title: Modify the message body from spring-cloud-stream app starter
body: <p>I am new to Spring Cloud Stream. My use case is to read from a file source and publish messages (to Kafka) for each line in the file. I have tried using the file source app starter (<a href="https://github.com/spring-cloud-stream-app-starters/file/tree/master/spring-cloud-starter-stream-source-file" rel="nofollow noreferrer">https://github.com/spring-cloud-stream-app-starters/file/tree/master/spring-cloud-starter-stream-source-file</a>) and have been able to publish messages.</p>
  
  <p>However the I now need to tweak the body of the message before publishing. The app starter generates generic messages and I need to modify the structure before publishing. I have tried searching on SO but haven't found any suitable example. Can anyone please suggest on how to achieve this?</p>
  
  <p>Many thanks.</p>