Class Level @KafkaListener

How classes with @KafkaListener annotation are selected?

Found class will be selected for further processing only if:

It has @KafkaHandler annotation on one of its methods - Spring Documentation: Class level KafkaListener

When you use @KafkaListener at the class-level, you must specify @KafkaHandler at the method level. If no @KafkaHandler on any methods of this class or its sub-classes, the framework will reject such a configuration.

When class will be ignored?

Here are the examples of classes which will be ignored:

Class without @KafkaHandler annotations

This class will be ignored because it has no @KafkaHandler annotation on any of its methods.

@KafkaListener(topics = {"messages.blue"})
@KafkaListener(topics = {"messages.green"})
public class MultiMessagesListener {

    public void listen(Object message) {}

}

Class with empty @KafkaListener annotation

This class will be ignored because it has empty @KafkaListener annotation.

@KafkaListener
public class MultiMessagesListener {

    @KafkaHandler
    public void listen(Object message) {}

}

Class with unclear @KafkaHandler

When class contains @KafkaHandler with unclear message schema

@KafkaListener(topics = {"messages.green"})
public class MultiMessagesListener {

    // (1)
    @KafkaHandler
    public void stringMessage() {}

    // (2)
    @KafkaHandler
    public void integerMessage(Object obj1, Object obj2) {}

}

• (1) - handler doesn't have any arguments
• (2) - handler has one or more arguments, so it's impossible to understand which argument is message and header, for example