在ConsumerConfig这个类中定义了这个属性的默认值，如下图

也就是默认值为latest,它的含义是：如果没有客户端提交过offset的话，当新的客户端消费时，把最新的offset设置为当前消费的offset.

默认是自动提交位移的，每5秒进行一次提交。可以通过参数配置手动提交。

手动提交offset的示例

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.clients.producer.ProducerConfig;
import org.apache.kafka.common.PartitionInfo;
import org.apache.kafka.common.TopicPartition;
import org.apache.kafka.common.serialization.StringDeserializer;
import org.apache.kafka.common.serialization.StringSerializer;

import java.time.Duration;
import java.time.temporal.TemporalUnit;
import java.util.*;
import java.util.concurrent.TimeUnit;
/*
设置手动提交offset
 */

public class KafkaTest08 {

    private static Properties getProperties(){
        Properties properties=new Properties();

        properties.setProperty(ConsumerConfig.KEY_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class.getName());
        properties.setProperty(ConsumerConfig.VALUE_DESERIALIZER_CLASS_CONFIG, StringDeserializer.class.getName());
        properties.setProperty(ConsumerConfig.BOOTSTRAP_SERVERS_CONFIG,"xx.xx.xx.xx:9092");
        properties.setProperty(ConsumerConfig.GROUP_ID_CONFIG,"testGroup8");
        //设置手动提交位移
        properties.setProperty(ConsumerConfig.ENABLE_AUTO_COMMIT_CONFIG,"false");
        return properties;
    }
    public static void main(String[] args) {

        KafkaConsumer<String,String> myConsumer=new KafkaConsumer<String, String>(getProperties());
        myConsumer.subscribe(Arrays.asList("student"));

        int i=0;


        while(true){
            ConsumerRecords<String,String> consumerRecords=myConsumer.poll(Duration.ofMillis(5000));
            for(ConsumerRecord record: consumerRecords){
                System.out.println(record.value());
                System.out.println("record offset is: "+record.offset());

            }

            myConsumer.commitSync();
            
            if(i==0){
                //myConsumer.commitSync();
                i ++;
            }
            else {
                i ++;
            }

        }



    }
}