GH-597: Add default impls for listener methods

Resolves #597

Add default no-op methods as a convenience so only those that are needed
have to be implemented.

Author: garyrussell

spring-kafka/src/main/java/org/springframework/kafka/listener/ConsumerAwareRebalanceListener.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017 the original author or authors.
+ * Copyright 2017-2018 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.
@@ -23,7 +23,9 @@
 import org.apache.kafka.common.TopicPartition;
 
 /**
- * A rebalance listener that provides access to the consumer object.
+ * A rebalance listener that provides access to the consumer object. Starting with version
+ * 2.1.5, as a convenience, default no-op implementations are provided for all methods,
+ * allowing the user to implement just those (s)he is interested in.
  *
  * @author Gary Russell
  * @since 2.0
@@ -37,23 +39,29 @@ public interface ConsumerAwareRebalanceListener extends ConsumerRebalanceListene
 	 * @param consumer the consumer.
 	 * @param partitions the partitions.
 	 */
-	void onPartitionsRevokedBeforeCommit(Consumer<?, ?> consumer, Collection<TopicPartition> partitions);
+	default void onPartitionsRevokedBeforeCommit(Consumer<?, ?> consumer, Collection<TopicPartition> partitions) {
+		// do nothing
+	}
 
 	/**
 	 * The same as {@link #onPartitionsRevoked(Collection)} with the additional consumer
 	 * parameter. It is invoked by the container after any pending offsets are committed.
 	 * @param consumer the consumer.
 	 * @param partitions the partitions.
 	 */
-	void onPartitionsRevokedAfterCommit(Consumer<?, ?> consumer, Collection<TopicPartition> partitions);
+	default void onPartitionsRevokedAfterCommit(Consumer<?, ?> consumer, Collection<TopicPartition> partitions) {
+		// do nothing
+	}
 
 	/**
 	 * The same as {@link #onPartitionsAssigned(Collection)} with the additional consumer
 	 * parameter.
 	 * @param consumer the consumer.
 	 * @param partitions the partitions.
 	 */
-	void onPartitionsAssigned(Consumer<?, ?> consumer, Collection<TopicPartition> partitions);
+	default void onPartitionsAssigned(Consumer<?, ?> consumer, Collection<TopicPartition> partitions) {
+		// do nothing
+	}
 
 	@Override
 	default void onPartitionsRevoked(Collection<TopicPartition> partitions) {