Kafka producer JSON serialization - spring

Question

I'm trying to use Spring Cloud Stream to integrate with Kafka. The message being written is a Java POJO and while it works as expected (the message is being written to the topic and I can read off with a consumer app), there are some unknown characters being added to the start of the message which are causing trouble when trying to integrate Kafka Connect to sink the messages from the topic.
With the default setup this is the message being pushed to Kafka:
 contentType "text/plain"originalContentType "application/json;charset=UTF-8"{"payload":{"username":"john"},"metadata":{"eventName":"Login","sessionId":"089acf50-00bd-47c9-8e49-dc800c1daf50","username":"john","hasSent":null,"createDate":1511186145471,"version":null}}
If I configure the Kafka producer within the Java app then the message is written to the topic without the leading characters / headers:
#Configuration
public class KafkaProducerConfig {
#Bean
public ProducerFactory<String, Object> producerFactory() {
Map<String, Object> configProps = new HashMap<>();
configProps.put(
ProducerConfig.BOOTSTRAP_SERVERS_CONFIG,
"localhost:9092");
configProps.put(
ProducerConfig.KEY_SERIALIZER_CLASS_CONFIG,
StringSerializer.class);
configProps.put(
ProducerConfig.VALUE_SERIALIZER_CLASS_CONFIG,
JsonSerializer.class);
return new DefaultKafkaProducerFactory<String, Object>(configProps);
}
#Bean
public KafkaTemplate<String, Object> kafkaTemplate() {
return new KafkaTemplate<>(producerFactory());
}
}
Message on Kafka:
{"payload":{"username":"john"},"metadata":{"eventName":"Login","sessionId":"089acf50-00bd-47c9-8e49-dc800c1daf50","username":"john","hasSent":null,"createDate":1511186145471}
Since I'm just setting the key/value serializers I would've expected to be able to do this within the application.yml properties file, rather than doing it through the code.
However, when the yml is updated to specify the serializers it's not working as I would expect i.e. it's not generating the same message as the producer configured in Java (above):
spring:
profiles: local
cloud:
stream:
bindings:
session:
destination: session
contentType: application/json
kafka:
binder:
brokers: localhost
zkNodes: localhost
defaultZkPort: 2181
defaultBrokerPort: 9092
bindings:
session:
producer:
configuration:
value:
serializer: org.springframework.kafka.support.serializer.JsonSerializer
key:
serializer: org.apache.kafka.common.serialization.StringSerializer
Message on Kafka:
"/wILY29udGVudFR5cGUAAAAMInRleHQvcGxhaW4iE29yaWdpbmFsQ29udGVudFR5cGUAAAAgImFwcGxpY2F0aW9uL2pzb247Y2hhcnNldD1VVEYtOCJ7InBheWxvYWQiOnsidXNlcm5hbWUiOiJqb2huIn0sIm1ldGFkYXRhIjp7ImV2ZW50TmFtZSI6IkxvZ2luIiwic2Vzc2lvbklkIjoiNGI3YTBiZGEtOWQwZS00Nzg5LTg3NTQtMTQyNDUwYjczMThlIiwidXNlcm5hbWUiOiJqb2huIiwiaGFzU2VudCI6bnVsbCwiY3JlYXRlRGF0ZSI6MTUxMTE4NjI2NDk4OSwidmVyc2lvbiI6bnVsbH19"
Should it be possible to configure this solely through the application yml? Are there additional settings that are missing?

Answer 1

Credit to #Gary for the answer above!
For completeness, the configuration which is now working for me is below.
spring:
profiles: local
cloud:
stream:
bindings:
session:
producer:
useNativeEncoding: true
destination: session
contentType: application/json
kafka:
binder:
brokers: localhost
zkNodes: localhost
defaultZkPort: 2181
defaultBrokerPort: 9092
bindings:
session:
producer:
configuration:
value:
serializer: org.springframework.kafka.support.serializer.JsonSerializer
key:
serializer: org.apache.kafka.common.serialization.StringSerializer

Answer 2

See headerMode and useNativeEncoding in the producer properties (....session.producer.useNativeEncoding).
headerMode
When set to raw, disables header embedding on output. Effective only for messaging middleware that does not support message headers natively and requires header embedding. Useful when producing data for non-Spring Cloud Stream applications.
Default: embeddedHeaders.
useNativeEncoding
When set to true, the outbound message is serialized directly by client library, which must be configured correspondingly (e.g. setting an appropriate Kafka producer value serializer). When this configuration is being used, the outbound message marshalling is not based on the contentType of the binding. When native encoding is used, it is the responsibility of the consumer to use appropriate decoder (ex: Kafka consumer value de-serializer) to deserialize the inbound message. Also, when native encoding/decoding is used the headerMode property is ignored and headers will not be embedded into the message.
Default: false.

Answer 3

Now, spring.kafka.producer.value-serializer property can be used
yml:
spring:
kafka:
producer:
value-serializer: org.springframework.kafka.support.serializer.JsonSerializer
properties:
spring.kafka.producer.value-serializer=org.springframework.kafka.support.serializer.JsonSerializer