Auto-configure listener container factory without consumer factory

Previously, the presence of a `ConsumerFactory` bean would make the
auto-configured one to back off, leading to a failure down the line if
no available instance matches the generics criterion. This commit
improves the auto-configuration to create a `ConsumerFactory<?,?>`
behind the scenes if none is available.

Closes gh-19221

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/kafka/KafkaAnnotationDrivenConfiguration.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,6 +25,7 @@ import org.springframework.kafka.annotation.EnableKafka;
 import org.springframework.kafka.config.ConcurrentKafkaListenerContainerFactory;
 import org.springframework.kafka.config.KafkaListenerConfigUtils;
 import org.springframework.kafka.core.ConsumerFactory;
+import org.springframework.kafka.core.DefaultKafkaConsumerFactory;
 import org.springframework.kafka.core.KafkaTemplate;
 import org.springframework.kafka.listener.AfterRollbackProcessor;
 import org.springframework.kafka.listener.ErrorHandler;
@@ -84,9 +85,10 @@ class KafkaAnnotationDrivenConfiguration {
 	@ConditionalOnMissingBean(name = "kafkaListenerContainerFactory")
 	public ConcurrentKafkaListenerContainerFactory<?, ?> kafkaListenerContainerFactory(
 			ConcurrentKafkaListenerContainerFactoryConfigurer configurer,
-			ConsumerFactory<Object, Object> kafkaConsumerFactory) {
+			ObjectProvider<ConsumerFactory<Object, Object>> kafkaConsumerFactory) {
 		ConcurrentKafkaListenerContainerFactory<Object, Object> factory = new ConcurrentKafkaListenerContainerFactory<>();
-		configurer.configure(factory, kafkaConsumerFactory);
+		configurer.configure(factory, kafkaConsumerFactory
+				.getIfAvailable(() -> new DefaultKafkaConsumerFactory<>(this.properties.buildConsumerProperties())));
 		return factory;
 	}
 

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/kafka/KafkaAutoConfigurationTests.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -48,6 +48,7 @@ import org.springframework.kafka.config.ConcurrentKafkaListenerContainerFactory;
 import org.springframework.kafka.config.KafkaListenerContainerFactory;
 import org.springframework.kafka.config.KafkaStreamsConfiguration;
 import org.springframework.kafka.config.StreamsBuilderFactoryBean;
+import org.springframework.kafka.core.ConsumerFactory;
 import org.springframework.kafka.core.DefaultKafkaConsumerFactory;
 import org.springframework.kafka.core.DefaultKafkaProducerFactory;
 import org.springframework.kafka.core.KafkaAdmin;
@@ -476,6 +477,16 @@ public class KafkaAutoConfigurationTests {
 		});
 	}
 
+	@Test
+	public void testConcurrentKafkaListenerContainerFactoryWithCustomConsumerFactory() {
+		this.contextRunner.withUserConfiguration(ConsumerFactoryConfiguration.class).run((context) -> {
+			ConcurrentKafkaListenerContainerFactory<?, ?> kafkaListenerContainerFactory = context
+					.getBean(ConcurrentKafkaListenerContainerFactory.class);
+			assertThat(kafkaListenerContainerFactory.getConsumerFactory())
+					.isNotSameAs(context.getBean(ConsumerFactoryConfiguration.class).consumerFactory);
+		});
+	}
+
 	@Configuration
 	protected static class MessageConverterConfiguration {
 
@@ -520,6 +531,18 @@ public class KafkaAutoConfigurationTests {
 
 	}
 
+	@Configuration
+	protected static class ConsumerFactoryConfiguration {
+
+		private final ConsumerFactory<String, Object> consumerFactory = mock(ConsumerFactory.class);
+
+		@Bean
+		public ConsumerFactory<String, Object> myConsumerFactory() {
+			return this.consumerFactory;
+		}
+
+	}
+
 	@Configuration
 	@EnableKafkaStreams
 	protected static class EnableKafkaStreamsConfiguration {