1. 消费消息 poll方法

//3. 循环消费消息
while (true) {
    //这里返回的ConsumerRecords，并不是ConsumerRecord，可以认为是一批ConsumerRecord
    //一个ConsumerRecord 和 ProducerRecorde对应
    //使用for循环遍历
    ConsumerRecords<String, String> records = consumer.poll(Duration.ofSeconds(5));
    for (ConsumerRecord<String, String> record : records) {
        System.out.println("--------------->:" + record.value());
    }  
    
    //使用Iterator遍历
    Iterator<ConsumerRecord<String, String>> iterator = records.iterator();
    while(iterator.hasNext()) {
        ConsumerRecord<String,String> next = iterator.next();
        System.out.println("--------------->:" + next.value());
    }
}

ConsumerRecord代码如下：

import org.apache.kafka.common.header.Headers;
import org.apache.kafka.common.header.internals.RecordHeaders;
import org.apache.kafka.common.record.DefaultRecord;
import org.apache.kafka.common.record.RecordBatch;
import org.apache.kafka.common.record.TimestampType;

/**
 * A key/value pair to be received from Kafka. This also consists of a topic name and 
 * a partition number from which the record is being received, an offset that points 
 * to the record in a Kafka partition, and a timestamp as marked by the corresponding ProducerRecord.
 */
public class ConsumerRecord<K, V> {
    public static final long NO_TIMESTAMP = RecordBatch.NO_TIMESTAMP;
    public static final int NULL_SIZE = -1;
    public static final int NULL_CHECKSUM = -1;

    private final String topic; //主题
    private final int partition;//分区
    private final long offset;//偏移量
    private final long timestamp;//时间戳
    private final TimestampType timestampType;//时间戳
    private final int serializedKeySize;//序列化的key的大小
    private final int serializedValueSize;
    private final Headers headers;//头信息
    private final K key;//key
    private final V value;//value

    private volatile Long checksum;//校验信息
}

按照分区维度来消费消息

ConsumerRecords<String, String> records = consumer.poll(Duration.ofSeconds(5));
//records.partitions(): 获取消息集中所有分区
//按照分区维度来消费消息
for (TopicPartition tp : records.partitions()) {
    //public List<ConsumerRecord<K, V>> records(TopicPartition partition)
    //按照TopicPartition 来获取消息
    List<ConsumerRecord<String, String>> consumerRecordList = records.records(tp);
    consumerRecordList.forEach(r -> System.out.println(r.partition()+"---->"+r.value()));
}

按照主题维度来消费消息

在ConsumerRecords类中并没有一个方法topics()来获取消费者所订阅的所有主题。因此按照主题消费，就只能根据消费者订阅的主题来处理。

//2. 订阅主题
List<String> asList = Arrays.asList("topic1","topic2");
consumer.subscribe(asList);
while (true) {
     ConsumerRecords<String, String> records = consumer.poll(Duration.ofSeconds(5));
    for (String s : asList) {
        //按照主题消费消息
        for (ConsumerRecord<String, String> record : records.records(s)) {
            System.out.println(record.partition()+"---->"+record.value());
        }
    }
}