Fix NPE in QueryMapper when trying to apply target type on null value.

Closes #3633
Original pull request: #3643.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/QueryMapper.java

@@ -756,12 +756,12 @@ public class QueryMapper {
 	 * converted one by one.
 	 *
 	 * @param documentField the field and its meta data
-	 * @param value the actual value
+	 * @param value the actual value. Can be {@literal null}.
 	 * @return the potentially converted target value.
 	 */
-	private Object applyFieldTargetTypeHintToValue(Field documentField, Object value) {
+	private Object applyFieldTargetTypeHintToValue(Field documentField, @Nullable Object value) {
 
-		if (documentField.getProperty() == null || !documentField.getProperty().hasExplicitWriteTarget()) {
+		if (value == null || documentField.getProperty() == null || !documentField.getProperty().hasExplicitWriteTarget()) {
 			return value;
 		}
 

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/QueryMapperUnitTests.java

@@ -1255,6 +1255,17 @@ public class QueryMapperUnitTests {
 		assertThat(document).isEqualTo(new org.bson.Document("double_underscore.renamed", new org.bson.Document("$exists", true)));
 	}
 
+	@Test // GH-3633
+	void mapsNullValueForFieldWithCustomTargetType() {
+
+		Query query = query(where("stringAsOid").is(null));
+
+		org.bson.Document document = mapper.getMappedObject(query.getQueryObject(),
+				context.getPersistentEntity(NonIdFieldWithObjectIdTargetType.class));
+
+		assertThat(document).isEqualTo(new org.bson.Document("stringAsOid", null));
+	}
+
 	class WithDeepArrayNesting {
 
 		List<WithNestedArray> level0;

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/repository/AbstractPersonRepositoryIntegrationTests.java

@@ -61,7 +61,9 @@ import org.springframework.data.mongodb.core.MongoOperations;
 import org.springframework.data.mongodb.core.aggregation.AggregationResults;
 import org.springframework.data.mongodb.core.geo.GeoJsonPoint;
 import org.springframework.data.mongodb.core.query.BasicQuery;
+import org.springframework.data.mongodb.core.query.Criteria;
 import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.data.mongodb.core.query.Update;
 import org.springframework.data.mongodb.repository.Person.Sex;
 import org.springframework.data.mongodb.repository.SampleEvaluationContextExtension.SampleSecurityContextHolder;
 import org.springframework.data.mongodb.test.util.EnableIfMongoServerVersion;
@@ -1435,6 +1437,15 @@ public abstract class AbstractPersonRepositoryIntegrationTests {
 		assertThat(target.getFirstname()).isEqualTo(alicia.getFirstname().toUpperCase());
 	}
 
+	@Test // GH-3633
+	void annotatedQueryWithNullEqualityCheckShouldWork() {
+
+		operations.updateFirst(Query.query(Criteria.where("id").is(dave.getId())), Update.update("age", null), Person.class);
+
+		Person byQueryWithNullEqualityCheck = repository.findByQueryWithNullEqualityCheck();
+		assertThat(byQueryWithNullEqualityCheck.getId()).isEqualTo(dave.getId());
+	}
+
 	@Test // GH-3602
 	void executesQueryWithDocumentReferenceCorrectly() {
 

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/repository/PersonRepository.java

@@ -417,5 +417,8 @@ public interface PersonRepository extends MongoRepository<Person, String>, Query
 
 	List<Person> findByUnwrappedUser(User user);
 
+	@Query("{ 'age' : null }")
+	Person findByQueryWithNullEqualityCheck();
+
 	List<Person> findBySpiritAnimal(User user);
 }

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/util/json/ParameterBindingJsonReaderUnitTests.java

@@ -383,6 +383,13 @@ class ParameterBindingJsonReaderUnitTests {
 				.parse("{ 'stores.location' : { $geoWithin: { $centerSphere: [ [ 1.948516, 48.799029 ] , 0.004 ] } } }"));
 	}
 
+	@Test // GH-3633
+	void parsesNullValue() {
+
+		Document target = parse("{ 'parent' : null }");
+		assertThat(target).isEqualTo(new Document("parent", null));
+	}
+
 	private static Document parse(String json, Object... args) {
 
 		ParameterBindingJsonReader reader = new ParameterBindingJsonReader(json, args);