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.