Kotlin extensions provide the ability to extend existing classes with additional functionality. Spring Data Kotlin APIs use these extensions to add new Kotlin-specific conveniences to existing Spring APIs.

Keep in mind that Kotlin extensions need to be imported to be used. Similar to static imports, an IDE should automatically suggest the import in most cases.

For example, Kotlin reified type parameters provide a workaround for JVM generics type erasure, and Spring Data provides some extensions to take advantage of this feature. This allows for a better Kotlin API.

To retrieve a list of SWCharacter objects in Java, you would normally write the following:

Flux<SWCharacter> characters = template.query(SWCharacter.class).inTable("star-wars").all()

With Kotlin and the Spring Data extensions, you can instead write the following:

val characters = template.query<SWCharacter>().inTable("star-wars").all()
// or (both are equivalent)
val characters : Flux<SWCharacter> = template.query().inTable("star-wars").all()

As in Java, characters in Kotlin is strongly typed, but Kotlin’s clever type inference allows for shorter syntax.

Type-safe Queries for Kotlin

Kotlin embraces domain-specific language creation through its language syntax and its extension system. Spring Data MongoDB ships with a Kotlin Extension for Criteria using Kotlin property references to build type-safe queries. Queries using this extension are typically benefit from improved readability. Most keywords on Criteria have a matching Kotlin extension, such as inValues and regex.

Consider the following example explaining Type-safe Queries:

import org.springframework.data.mongodb.core.query.*

  Query(Book::title isEqualTo "Moby-Dick")               (1)

  Query(titlePredicate = Book::title exists true)

      Book::price gt 5,
      Book::price lt 10

// Binary operators
  Query(BinaryMessage::payload bits { allClear(0b101) }) (2)

// Nested Properties (i.e. refer to "book.author")
  Query(Book::author / Author::name regex "^H")          (3)
1 isEqualTo() is an infix extension function with receiver type KProperty<T> that returns Criteria.
2 For bitwise operators, pass a lambda argument where you call one of the methods of Criteria.BitwiseCriteriaOperators.
3 To construct nested properties, use the / character (overloaded operator div).