查看“Java 客户端消费者的实现细节”的源代码

https://www.bilibili.com/video/BV1Xy4y1G7zA?p=18

=== 1.消费者的基本实现 ===
<syntaxhighlight lang="java">
package org.example.kafka;

import org.apache.kafka.clients.consumer.ConsumerConfig;
import org.apache.kafka.clients.consumer.ConsumerRecord;
import org.apache.kafka.clients.consumer.ConsumerRecords;
import org.apache.kafka.clients.consumer.KafkaConsumer;
import org.apache.kafka.common.serialization.StringDeserializer;

import java.time.Duration;
import java.util.Arrays;
import java.util.Properties;

public class MySimpleConsumer {

    private final static String TOPIC_NAME = "my-replicated-topic";

    private final static String CONSUMER_GROUP_NAME = "testGroup";

    public static void main(String[] args) {

        Properties props = new Properties();

        props.put(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG, "192.168.137.200:9092,192.168.137.200:9093,192.168.137" +
                ".200:9094");

        // 消费分组名
        props.put(ConsumerConfig.GROUP_ID_CONFIG, CONSUMER_GROUP_NAME);
        props.put(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class.getName());
        props.put(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class.getName());

        //创建一个消费者的客户端
        KafkaConsumer<String, String> consumer = new KafkaConsumer<String, String>(props);

        //消费者订阅主题列表
        consumer.subscribe(Arrays.asList(TOPIC_NAME));

        while (true) {

            /*
             * poll() API 是拉取消息的长轮询
             */
            ConsumerRecords<String, String> records = consumer.poll(Duration.ofMillis(1000));

            for (ConsumerRecord<String, String> record:records) {
                System.out.printf("收到消息：partition = %d, offset = %d, key = %s, value = %s%n", record.partition(),
                        record.offset(), record.key(), record.value());
            }

        }// end while



    }// end main

}//end class

</syntaxhighlight>

=== 2.关于消费者自动提交和手动提交 offset ===
https://www.bilibili.com/video/BV1Xy4y1G7zA?p=19

==== 1）提交的内容 ====
消费者无论是自动提交还是手动提交，都需要把所属的消费组+消费的某个主题+消费的某个分区及消费的偏移量，这样的信息提交到集群的 _consumer_offsets 主题里面。

==== 2）自动提交 ====
消费者 poll 消息下来以后就会自动提交 offset<syntaxhighlight lang="java">
//是否自动提交 offset，默认就是 true
props.put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, "true");

//自动提交 offset 的时间间隔
props.put(ConsumerConfig.AUTO_COMMIT_INTERVAL_MS_CONFIG, "1000");
</syntaxhighlight>注意：自动提交会丢消息。因为消费者在消费前提交 offset，有可能提交完成后还没消费时消费者挂了。

==== 3）手动提交 ====
需要把自动提交的配置改成 false<syntaxhighlight lang="java">
props.put(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG, "false");
</syntaxhighlight>手动提交又分成了两种：

* 手动同步提交
* 手动异步提交