Spring Boot supports Kotlin 1.2.x. To use Kotlin, org.jetbrains.kotlin:kotlin-stdlib and org.jetbrains.kotlin:kotlin-reflect must be present on the classpath. The kotlin-stdlib variants kotlin-stdlib-jdk7 and kotlin-stdlib-jdk8 can also be used.

Since Kotlin classes are final by default, you are likely to want to configure kotlin-spring plugin in order to automatically open Spring-annotated classes so that they can be proxied.

Jackson’s Kotlin module is required for serializing / deserializing JSON data in Kotlin. It is automatically registered when found on the classpath. A warning message is logged if Jackson and Kotlin are present but the Jackson Kotlin module is not.

	Tip
	These dependencies and plugins are provided by default if one bootstraps a Kotlin project on start.spring.io.

One of Kotlin’s key features is null-safety. It deals with null values at compile time rather than deferring the problem to runtime and encountering a NullPointerException. This helps to eliminate a common source of bugs without paying the cost of wrappers like Optional. Kotlin also allows using functional constructs with nullable values as described in this comprehensive guide to null-safety in Kotlin.

Although Java does not allow one to express null-safety in its type system, Spring Framework, Spring Data, and Reactor now provide null-safety of their API via tooling-friendly annotations. By default, types from Java APIs used in Kotlin are recognized as platform types for which null-checks are relaxed. Kotlin’s support for JSR 305 annotations combined with nullability annotations provide null-safety for the related Spring API in Kotlin.

The JSR 305 checks can be configured by adding the -Xjsr305 compiler flag with the following options: -Xjsr305={strict|warn|ignore}. The default behavior is the same as -Xjsr305=warn. The strict value is required to have null-safety taken in account in Kotlin types inferred from Spring API but should be used with the knowledge that Spring API nullability declaration could evolve even between minor releases and more checks may be added in the future).

	Warning
	Generic type arguments, varargs and array elements nullability are not yet supported. See SPR-15942 for up-to-date information. Also be aware that Spring Boot’s own API is not yet annotated.

In order to avoid mixing different version of Kotlin dependencies on the classpath, dependency management of the following Kotlin dependencies is provided:

With Maven, the Kotlin version can be customized via the kotlin.version property and plugin management is provided for kotlin-maven-plugin. With Gradle, the Spring Boot plugin automatically aligns the kotlin.version with the version of the Kotlin plugin.

@ConfigurationProperties currently only works with lateinit or nullable var properties (the former is recommended), since immutable classes initialized by constructors are not yet supported.

@ConfigurationProperties("example.kotlin")
class KotlinExampleProperties {

	lateinit var name: String

	lateinit var description: String

	val myService = MyService()

	class MyService {

		lateinit var apiToken: String

		lateinit var uri: URI

	}

}

	Tip
	To generate your own metadata using the annotation processor, kapt should be configured with the spring-boot-configuration-processor dependency.

While it is possible to use JUnit 4 (the default provided by spring-boot-starter-test) to test Kotlin code, JUnit 5 is recommended. JUnit 5 enables a test class to be instantiated once and reused for all of the class’s tests. This makes it possible to use @BeforeAll and @AfterAll annotations on non-static methods, which is a good fit for Kotlin.

To use JUnit 5, exclude junit:junit dependency from spring-boot-starter-test, add JUnit 5 dependencies, and configure the Maven or Gradle plugin accordingly. See the JUnit 5 documentation for more details. You also need to switch test instance lifecycle to "per-class".

To mock Kotlin classes, MockK is recommended. If you need the Mockk equivalent of the Mockito specific @MockBean and @SpyBean annotations, you can use SpringMockK which provides similar @MockkBean and @SpykBean annotations.