4. kafka生产者&消费者

原生方式

无论是生产者还是消费者，引入的依赖都是kafka-clients，maven坐标如下：

<dependency>
    <groupId>org.apache.kafka</groupId>
    <artifactId>kafka-clients</artifactId>
    <version>1.1.0</version>
</dependency>

生产者

kafka生产者对象就是KafkaProducer，构造方式如下：

Properties props = new Properties();
// kafka集群地址
props.put("bootstrap.servers", "10.0.55.229:9092");
// kafka消息key的序列化方式
props.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer");
// kafka消息value的序列化方式
props.put("value.serializer", "org.apache.kafka.common.serialization.StringSerializer");
KafkaProducer<String, String> kafkaProducer = new KafkaProducer<>(props);

消息

-创建生产者
KafkaProducer构造好后，需要构造待发送的消息。kafka消息对象是ProducerRecord，根据源码可知，构造方式有多种：

public class ProducerRecord<K, V> {
   

    /**
     * 所有构造方法最后都是调用这个构造方法, 所以弄明白这个构造方法所有参数含义就可以了
     * Creates a record with a specified timestamp to be sent to a specified topic and partition
     * @param topic - The topic the record will be appended to, topic名称
     * @param partition - The partition to which the record should be sent, 消息发送的目标分区名称, 如果不指定, kafka会根据Partitioner计算目标分区
     * @param timestamp - The timestamp of the record, in milliseconds since epoch. If null, the producer will assign
     *                  the timestamp using System.currentTimeMillis(). 消息发送的指定时间戳, 默认为当前时间
     * @param key - The key that will be included in the record, 消息的key, kafka根据这个key计算分区
     * @param value - The record contents 消息的内容
     * @param headers - the headers that will be included in the record
     */
    public ProducerRecord(String topic, Integer partition, Long timestamp, K key, V value, Iterable<Header> headers) {
        // topic是构造ProducerRecord的必传参数
        if (topic == null)
            throw new IllegalArgumentException("Topic cannot be null.");
        // 发送的时间戳不能为负数
        if (timestamp != null && timestamp < 0)
            throw new IllegalArgumentException(
                    String.format("Invalid timestamp: %d. Timestamp should always be non-negative or null.", timestamp));
        // 分区值不能为负数
        if (partition != null && partition < 0)
            throw new IllegalArgumentException(
                    String.format("Invalid partition: %d. Partition number should always be non-negative or null.", partition));
        this.topic = topic;
        this.partition = partition;
        this.key = key;
        this.value = value;
        this.timestamp = timestamp;
        this.headers = new RecordHeaders(headers);
    }

    public ProducerRecord(String topic, Integer partition, Long timestamp, K key, V value) {
        this(topic, partition, timestamp, key, value, null);
    }

    public ProducerRecord(String topic, Integer partition, K key, V value, Iterable<Header> headers) {
        this(topic, partition, null, key, value, headers);
    }

    public ProducerRecord(String topic, Integer partition, K key, V value) {
        this(topic, partition, null, key, value, null);
    }

    public ProducerRecord(String topic, K key, V value) {
        this(topic, null, null, key, value, null);
    }

    public ProducerRecord(String topic, V value) {
        this(topic, null, null, null, value, null);
    }

    ... ...
}

• 创建消息
  下面构造一个最常用的ProducerRecord，只指定topic和value，由kafka去决定分区：

// ProducerRecord就是发送的信息对象, 包括: topic名称, key（可选）, value（发送的内容）
// key的用途主要是：消息的附加信息，用来决定消息被写到哪个分区，拥有相同key的消息会被写到同一个分区
ProducerRecord<String, String> record = new ProducerRecord<>("ORDER-DETAIL",
    JSON.toJSONString(new Order(201806260001L, new Date(), 98000, "desc", "165120001")));

消费者

• 创建消费者
  kafka消费者者对象就是KafkaConsumer，构造方式如下：

Properties props = new Properties();
// kafka集群地址
props.put("bootstrap.servers", "10.0.55.229:9092");
// ConsumerGroup即消费者组名称
props.put("group.id", "afei");
// kafka消息key的反序列化方式
props.put("key.deserializer",   "org.apache.kafka.common.serialization.StringDeserializer");
// kafka消息value的序列化方式
props.put("value.deserializer", "org.apache.kafka.common.serialization.StringDeserializer"