Add mapping adapter to ItemWriter

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

Author: scordio

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

@@ -16,6 +16,8 @@
 
 package org.springframework.batch.item;
 
+import java.util.function.Function;
+
 import org.springframework.lang.NonNull;
 
 /**
@@ -37,6 +39,7 @@
  * @author Lucas Ward
  * @author Taeik Lim
  * @author Mahmoud Ben Hassine
+ * @author Stefano Cordio
  */
 @FunctionalInterface
 public interface ItemWriter<T> {
@@ -50,4 +53,27 @@ public interface ItemWriter<T> {
 	 */
 	void write(@NonNull Chunk<? extends T> chunk) throws Exception;
 
+	/**
+	 * Adapts an {@code ItemWriter} accepting items of type {@link U} to one accepting
+	 * items of type {@link T} by applying a mapping function to each item before writing.
+	 * <p>
+	 * The {@code mapping()} item writers are most useful when used in combination with a
+	 * {@link org.springframework.batch.item.support.CompositeItemWriter}, where the
+	 * mapping function in front of the downstream writer can be a getter of the input
+	 * item or a more complex transformation logic.
+	 * <p>
+	 * This adapter mimics the behavior of
+	 * {@link java.util.stream.Collectors#mapping(Function, java.util.stream.Collector)}.
+	 * @param <T> the type of the input items
+	 * @param <U> type of items accepted by downstream item writer
+	 * @param mapper a function to be applied to the input items
+	 * @param downstream an item writer which will accept mapped items
+	 * @return an item writer which applies the mapping function to the input items and
+	 * provides the mapped results to the downstream item writer
+	 * @since 6.0
+	 */
+	static <T, U> ItemWriter<T> mapping(Function<? super T, ? extends U> mapper, ItemWriter<? super U> downstream) {
+		return chunk -> downstream.write(new Chunk<>(chunk.getItems().stream().map(mapper).toList()));
+	}
+
 }

spring-batch-infrastructure/src/test/java/org/springframework/batch/item/ItemWriterIntegrationTests.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2006-2022 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.batch.item;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.batch.item.support.CompositeItemWriter;
+import org.springframework.batch.item.support.ListItemWriter;
+
+import java.util.List;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.springframework.batch.item.ItemWriter.mapping;
+
+class ItemWriterIntegrationTests {
+
+	@Test
+	void testMappingWithCompositeItemWriter() throws Exception {
+		ListItemWriter<String> nameItemWriter = new ListItemWriter<>();
+		ListItemWriter<Integer> ageItemWriter = new ListItemWriter<>();
+
+		record Person(String name, int age) {
+		}
+
+		ItemWriter<Person> personItemWriter = new CompositeItemWriter<>(List.of( //
+				mapping(Person::name, nameItemWriter), //
+				mapping(Person::age, ageItemWriter)));
+
+		personItemWriter.write(Chunk.of(new Person("Foo", 42), new Person("Bar", 24)));
+		personItemWriter.write(Chunk.of(new Person("Baz", 21), new Person("Qux", 12)));
+
+		assertThat(nameItemWriter.getWrittenItems()).containsExactly("Foo", "Bar", "Baz", "Qux");
+		assertThat(ageItemWriter.getWrittenItems()).containsExactly(42, 24, 21, 12);
+	}
+
+}