I'm writing a Spring Boot app with Spring Data JPA and Kotlin, and I've noticed that in CrudRepository there is the following method:
Optional<T> findById(ID id);
不过，我使用的是Kotlin，它处理空值的方式比Optional更流畅.有人知道我该如何把这个方法转换成这样吗？
fun findById(id: ID): T?
When I extend Repository itself and create a repo with that signature I get the error:
java.lang.ClassCastException: java.util.Optional cannot be cast to com.books.Book
As of Spring Data Lovelace SR4 / Spring Boot 2.1.2, a CrudRepository.findByIdOrNull(id: ID): T? = findById(id).orElse(null) Kotlin extension now provides out of the box a way to retrieve nullable entities in Spring Data.
If for performance reasons you would like to avoid the usage of Optional<T> wrapper, be aware that you have also the possibility to create a custom interface with a findFooById(id: ID): T? function. Query execution is store specific, but and most are using internally nullable values and will avoid the cost of Optional<T> wrapper. Notice this overhead should be negligible for most use cases, so using the builtin extension is recommended method.
See DATACMNS-1346 for more details.