Kafka-ConsumerRecord

ConsumerRecord 是 Apache Kafka 消费者从主题中读取消息时的核心数据结构，每条消息都会被封装为一个 ConsumerRecord 对象。它包含了消息的元数据（如来源、位置）和实际内容，是消费者处理消息的基础单元。以下是其核心要点：

ConsumerRecord 的结构与字段

每个 ConsumerRecord 包含以下关键信息：

topic：消息所属的 Kafka 主题名称（字符串）

partition：消息所在分区的编号（整数）

offset：消息在分区中的唯一位置标识（长整数），用于追踪消费进度

key：消息的键（泛型），通常用于分区路由或业务标识（如订单ID

value：消息的值（泛型），即实际传输的数据（如 JSON 字符串、二进制数据等）

timestamp：消息的时间戳（长整数），表示消息生成或追加到分区的时间

headers：消息头（键值对集合），用于传递业务元数据（如消息类型、版本号）

checksum：校验和（长整数，已逐步弃用），用于验证消息完整性

在代码中的使用示例

在 Java 或 Spring Kafka 中，ConsumerRecord 通常通过消费者监听器（如 @KafkaListener）接收并处理：

@KafkaListener(topics = "orders")
public void handleOrder(ConsumerRecord<String, Order> record) {
    String key = record.key();         // 获取消息键（如订单ID）
    Order value = record.value();     // 获取消息值（如订单对象）
    String topic = record.topic();     // 获取主题名称
    long offset = record.offset();     // 获取消息偏移量
    // 处理业务逻辑...
}

源码

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package org.apache.kafka.clients.consumer;

import java.util.Optional;
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.TimestampType;

public class ConsumerRecord<K, V> {
    public static final long NO_TIMESTAMP = -1L;
    public static final int NULL_SIZE = -1;
    public static final int NULL_CHECKSUM = -1;
    private final String topic;
    private final int partition;
    private final long offset;
    private final long timestamp;
    private final TimestampType timestampType;
    private final int serializedKeySize;
    private final int serializedValueSize;
    private final Headers headers;
    private final K key;
    private final V value;
    private final Optional<Integer> leaderEpoch;
    private volatile Long checksum;

    public ConsumerRecord(String topic, int partition, long offset, K key, V value) {
        this(topic, partition, offset, -1L, TimestampType.NO_TIMESTAMP_TYPE, -1L, -1, -1, key, value);
    }

    public ConsumerRecord(String topic, int partition, long offset, long timestamp, TimestampType timestampType, long checksum, int serializedKeySize, int serializedValueSize, K key, V value) {
        this(topic, partition, offset, timestamp, timestampType, checksum, serializedKeySize, serializedValueSize, key, value, new RecordHeaders());
    }

    public ConsumerRecord(String topic, int partition, long offset, long timestamp, TimestampType timestampType, Long checksum, int serializedKeySize, int serializedValueSize, K key, V value, Headers headers) {
        this(topic, partition, offset, timestamp, timestampType, checksum, serializedKeySize, serializedValueSize, key, value, headers, Optional.empty());
    }

    public ConsumerRecord(String topic, int partition, long offset, long timestamp, TimestampType timestampType, Long checksum, int serializedKeySize, int serializedValueSize, K key, V value, Headers headers, Optional<Integer> leaderEpoch) {
        if (topic == null) {
            throw new IllegalArgumentException("Topic cannot be null");
        } else if (headers == null) {
            throw new IllegalArgumentException("Headers cannot be null");
        } else {
            this.topic = topic;
            this.partition = partition;
            this.offset = offset;
            this.timestamp = timestamp;
            this.timestampType = timestampType;
            this.checksum = checksum;
            this.serializedKeySize = serializedKeySize;
            this.serializedValueSize = serializedValueSize;
            this.key = key;
            this.value = value;
            this.headers = headers;
            this.leaderEpoch = leaderEpoch;
        }
    }

    public String topic() {
        return this.topic;
    }

    public int partition() {
        return this.partition;
    }

    public Headers headers() {
        return this.headers;
    }

    public K key() {
        return this.key;
    }

    public V value() {
        return this.value;
    }

    public long offset() {
        return this.offset;
    }

    public long timestamp() {
        return this.timestamp;
    }

    public TimestampType timestampType() {
        return this.timestampType;
    }

    /** @deprecated */
    @Deprecated
    public long checksum() {
        if (this.checksum == null) {
            this.checksum = DefaultRecord.computePartialChecksum(this.timestamp, this.serializedKeySize, this.serializedValueSize);
        }

        return this.checksum;
    }

    public int serializedKeySize() {
        return this.serializedKeySize;
    }

    public int serializedValueSize() {
        return this.serializedValueSize;
    }

    public Optional<Integer> leaderEpoch() {
        return this.leaderEpoch;
    }

    public String toString() {
        return "ConsumerRecord(topic = " + this.topic + ", partition = " + this.partition + ", leaderEpoch = " + this.leaderEpoch.orElse((Object)null) + ", offset = " + this.offset + ", " + this.timestampType + " = " + this.timestamp + ", serialized key size = " + this.serializedKeySize + ", serialized value size = " + this.serializedValueSize + ", headers = " + this.headers + ", key = " + this.key + ", value = " + this.value + ")";
    }
}