@KafkaListener on a Class

当您在类级别使用 @KafkaListener 时，必须在方法级别指定 @KafkaHandler。当传递消息时，转换的消息有效负载类型用于确定要调用的方法。以下示例演示如何操作：

When you use @KafkaListener at the class-level, you must specify @KafkaHandler at the method level. When messages are delivered, the converted message payload type is used to determine which method to call. The following example shows how to do so:

@KafkaListener(id = "multi", topics = "myTopic")
static class MultiListenerBean {

    @KafkaHandler
    public void listen(String foo) {
        ...
    }

    @KafkaHandler
    public void listen(Integer bar) {
        ...
    }

    @KafkaHandler(isDefault = true)
    public void listenDefault(Object object) {
        ...
    }

}

从第 2.1.3 版开始，你可以指定一个 @KafkaHandler 方法作为默认方法，该方法如果在其他方法中没有匹配项，则会被调用。最多可以指定一个方法。在使用 @KafkaHandler 方法时，有效负载必须已转换为域对象（因此可以执行匹配）。使用自定义反序列化程序、 JsonDeserializer 或 JsonMessageConverter，其中其 TypePrecedence 设置为 TYPE_ID。请参阅 Serialization, Deserialization, and Message Conversion 了解更多信息。

Starting with version 2.1.3, you can designate a @KafkaHandler method as the default method that is invoked if there is no match on other methods. At most, one method can be so designated. When using @KafkaHandler methods, the payload must have already been converted to the domain object (so the match can be performed). Use a custom deserializer, the JsonDeserializer, or the JsonMessageConverter with its TypePrecedence set to TYPE_ID. See Serialization, Deserialization, and Message Conversion for more information.

由于 Spring 解析方法参数的方式存在一些限制，所以默认的 @KafkaHandler 不能接收离散的标头；它必须使用 Consumer Record Metadata 中讨论的 ConsumerRecordMetadata。

Due to some limitations in the way Spring resolves method arguments, a default @KafkaHandler cannot receive discrete headers; it must use the ConsumerRecordMetadata as discussed in Consumer Record Metadata.

例如：

For example:

@KafkaHandler(isDefault = true)
public void listenDefault(Object object, @Header(KafkaHeaders.RECEIVED_TOPIC) String topic) {
    ...
}

如果对象是 String，那么这将不起作用；topic 参数也会获取对 object 的引用。

This won’t work if the object is a String; the topic parameter will also get a reference to object.

如果您需要有关默认方法中记录的元数据，请使用以下内容：

If you need metadata about the record in a default method, use this:

@KafkaHandler(isDefault = true)
void listen(Object in, @Header(KafkaHeaders.RECORD_METADATA) ConsumerRecordMetadata meta) {
    String topic = meta.topic();
    ...
}