Add MappingItemWriter

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

Author: scordio

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

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2025 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.infrastructure.item.support;
+
+import org.springframework.batch.infrastructure.item.Chunk;
+import org.springframework.batch.infrastructure.item.ItemWriter;
+
+import java.util.function.Function;
+
+/**
+ * Adapts an {@link 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 MappingItemWriter} is most useful when used in combination with a
+ * {@link org.springframework.batch.infrastructure.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
+ * @author Stefano Cordio
+ * @since 6.0
+ */
+public class MappingItemWriter<T, U> implements ItemWriter<T> {
+
+	private final Function<? super T, ? extends U> mapper;
+
+	private final ItemWriter<? super U> downstream;
+
+	/**
+	 * Create a new {@link MappingItemWriter}.
+	 * @param mapper the mapping function to apply to the input items
+	 * @param downstream the downstream item writer that accepts mapped items
+	 */
+	public MappingItemWriter(Function<? super T, ? extends U> mapper, ItemWriter<? super U> downstream) {
+		this.mapper = mapper;
+		this.downstream = downstream;
+	}
+
+	@Override
+	public void write(Chunk<? extends T> chunk) throws Exception {
+		downstream.write(new Chunk<>(chunk.getItems().stream().map(mapper).toList()));
+	}
+
+}

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

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+/*
+ * Copyright 2025 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.infrastructure.item.support;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.batch.infrastructure.item.Chunk;
+import org.springframework.batch.infrastructure.item.ItemWriter;
+
+import java.util.List;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Stefano Cordio
+ */
+class MappingItemWriterIntegrationTests {
+
+	@Test
+	void testWithCompositeItemWriter() throws Exception {
+		// given
+		record Person(String name, int age) {
+		}
+
+		ListItemWriter<String> nameItemWriter = new ListItemWriter<>();
+		ListItemWriter<Integer> ageItemWriter = new ListItemWriter<>();
+
+		ItemWriter<Person> underTest = new CompositeItemWriter<>(List.of( //
+				new MappingItemWriter<>(Person::name, nameItemWriter), //
+				new MappingItemWriter<>(Person::age, ageItemWriter)));
+
+		// when
+		underTest.write(Chunk.of(new Person("Foo", 42), new Person("Bar", 24)));
+		underTest.write(Chunk.of(new Person("Baz", 21), new Person("Qux", 12)));
+
+		// then
+		assertThat(nameItemWriter.getWrittenItems()).containsExactly("Foo", "Bar", "Baz", "Qux");
+		assertThat(ageItemWriter.getWrittenItems()).containsExactly(42, 24, 21, 12);
+	}
+
+}

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

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+/*
+ * Copyright 2025 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.infrastructure.item.support;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.batch.infrastructure.item.Chunk;
+
+import java.util.function.Function;
+
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+
+/**
+ * @author Stefano Cordio
+ */
+class MappingItemWriterTests {
+
+	@Test
+	void testWithMapperAcceptingItemSuperclass() throws Exception {
+		// given
+		Function<Entity, String> mapper = Entity::name;
+		ListItemWriter<String> downstream = mock();
+		MappingItemWriter<Person, String> underTest = new MappingItemWriter<>(mapper, downstream);
+
+		// when
+		underTest.write(Chunk.of(new Person("Foo", 42)));
+
+		// then
+		verify(downstream).write(Chunk.of("Foo"));
+	}
+
+	@Test
+	void testWithMapperProducingMappedItemSubclass() throws Exception {
+		// given
+		Function<Person, String> mapper = Person::name;
+		ListItemWriter<CharSequence> downstream = mock();
+		MappingItemWriter<Person, CharSequence> underTest = new MappingItemWriter<>(mapper, downstream);
+
+		// when
+		underTest.write(Chunk.of(new Person("Foo", 42)));
+
+		// then
+		verify(downstream).write(Chunk.of("Foo"));
+	}
+
+	@Test
+	void testWithDownstreamAcceptingMappedItemSuperclass() throws Exception {
+		// given
+		Function<Person, String> mapper = Person::name;
+		ListItemWriter<CharSequence> downstream = mock();
+		MappingItemWriter<Person, String> underTest = new MappingItemWriter<>(mapper, downstream);
+
+		// when
+		underTest.write(Chunk.of(new Person("Foo", 42)));
+
+		// then
+		verify(downstream).write(Chunk.of("Foo"));
+	}
+
+	private interface Entity {
+
+		String name();
+
+	}
+
+	private record Person(String name, int age) implements Entity {
+	}
+
+}