokerList) {
        Properties props = new Properties();
        props.put("bootstrap.servers", brokerList);
        props.put("group.id", "test-group");
        props.put("enable.auto.commit", "false");
        props.put("key.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");
        props.put("value.deserializer", "org.apache.kafka.common.serialization.StringDeserializer");

        try (KafkaConsumer<String, String> consumer = new KafkaConsumer<>(props)) {
            List<TopicPartition> tps = Optional.ofNullable(consumer.partitionsFor(topic))
                    .orElse(Collections.emptyList())
                    .stream()
                    .map(info -> new TopicPartition(info.topic(), info.partition()))
                    .collect(Collectors.toList());
            Map<TopicPartition, Long> beginOffsets = consumer.beginningOffsets(tps);
            Map<TopicPartition, Long> endOffsets = consumer.endOffsets(tps);

            return tps.stream().mapToLong(tp -> endOffsets.get(tp) - beginOffsets.get(tp)).sum();
        }
    }

Java API獲取非compacted topic總消息數