1、消费者

指定消费者读取配置文件中
topic = "$kafka.topic.name",groupId="${kafka.consumer.group}"下的数据。

package com.power.consumer;

import com.power.model.User;
import com.power.util.JSONUtils;
import org.apache.kafka.clients.consumer.ConsumerRecord;
import org.springframework.kafka.annotation.KafkaListener;
import org.springframework.kafka.annotation.PartitionOffset;
import org.springframework.kafka.annotation.TopicPartition;
import org.springframework.kafka.support.Acknowledgment;
import org.springframework.kafka.support.KafkaHeaders;
import org.springframework.messaging.handler.annotation.Header;
import org.springframework.messaging.handler.annotation.Payload;
import org.springframework.stereotype.Component;

import java.util.function.Consumer;

@Component
public class EventConsumer { 	

    @KafkaListener(groupId="${kafka.consumer.group}",
            topicPartitions = {
                @TopicPartition(
                        topic = "${kafka.topic.name}",
                        partitions={"0","1","2"},
                        partitionOffsets = {
                                @PartitionOffset(partition="3",initialOffset = "3"),
                                @PartitionOffset(partition="4",initialOffset = "3")
                        })
            })
    public void onEvent5(String userJson,
                         @Header(value=KafkaHeaders.RECEIVED_TOPIC) String topic,
                         @Header(value=KafkaHeaders.RECEIVED_PARTITION_ID) String partition,
                         ConsumerRecord<String,String> record,
                         Acknowledgment ack){
        try {
            User user =JSONUtils.toBean(userJson,User.class);
            System.out.println("读取到的事件5："+user+",topic："+topic+",partition："+partition);

            //业务确认完成，给kafka服务器反馈确认
            ack.acknowledge();//手动确认消息，就是告诉kafka服务器，该消息我已经接收到了，默认情况下是自动确认
            //手动确认后，下次启动消费者，偏移量会从新的位置开始；没有手动确认，下次启动消费者，偏移量还是从老位置开始
        }catch (Exception e){
            e.printStackTrace();
        }
    }

}

2、生产者

package com.power.producer;

import com.power.model.User;
import com.power.util.JSONUtils;
import org.springframework.kafka.core.KafkaTemplate;
import org.springframework.stereotype.Component;

import javax.annotation.Resource;
import java.util.Date;

@Component
public class EventProducer {

    @Resource
    private KafkaTemplate<String,Object> kafkaTemplate;

    public void sendEvent3(){
        for (int i = 0; i < 25; i++) {
            User user = User.builder().id(i).phone("1567676767"+i).birthday(new Date()).build();
            String userJson = JSONUtils.toJSON(user);
            kafkaTemplate.send("helloTopic","k"+i,userJson);
        }
    }

}

3、配置类

package com.power.config;

import org.apache.kafka.clients.admin.NewTopic;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class KafkaConfig {

    @Bean
    public NewTopic newTopic(){
        return new NewTopic("helloTopic",5,(short)1);
    }
}

4、配置文件

spring:
  application:
    #应用名称
    name: spring-boot-02-kafka-base

  #kafka连接地址（ip+port）
  kafka:
    bootstrap-servers: <你的服务器IP>:9092

    #消费者
    consumer:
      auto-offset-reset: earliest


    #配置消息监听器
    listener:
      ack-mode: manual

#自定义配置
kafka:
  topic:
    name: helloTopic
  consumer:
    group: helloGroup

5、实体类

package com.power.model;

import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.util.Date;

@Builder
@AllArgsConstructor
@NoArgsConstructor
@Data
public class User {

    private Integer id;

    private String phone;

    private Date birthday;

}

6、工具类

package com.power.util;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class JSONUtils {

    private static final ObjectMapper OBJECTMAPPER = new ObjectMapper();

    public static String toJSON(Object object){
        try {
            return OBJECTMAPPER.writeValueAsString(object);
        } catch (JsonProcessingException e) {
            throw new RuntimeException(e);
        }
    }

    public static <T> T toBean(String json,Class<T> clazz){
        try {
            return OBJECTMAPPER.readValue(json,clazz);
        } catch (JsonProcessingException e) {
            throw new RuntimeException(e);
        }
    }
}

7、测试类

package com.power;

import com.power.producer.EventProducer;
import org.junit.jupiter.api.Test;
import org.springframework.boot.test.context.SpringBootTest;

import javax.annotation.Resource;

@SpringBootTest
public class SpringBoot02KafkaBaseApplication {

    @Resource
    private EventProducer eventProducer;

    @Test
    void sendEvent5(){
        eventProducer.sendEvent5();
    }

}

8、第一次测试（读取到19条信息）

总共读取1、2、3分区全部数据和4分区从4偏移量开始读，5分区从4开始读取。
6+4+6+3+0=19条数据。

9、第二次测试（读取到3条信息）

因为第一次读取已经记下来偏移量offset，即便在配置文件中指定了消费者从最开始读，也依然读取不到的。

此时可以通过修改组id来从头开始读取