Kotlin is a statically-typed language targeting the JVM (and other
platforms) which allows writing concise and elegant code while providing
interoperability with existing libraries written
in Java.
Spring Boot provides Kotlin support by leveraging the support in other Spring projects
such as Spring Framework, Spring Data, and Reactor. See the
Spring Framework Kotlin support documentation
for more information.
The easiest way to start with Spring Boot and Kotlin is to follow
this comprehensive tutorial. You
can create new Kotlin projects via
start.spring.io. Feel free to join the #spring
channel of Kotlin Slack or ask a question with the spring
and kotlin
tags on Stack
Overflow if you need support.
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]](images/tip.png) | Tip |
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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]](images/warning.png) | Warning |
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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. |
Spring Boot provides an idiomatic way to run an application with
runApplication<MyApplication>(*args)
as shown in the following example:
import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication
@SpringBootApplication
class MyApplication
fun main(args: Array<String>) {
runApplication<MyApplication>(*args)
}
This is a drop-in replacement for
SpringApplication.run(MyApplication::class.java, *args)
. It also allows customization
of the application as shown in the following example:
runApplication<MyApplication>(*args) {
setBannerMode(OFF)
}
Kotlin extensions provide the ability
to extend existing classes with additional functionality. The Spring Boot Kotlin API makes
use of these extensions to add new Kotlin specific conveniences to existing APIs.
TestRestTemplate
extensions, similar to those provided by Spring Framework for
RestOperations
in Spring Framework, are provided. Among other things, the extensions
make it possible to take advantage of Kotlin reified type parameters.
47.4 Dependency management
In order to avoid mixing different version of Kotlin dependencies on the classpath,
dependency management of the following Kotlin dependencies is provided:
kotlin-reflect
kotlin-runtime
kotlin-stdlib
kotlin-stdlib-jdk7
kotlin-stdlib-jdk8
kotlin-stdlib-jre7
kotlin-stdlib-jre8
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.
47.5 @ConfigurationProperties
@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
}
}
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".