This version is still in development and is not considered stable yet. For the latest stable version, please use Spring Framework 6.1.14!

Annotations

The Spring Framework also takes advantage of Kotlin null-safety to determine if an HTTP parameter is required without having to explicitly define the required attribute. That means @RequestParam name: String? is treated as not required and, conversely, @RequestParam name: String is treated as being required. This feature is also supported on the Spring Messaging @Header annotation.

In a similar fashion, Spring bean injection with @Autowired, @Bean, or @Inject uses this information to determine if a bean is required or not.

For example, @Autowired lateinit var thing: Thing implies that a bean of type Thing must be registered in the application context, while @Autowired lateinit var thing: Thing? does not raise an error if such a bean does not exist.

Following the same principle, @Bean fun play(toy: Toy, car: Car?) = Baz(toy, Car) implies that a bean of type Toy must be registered in the application context, while a bean of type Car may or may not exist. The same behavior applies to autowired constructor parameters.

If you use bean validation on classes with properties or a primary constructor parameters, you may need to use annotation use-site targets, such as @field:NotNull or @get:Size(min=5, max=15), as described in this Stack Overflow response.