Introduce JsonLineAggregator for FlatFileItemWriter to write JSON Lines format file

spring-batch-infrastructure/src/main/java/org/springframework/batch/infrastructure/item/file/transform/JsonLineAggregator.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2006-2026 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.file.transform;
+
+import org.springframework.batch.infrastructure.item.file.mapping.JsonLineMapper;
+import org.springframework.batch.infrastructure.item.json.JsonObjectMarshaller;
+
+/**
+ * A {@link LineAggregator} implementation that converts objects into
+ * <a href="https://jsonlines.org">JSON Lines</a>.
+ *
+ * @author Yanming Zhou
+ * @see JsonLineMapper
+ */
+public class JsonLineAggregator<T> implements LineAggregator<T> {
+
+	private final JsonObjectMarshaller<T> jsonObjectMarshaller;
+
+	public JsonLineAggregator(JsonObjectMarshaller<T> jsonObjectMarshaller) {
+		this.jsonObjectMarshaller = jsonObjectMarshaller;
+	}
+
+	/**
+	 * Marshal the provided item into JSON string.
+	 * @param item value to be converted
+	 * @return JSON string
+	 */
+	@Override
+	public String aggregate(T item) {
+		return jsonObjectMarshaller.marshal(item);
+	}
+
+}

spring-batch-infrastructure/src/test/java/org/springframework/batch/infrastructure/item/file/transform/JsonLineAggregatorTests.java

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+/*
+ * Copyright 2006-2026 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.file.transform;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.springframework.batch.infrastructure.item.json.JacksonJsonObjectMarshaller;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * @author Yanming Zhou
+ *
+ */
+class JsonLineAggregatorTests {
+
+	private JsonLineAggregator<User> aggregator;
+
+	@BeforeEach
+	void setup() {
+		this.aggregator = new JsonLineAggregator<>(new JacksonJsonObjectMarshaller<>());
+	}
+
+	@Test
+	void testAggregate() {
+		assertEquals("""
+				{"name":"foo","email":"[email protected]","introduction":"I'm\\npowerful\\nman"}""",
+				aggregator.aggregate(new User("foo", "[email protected]", """
+						I'm
+						powerful
+						man""")));
+	}
+
+	record User(String name, String email, String introduction) {
+
+	}
+
+}