Spring-Data JPA CrudRepository returns Iterable, is it OK to cast this to List?

Question

I'm writing a code-gen tool to generate backend wiring code for Spring Boot applications using Spring Data JPA and it's mildly annoying me that the methods in the CrudRepository return Iterable rather than List, as Iterable doesn't provide quite enough functionality, but List does, so I'm looking for the best way to convert the Iterable into a List.

I saw this post on changing an Iterable to a Collection and I was wondering, rather than using a library like Guava or implementing my own function to do the conversion, why not just cast it to List? Is there something wrong with doing that that I don't know about?

Edit: I ask because since it's a code-gen tool it's not reasonable to make it generate code that introduces dependencies on 3rd party libraries, and writing my own function to do the conversion also isn't really reasonable because it would have to live somewhere and I'd rather not have that in the generated code. A simple cast will work, if a little ugly, but just wondered if there's something I'm missing?

Answer 1

You mentioned [spring-data-jpa] so i guess you use JPA. In this case use JpaRepository instead of CrudRepository where the methods return List's like you want it.

Answer 2

No, I don't think it's OK.

While a List is guaranteed to be an Iterable an Iterable may not be a List. This means that if you do cast an Iterable to a List it may fail at runtime. Even if it works, there's no guarantee that it will continue to work in the future as it could change in new versions of Spring Data JPA without breaking the interface's contract.

Instead of using a cast, you should declare your own query methods that return List.

Alternatively you may use Streamable.of(iterable).toList() for doing the conversion. This answer also contains some background why Iterable was chosen as the return type for these methods.

Answer 3

Your interface can still extends the CrudRepository, you can just add a new method findAll returning a list. Like example below:

@Repository
public interface DataRepository extends CrudRepository<Data, Long> {

    @Override
    List<Data> findAll();

}

If you have an "abstract" repository to be extended by all your repositories, you can add this method too, so it will has effect to all your repositories. Like example below:

@NoRepositoryBean
public interface GenericRepository<T> extends CrudRepository<T, Long> {

    @Override
    List<T> findAll();

}

Answer 4

Starting from Spring Data 3 there is another option: ListCrudRepository.

This blog post covers why CrudRepository chose Iterable instead of List.

https://spring.io/blog/2022/02/22/announcing-listcrudrepository-friends-for-spring-data-3-0

Answer 5

Hi I know I am late to the party here but if you want to convert an Iterable to a Collection you can do it this way.

private <T> Collection<T> map(final Iterable<T> iterable) {
        return StreamSupport.stream(iterable.spliterator(), false)
                            .collect(Collectors.toCollection(HashSet::new));
    }

After that, du can get a List by .stream().toList(); on the Collection. Hope it helps

Answer 6

Simply, you can make your repository to extend JpaRepository not CrudRepository. This will work perfectly. Here findAll returns a List not an Iterable.