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Class Level @KafkaListeners ​

How classes with @KafkaListeners annotation are selected? ​

Found class will be selected for further processing only if:

  1. @KafkaListeners is no empty
  2. 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.

java
@KafkaListeners({
        @KafkaListener(topics = {"messages.blue"}),
        @KafkaListener(topics = {"messages.green"})
})
public class MultiMessagesListener {
    
    public void listen(Object message) {}

}

Class with empty @KafkaListeners annotation ​

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

java
@KafkaListeners
public class MultiMessagesListener {

    @KafkaHandler
    public void listen(Object message) {}

}

Class with not configured @KafkaListener ​

When class was annotated properly, but one of @KafkaListener is not configured, class will be ignored. Because such configuraiton on method level will throw an exception – java.lang.IllegalStateException: topics, topicPattern, or topicPartitions must be provided

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

    @KafkaHandler
    public void stringMessage(String string) {}

    @KafkaHandler
    public void integerMessage(Integer integer) {}

    @KafkaHandler
    public void publicMessage(SendPublicMessageRequest message) {}

}

Class with unclear @KafkaHandler ​

When class contains @KafkaHandler with unclear message schema

java
@KafkaListeners({
        @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