spring data findBy return type

Question

What are the possible return types of spring data findBy methods? I haven't found any consolidated list anywhere.

like findByXXX can return

1) Entity
2) Optional<Entity>
3) List<Entity>
4) Stream<Entity>

and many more, but do we have a definite list?

Also, List<Entity> returns emptyList when no data found but Stream<Entity> returns null when no data found, so when you apply map/filter etc on stream, you get NPE.

Is there a better way to use Stream without causing NPE, other than physically checking null before calling stream functions.

Answer 1

Your list of possible return type is corret. You can use too the type Page and Slice when you need pagination the result. See example:

Page<User> findByLastname(String lastname, Pageable pageable);

Slice<User> findByLastname(String lastname, Pageable pageable);

"The first method allows you to pass an org.springframework.data.domain.Pageable instance to the query method to dynamically add paging to your statically defined query. A Page knows about the total number of elements and pages available. It does so by the infrastructure triggering a count query to calculate the overall number. As this might be expensive depending on the store used, Slice can be used as return instead. A Slice only knows about whether there’s a next Slice available which might be just sufficient when walking thought a larger result set." - See more in: Spring Data Documentation

For another question, you can use Guava/Java 8 Optional. This way, your query method will return an Optional that contains the found object or an empty Optional. Optional is a way of replacing a nullable T reference with a non-null value, see example:

Optional<User> findById(Long id);

I hope I have help you. =)

Answer 2

Though a bit late, but the supported return types are documented here:

https://docs.spring.io/spring-data/jpa/docs/current/reference/html/#repository-query-return-types

Generally, the return types could be any of the following categories:

1. a (Optional) single item, could be a primitive, wrapped, or object, or void
2. a collection of items, or paged list
3. a stream, Iterator, Mono/Flux, or those similar.
4. Future, or its variants

And, the complete list of currently supported return types:

• void
• Primitives
• Wrapper types
• T
• Iterator
• Collection
• List
• Optional
• Option
• Stream
• Streamable
• Types that implement Streamable and take a Streamable constructor or factory method argument
• Vavr Seq, List, Map, Set
• Future
• CompletableFuture
• ListenableFuture
• Slice
• Page
• GeoResult
• GeoResults
• GeoPage
• Mono
• Flux
• Single
• Maybe
• Flowable