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Class Level @KafkaListener ​

Selection Criteria ​

Found class will be selected for further processing only if:

It has @KafkaHandler annotation on one of its methods

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.

Spring Documentation: Class level KafkaListener

Registration process ​

Operations registration process depends on @SendTo annotation usage.

With @SendTo ​

See Receive & Reply Operations

Without @SendTo ​

When listener is registered without @SendTo annotation, one-to-one operation will be created for each @KafkaHandler method.

java
@KafkaListener(topics = {"messages.blue"})
@KafkaListener(topics = {"messages.green", "messages.yellow"}, groupId = "group-id")
public class MultiMessagesListener {

    @KafkaHandler
    public void listen(Object message) {}

}

The next operations will be created:

  1. receive Object from messages.blue
  2. receive Object from messages.green with groupId group-id
  3. receive Object from messages.yellow with groupId group-id

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.

java
@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.

java
@KafkaListener
public class MultiMessagesListener {

    @KafkaHandler
    public void listen(Object message) {}

}

Class with unclear @KafkaHandler ​

When class contains @KafkaHandler with unclear message schema

java
@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