Fix writing collections where first element is null

spring-data-jdbc/src/test/java/org/springframework/data/jdbc/repository/JdbcRepositoryCustomConversionIntegrationTests.java

@@ -21,6 +21,7 @@
 
 import java.math.BigDecimal;
 import java.sql.JDBCType;
+import java.util.ArrayList;
 import java.util.Date;
 import java.util.List;
 import java.util.Set;
@@ -68,6 +69,11 @@ EntityWithDirectionsRepository repositoryWithDirections(JdbcRepositoryFactory fa
 			return factory.getRepository(EntityWithDirectionsRepository.class);
 		}
 
+		@Bean
+		EntityWithIdsRepository repositoryWithIds(JdbcRepositoryFactory factory) {
+			return factory.getRepository(EntityWithIdsRepository.class);
+		}
+
 		@Bean
 		JdbcCustomConversions jdbcCustomConversions() {
 			return new JdbcCustomConversions(asList(StringToBigDecimalConverter.INSTANCE, BigDecimalToString.INSTANCE,
@@ -78,6 +84,7 @@ JdbcCustomConversions jdbcCustomConversions() {
 
 	@Autowired EntityWithStringyBigDecimalRepository repository;
 	@Autowired EntityWithDirectionsRepository repositoryWithDirections;
+	@Autowired EntityWithIdsRepository repositoryWithIds;
 
 	/**
 	 * In PostrgreSQL this fails if a simple converter like the following is used.
@@ -182,6 +189,19 @@ void saveAndLoadListOfDirectionsAsArray() {
 		assertThat(reloaded).isEqualTo(saved);
 	}
 
+	@Test // GH-2215
+	@EnabledOnFeature(TestDatabaseFeatures.Feature.SUPPORTS_ARRAYS)
+	void saveAndLoadListOfNullAsArray() {
+		var list = new ArrayList<Integer>();
+		list.add(null);
+
+		EntityWithIds saved = repositoryWithIds.save(new EntityWithIds(null, list));
+
+		EntityWithIds reloaded = repositoryWithIds.findById(saved.id).orElseThrow();
+
+		assertThat(reloaded).isEqualTo(saved);
+	}
+
 	interface EntityWithStringyBigDecimalRepository extends CrudRepository<EntityWithStringyBigDecimal, CustomId> {
 
 		@Query("SELECT * FROM ENTITY_WITH_STRINGY_BIG_DECIMAL WHERE DIRECTION IN (:types)")
@@ -193,6 +213,8 @@ interface EntityWithStringyBigDecimalRepository extends CrudRepository<EntityWit
 
 	interface EntityWithDirectionsRepository extends CrudRepository<EntityWithDirections, Long> {}
 
+	interface EntityWithIdsRepository extends CrudRepository<EntityWithIds, Long> {}
+
 	private static class EntityWithStringyBigDecimal {
 
 		@Id CustomId id;
@@ -223,6 +245,9 @@ enum Direction {
 		LEFT, CENTER, RIGHT
 	}
 
+	record EntityWithIds(@Id Long id, List<Integer> ids) {
+	}
+
 	@WritingConverter
 	enum StringToBigDecimalConverter implements Converter<String, JdbcValue> {
 

spring-data-jdbc/src/test/resources/org.springframework.data.jdbc.repository/JdbcRepositoryCustomConversionIntegrationTests-h2.sql

@@ -1,3 +1,4 @@
 CREATE TABLE ENTITY_WITH_STRINGY_BIG_DECIMAL ( id IDENTITY PRIMARY KEY, Stringy_number DECIMAL(20,10), DIRECTION INTEGER);
 CREATE TABLE OTHER_ENTITY ( ID IDENTITY PRIMARY KEY, CREATED DATE, ENTITY_WITH_STRINGY_BIG_DECIMAL INTEGER);
 CREATE TABLE ENTITY_WITH_DIRECTIONS ( ID IDENTITY PRIMARY KEY, DIRECTIONS INTEGER ARRAY);
+CREATE TABLE ENTITY_WITH_IDS ( ID IDENTITY PRIMARY KEY, IDS INTEGER ARRAY);

spring-data-jdbc/src/test/resources/org.springframework.data.jdbc.repository/JdbcRepositoryCustomConversionIntegrationTests-hsql.sql

@@ -1,4 +1,5 @@
 CREATE TABLE ENTITY_WITH_STRINGY_BIG_DECIMAL ( id IDENTITY PRIMARY KEY, Stringy_number DECIMAL(20,10), DIRECTION INTEGER);
 CREATE TABLE OTHER_ENTITY ( ID IDENTITY PRIMARY KEY, CREATED DATE, ENTITY_WITH_STRINGY_BIG_DECIMAL INTEGER);
 CREATE TABLE ENTITY_WITH_DIRECTIONS ( ID IDENTITY PRIMARY KEY, DIRECTIONS INTEGER ARRAY);
+CREATE TABLE ENTITY_WITH_IDS ( ID IDENTITY PRIMARY KEY, IDS INTEGER ARRAY);
 

spring-data-jdbc/src/test/resources/org.springframework.data.jdbc.repository/JdbcRepositoryCustomConversionIntegrationTests-postgres.sql

@@ -1,3 +1,4 @@
 CREATE TABLE ENTITY_WITH_STRINGY_BIG_DECIMAL ( id SERIAL PRIMARY KEY, Stringy_number DECIMAL(20,10), DIRECTION INTEGER);
 CREATE TABLE OTHER_ENTITY ( ID SERIAL PRIMARY KEY, CREATED DATE, ENTITY_WITH_STRINGY_BIG_DECIMAL INTEGER);
 CREATE TABLE ENTITY_WITH_DIRECTIONS ( ID SERIAL PRIMARY KEY, DIRECTIONS INTEGER ARRAY);
+CREATE TABLE ENTITY_WITH_IDS ( ID SERIAL PRIMARY KEY, IDS INTEGER ARRAY);

spring-data-relational/src/main/java/org/springframework/data/relational/core/conversion/MappingRelationalConverter.java

@@ -815,12 +815,11 @@ private Object writeCollection(Iterable<?> value, TypeInformation<?> type) {
 		// if we succeeded converting the members of the collection, we actually ignore the fallback targetType since that
 		// was derived without considering custom conversions.
 		Class<?> targetType = type.getType();
-		if (!mapped.isEmpty()) {
+		if (!mapped.isEmpty() && mapped.get(0) != null) {
 
 			Class<?> targetComponentType = mapped.get(0).getClass();
 			targetType = Array.newInstance(targetComponentType, 0).getClass();
 		}
-
 		Object converted = getConversionService().convert(mapped, targetType);
 
 		Assert.state(converted != null, "Converted must not be null");