Clarify ItemWriter contract regarding empty chunks

Signed-off-by: Stefano Cordio <stefano.cordio@gmail.com>

Author: scordio

spring-batch-docs/modules/ROOT/pages/readers-and-writers/item-writer.adoc

@@ -26,5 +26,6 @@ interface accepts a list of items, rather than an item by itself. After writing
 list, any flushing that may be necessary can be performed before returning from the write
 method. For example, if writing to a Hibernate DAO, multiple calls to write can be made,
 one for each item. The writer can then call `flush` on the hibernate session before
-returning.
-
+returning. Note that the chunk passed to `write` may be empty if all items in the chunk
+were filtered out by an `ItemProcessor` or skipped during fault-tolerant step processing.
+Implementations are expected to handle empty chunks gracefully.

spring-batch-infrastructure/src/main/java/org/springframework/batch/infrastructure/item/ItemWriter.java

@@ -41,8 +41,10 @@ public interface ItemWriter<T> {
 
 	/**
 	 * Process the supplied data element. Will not be called with any null items in normal
-	 * operation.
-	 * @param chunk of items to be written. Must not be {@code null}.
+	 * operation but might be called with an empty chunk, for example when all items have
+	 * been filtered by an {@link ItemProcessor} or skipped by the fault-tolerant step
+	 * processing. Implementations are expected to handle empty chunks gracefully.
+	 * @param chunk of items to be written. Never {@code null} but may be empty.
 	 * @throws Exception if there are errors. The framework will catch the exception and
 	 * convert or rethrow it as appropriate.
 	 */