DATAMONGO-1525 - Improved creation of empty collections, esp. EnumSet.

We now use more type information to create a better empty collection in the first place. The previous algorithm always used an empty HashSet plus a subsequent conversion using the raw collection type. Especially the latter caused problems for EnumSets as the conversion into one requires the presence of component type information.

We now use Spring's collection factory and more available type information to create a proper collection in the first place and only rely on a subsequent conversion for arrays.

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

@@ -886,10 +886,6 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 
 		Class<?> collectionType = targetType.getType();
 
-		if (sourceValue.isEmpty()) {
-			return getPotentiallyConvertedSimpleRead(new HashSet<Object>(), collectionType);
-		}
-
 		TypeInformation<?> componentType = targetType.getComponentType();
 		Class<?> rawComponentType = componentType == null ? null : componentType.getType();
 
@@ -897,6 +893,10 @@ public class MappingMongoConverter extends AbstractMongoConverter implements App
 		Collection<Object> items = targetType.getType().isArray() ? new ArrayList<Object>()
 				: CollectionFactory.createCollection(collectionType, rawComponentType, sourceValue.size());
 
+		if (sourceValue.isEmpty()) {
+			return getPotentiallyConvertedSimpleRead(items, collectionType);
+		}
+
 		for (Object dbObjItem : sourceValue) {
 
 			if (dbObjItem instanceof DBRef) {

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

@@ -2086,6 +2086,17 @@ public class MappingMongoConverterUnitTests {
 		assertThat(result.sample, is("value"));
 	}
 
+	/**
+	 * @see DATAMONGO-1525
+	 */
+	@Test
+	public void readsEmptyEnumSet() {
+
+		DBObject source = new BasicDBObject("enumSet", new BasicDBList());
+
+		assertThat(converter.read(ClassWithEnumProperty.class, source).enumSet, is(EnumSet.noneOf(SampleEnum.class)));
+	}
+
 	static class GenericType<T> {
 		T content;
 	}