Spring Integration - kafka Outbound adapter not taking topic value exposed as spring bean - spring

Question

I have successfully integrated kafka outbound channle adapter with fixed topic name. Now, i want to make the topic name configurable and hence, want to expose it via application properties.
application.properties contain one of the following entry:
kafkaTopic:testNewTopic
My configuration class looks like below:
#Configuration
#Component
public class KafkaConfig {
#Value("${kafkaTopic}")
private String kafkaTopicName;
#Bean
public String getTopic(){
return kafkaTopicName;
}
#Bean
public KafkaTemplate<String, String> kafkaTemplate() {
return new KafkaTemplate<>(producerFactory());
}
#Bean
public ProducerFactory<String, String> producerFactory() {
Map<String, Object> props = new HashMap<>();
props.put(ProducerConfig.BOOTSTRAP_SERVERS_CONFIG, "localhost:9092");//this.brokerAddress);
props.put(ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG, "org.apache.kafka.common.serialization.StringSerializer");
props.put(ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG, "org.apache.kafka.common.serialization.StringSerializer");
// set more properties
return new DefaultKafkaProducerFactory<>(props);
}
}
and in my si-config.xml, i have used the following (ex: topic="getTopic") :
<int-kafka:outbound-channel-adapter
id="kafkaOutboundChannelAdapter" kafka-template="kafkaTemplate"
auto-startup="true" sync="true" channel="inputToKafka" topic="getTopic">
</int-kafka:outbound-channel-adapter>
However, the configuration is unable to pick up the topic name when exposed via bean. But it works fine when i hard code the value of the topic name.
Can someone please suggest what i am doing wrong here?
Does topic within kafka outbound channel accept the value referred as bean?
How do i externalize it as every application using my utility will supply different kafka topic names

Answer 1

The topic attribute is for string value.
However it supports property placeholder resolution:
topic="${kafkaTopic}"
and also SpEL evaluation for aforementioned bean:
topic="#{getTopic}"
Just because this is allowed by the XML parser configuration.
However you may pay attention that KafkaTemplate, which you inject into the <int-kafka:outbound-channel-adapter> has defaultTopic property. Therefore you won't need to worry about that XML.
And one more option available for you is Spring Integration Annotations configuration. Where you can define a #ServiceActivator for the KafkaProducerMessageHandler #Bean:
#ServiceActivator(inputChannel = "inputToKafka")
#Bean
KafkaProducerMessageHandler kafkaOutboundChannelAdapter() {
kafkaOutboundChannelAdapter adapter = new kafkaOutboundChannelAdapter( kafkaTemplate());
adapter.setSync(true);
adapter.setTopicExpression(new LiteralExpression(this.kafkaTopicName));
return adapter;
}