title: How to set groupId to null in @KafkaListeners
body: <p>In relation with : <a href="https://stackoverflow.com/questions/56017730/how-to-load-a-compacted-topic-in-memory-before-starting-the-context/56023409">this question</a></p>
  
  <p>I'm trying to read a compacted topic via a @KafkaListener.
  I'd like every consumers to read the whole topic each time.</p>
  
  <p>I can not generate an unique groupId each for each consumers. So I wanted to use a null groupid.</p>
  
  <p>I've tried to configure the container and the consumer to set the groupId to null, but neither worked.</p>
  
  <p>Here is my Container configuration :</p>
  
  <pre><code> ConcurrentKafkaListenerContainerFactory&lt;Object, Object&gt; factory = new ConcurrentKafkaListenerContainerFactory&lt;&gt;();
          configurer.configure(factory, kafkaConsumerFactory);
          // Set ackMode to manual and never commit, we are reading from the beginning each time
          factory.getContainerProperties().setAckMode(ContainerProperties.AckMode.MANUAL);
          factory.getContainerProperties().setAckOnError(false);
          // Remove groupId, we are consuming all partitions here
          factory.getContainerProperties().setGroupId(null);
          // Enable idle event in order to detect when init phase is over
          factory.getContainerProperties().setIdleEventInterval(1000L);
  </code></pre>
  
  <p>Also tried to force the consumer configuration :</p>
  
  <pre><code>Map&lt;String, Object&gt; consumerProperties = sprinfKafkaProperties.buildConsumerProperties();
          // Override group id property to force "null"
          consumerProperties.put(ConsumerConfig.GROUP_ID_CONFIG, null);
          ConsumerFactory&lt;Object, Object&gt; kafkaConsumerFactory = new DefaultKafkaConsumerFactory&lt;&gt;(consumerProperties);
  </code></pre>
  
  <p>When I'm setting the container groupId to null, a default with the listener id is used.</p>
  
  <p>When I'm forcing the consumer to a null groupId property, I've got an error :
  No group.id found in consumer config, container properties, or @KafkaListener annotation; a group.id is required when group management is used.</p>