Kotlin is a statically typed language that targets the JVM (and other platforms) which allows writing concise and elegant code while providing very good interoperability with existing libraries written in Java.
The Spring Framework provides first-class support for Kotlin and lets developers write Kotlin applications almost as if the Spring Framework was a native Kotlin framework.
The easiest way to build a Spring application with Kotlin is to leverage Spring Boot and its dedicated Kotlin support. This comprehensive tutorial will teach you how to build Spring Boot applications with Kotlin using start.spring.io.
As of Spring Framework 5.2, most of the code samples of the reference documentation are provided in Kotlin in addition to Java.
Feel free to join the #spring channel of Kotlin Slack
or ask a question with spring
and kotlin
as tags on
Stackoverflow if you need support.
Spring Framework supports Kotlin 1.3 and requires
kotlin-stdlib
(or one of its variants, such as kotlin-stdlib-jdk8
)
and kotlin-reflect
to be present on the classpath. They are provided by default if you bootstrap a Kotlin project on
start.spring.io.
Kotlin extensions provide the ability to extend existing classes with additional functionality. The Spring Framework Kotlin APIs use these extensions to add new Kotlin-specific conveniences to existing Spring APIs.
The {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/[Spring Framework KDoc API] lists and documents all available the Kotlin extensions and DSLs.
Note
|
Keep in mind that Kotlin extensions need to be imported to be used. This means,
for example, that the GenericApplicationContext.registerBean Kotlin extension
is available only if org.springframework.context.support.registerBean is imported.
That said, 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 the Spring Framework provides some extensions to take advantage of this feature.
This allows for a better Kotlin API RestTemplate
, for the new WebClient
from Spring
WebFlux, and for various other APIs.
Note
|
Other libraries, such as Reactor and Spring Data, also provide Kotlin extensions for their APIs, thus giving a better Kotlin development experience overall. |
To retrieve a list of User
objects in Java, you would normally write the following:
Flux<User> users = client.get().retrieve().bodyToFlux(User.class)
With Kotlin and the Spring Framework extensions, you can instead write the following:
val users = client.get().retrieve().bodyToFlux<User>()
// or (both are equivalent)
val users : Flux<User> = client.get().retrieve().bodyToFlux()
As in Java, users
in Kotlin is strongly typed, but Kotlin’s clever type inference allows
for shorter syntax.
One of Kotlin’s key features is null-safety,
which cleanly deals with null
values at compile time rather than bumping into the famous
NullPointerException
at runtime. This makes applications safer through nullability
declarations and expressing “value or no value” semantics without paying the cost of wrappers, such as Optional
.
(Kotlin allows using functional constructs with nullable values. See this
comprehensive guide to Kotlin null-safety.)
Although Java does not let you express null-safety in its type-system, the Spring Framework
provides null-safety of the whole Spring Framework API
via tooling-friendly annotations declared in the org.springframework.lang
package.
By default, types from Java APIs used in Kotlin are recognized as
platform types,
for which null-checks are relaxed.
Kotlin support for JSR-305 annotations
and Spring nullability annotations provide null-safety for the whole Spring Framework API to Kotlin developers,
with the advantage of dealing with null
-related issues at compile time.
Note
|
Libraries such as Reactor or Spring Data provide null-safe APIs to leverage this feature. |
You can configure JSR-305 checks by adding the -Xjsr305
compiler flag with the following
options: -Xjsr305={strict|warn|ignore}
.
For kotlin versions 1.1+, the default behavior is the same as -Xjsr305=warn
.
The strict
value is required to have Spring Framework API null-safety taken into 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 that more checks may
be added in the future.
Note
|
Generic type arguments, varargs, and array elements nullability are not supported yet, but should be in an upcoming release. See this discussion for up-to-date information. |
The Spring Framework supports various Kotlin constructs, such as instantiating Kotlin classes through primary constructors, immutable classes data binding, and function optional parameters with default values.
Kotlin parameter names are recognized through a dedicated KotlinReflectionParameterNameDiscoverer
,
which allows finding interface method parameter names without requiring the Java 8 -parameters
compiler flag to be enabled during compilation.
The Jackson Kotlin module, which is required for serializing or deserializing JSON data, is automatically registered when found in the classpath, and a warning message is logged if Jackson and Kotlin are detected without the Jackson Kotlin module being present.
You can declare configuration classes as top level or nested but not inner, since the later requires a reference to the outer class.
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.
Note
|
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.
|
Spring Framework supports registering beans in a functional way by using lambdas
as an alternative to XML or Java configuration (@Configuration
and @Bean
). In a nutshell,
it lets you register beans with a lambda that acts as a FactoryBean
.
This mechanism is very efficient, as it does not require any reflection or CGLIB proxies.
In Java, you can, for example, write the following:
class Foo {}
class Bar {
private final Foo foo;
public Bar(Foo foo) {
this.foo = foo;
}
}
GenericApplicationContext context = new GenericApplicationContext();
context.registerBean(Foo.class);
context.registerBean(Bar.class, () -> new Bar(context.getBean(Foo.class)));
In Kotlin, with reified type parameters and GenericApplicationContext
Kotlin extensions,
you can instead write the following:
class Foo
class Bar(private val foo: Foo)
val context = GenericApplicationContext().apply {
registerBean<Foo>()
registerBean { Bar(it.getBean()) }
}
When the class Bar
has a single constructor, you can even just specify the bean class,
the constructor parameters will be autowired by type:
val context = GenericApplicationContext().apply {
registerBean<Foo>()
registerBean<Bar>()
}
In order to allow a more declarative approach and cleaner syntax, Spring Framework provides
a {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.context.support/-bean-definition-dsl/[Kotlin bean definition DSL]
It declares an ApplicationContextInitializer
through a clean declarative API,
which lets you deal with profiles and Environment
for customizing
how beans are registered.
In the following example notice that:
-
Type inference usually allows to avoid specifying the type for bean references like
ref("bazBean")
-
It is possible to use Kotlin top level functions to declare beans using callable references like
bean(::myRouter)
in this example -
When specifying
bean<Bar>()
orbean(::myRouter)
, parameters are autowired by type -
The
FooBar
bean will be registered only if thefoobar
profile is active
class Foo
class Bar(private val foo: Foo)
class Baz(var message: String = "")
class FooBar(private val baz: Baz)
val myBeans = beans {
bean<Foo>()
bean<Bar>()
bean("bazBean") {
Baz().apply {
message = "Hello world"
}
}
profile("foobar") {
bean { FooBar(ref("bazBean")) }
}
bean(::myRouter)
}
fun myRouter(foo: Foo, bar: Bar, baz: Baz) = router {
// ...
}
Note
|
This DSL is programmatic, meaning it allows custom registration logic of beans
through an if expression, a for loop, or any other Kotlin constructs.
|
You can then use this beans()
function to register beans on the application context,
as the following example shows:
val context = GenericApplicationContext().apply {
myBeans.initialize(this)
refresh()
}
Note
|
Spring Boot is based on JavaConfig and
does not yet provide specific support for functional bean definition,
but you can experimentally use functional bean definitions through Spring Boot’s ApplicationContextInitializer support.
See this Stack Overflow answer
for more details and up-to-date information. See also the experimental Kofu DSL developed in Spring Fu incubator.
|
Spring Framework comes with a Kotlin router DSL available in 3 flavors:
-
WebMvc.fn DSL with {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.web.servlet.function/router.html[router { }]
-
WebFlux.fn Reactive DSL with {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.web.reactive.function.server/router.html[router { }]
-
WebFlux.fn Coroutines DSL with {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.web.reactive.function.server/co-router.html[coRouter { }]
These DSL let you write clean and idiomatic Kotlin code to build a RouterFunction
instance as the following example shows:
@Configuration
class RouterRouterConfiguration {
@Bean
fun mainRouter(userHandler: UserHandler) = router {
accept(TEXT_HTML).nest {
GET("/") { ok().render("index") }
GET("/sse") { ok().render("sse") }
GET("/users", userHandler::findAllView)
}
"/api".nest {
accept(APPLICATION_JSON).nest {
GET("/users", userHandler::findAll)
}
accept(TEXT_EVENT_STREAM).nest {
GET("/users", userHandler::stream)
}
}
resources("/**", ClassPathResource("static/"))
}
}
Note
|
This DSL is programmatic, meaning that it allows custom registration logic of beans
through an if expression, a for loop, or any other Kotlin constructs. That can be useful
when you need to register routes depending on dynamic data (for example, from a database).
|
See MiXiT project for a concrete example.
A Kotlin DSL is provided via MockMvc
Kotlin extensions in order to provide a more
idiomatic Kotlin API and to allow better discoverability (no usage of static methods).
val mockMvc: MockMvc = ...
mockMvc.get("/person/{name}", "Lee") {
secure = true
accept = APPLICATION_JSON
headers {
contentLanguage = Locale.FRANCE
}
principal = Principal { "foo" }
}.andExpect {
status { isOk }
content { contentType(APPLICATION_JSON) }
jsonPath("$.name") { value("Lee") }
content { json("""{"someBoolean": false}""", false) }
}.andDo {
print()
}
Spring Framework provides a
ScriptTemplateView
which supports JSR-223 to render templates by using script engines.
By leveraging kotlin-script-runtime
and scripting-jsr223-embeddable
dependencies, it
is possible to use such feature to render Kotlin-based templates with
kotlinx.html DSL or Kotlin multiline interpolated String
.
build.gradle.kts
dependencies {
compile("org.jetbrains.kotlin:kotlin-script-runtime:${kotlinVersion}")
runtime("org.jetbrains.kotlin:kotlin-scripting-jsr223-embeddable:${kotlinVersion}")
}
Configuration is usually done with ScriptTemplateConfigurer
and ScriptTemplateViewResolver
beans.
KotlinScriptConfiguration.kt
@Configuration
class KotlinScriptConfiguration {
@Bean
fun kotlinScriptConfigurer() = ScriptTemplateConfigurer().apply {
engineName = "kotlin"
setScripts("scripts/render.kts")
renderFunction = "render"
isSharedEngine = false
}
@Bean
fun kotlinScriptViewResolver() = ScriptTemplateViewResolver().apply {
setPrefix("templates/")
setSuffix(".kts")
}
}
See the kotlin-script-templating example project for more details.
As of Spring Framework 5.3, Kotlin multiplatform serialization is supported in Spring MVC. The builtin support currently only targets JSON format.
To enable it, follow those instructions and make sure neither Jackson, GSON or JSONB are in the classpath.
Note
|
For a typical Spring Boot web application, that can be achieved by excluding spring-boot-starter-json dependency.
|
If you need Jackson, GSON or JSONB for other purposes, you can keep them on the classpath and
configure message converters to remove MappingJackson2HttpMessageConverter
and add
KotlinSerializationJsonHttpMessageConverter
.
Kotlin Coroutines are Kotlin
lightweight threads allowing to write non-blocking code in an imperative way. On language side,
suspending functions provides an abstraction for asynchronous operations while on library side
kotlinx.coroutines provides functions like
async { }
and types like Flow
.
Spring Framework provides support for Coroutines on the following scope:
-
Deferred and Flow return values support in Spring WebFlux annotated
@Controller
-
Suspending function support in Spring WebFlux annotated
@Controller
-
Extensions for WebFlux {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.web.reactive.function.client/index.html[client] and {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.web.reactive.function.server/index.html[server] functional API.
-
WebFlux.fn {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.web.reactive.function.server/co-router.html[coRouter { }] DSL
-
Suspending function and
Flow
support in RSocket@MessageMapping
annotated methods -
Extensions for {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.messaging.rsocket/index.html[
RSocketRequester
]
Coroutines support is enabled when kotlinx-coroutines-core
and kotlinx-coroutines-reactor
dependencies are in the classpath:
build.gradle.kts
dependencies {
implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:${coroutinesVersion}")
implementation("org.jetbrains.kotlinx:kotlinx-coroutines-reactor:${coroutinesVersion}")
}
Version 1.3.0
and above are supported.
For return values, the translation from Reactive to Coroutines APIs is the following:
-
fun handler(): Mono<Void>
becomessuspend fun handler()
-
fun handler(): Mono<T>
becomessuspend fun handler(): T
orsuspend fun handler(): T?
depending on if theMono
can be empty or not (with the advantage of being more statically typed) -
fun handler(): Flux<T>
becomesfun handler(): Flow<T>
For input parameters:
-
If laziness is not needed,
fun handler(mono: Mono<T>)
becomesfun handler(value: T)
since a suspending functions can be invoked to get the value parameter. -
If laziness is needed,
fun handler(mono: Mono<T>)
becomesfun handler(supplier: suspend () → T)
orfun handler(supplier: suspend () → T?)
Flow
is Flux
equivalent in Coroutines world, suitable for hot or cold stream, finite or infinite streams, with the following main differences:
-
Flow
is push-based whileFlux
is push-pull hybrid -
Backpressure is implemented via suspending functions
-
Flow
has only a single suspendingcollect
method and operators are implemented as extensions -
Operators are easy to implement thanks to Coroutines
-
Extensions allow to add custom operators to
Flow
-
Collect operations are suspending functions
-
map
operator supports asynchronous operation (no need forflatMap
) since it takes a suspending function parameter
Read this blog post about Going Reactive with Spring, Coroutines and Kotlin Flow for more details, including how to run code concurrently with Coroutines.
Here is an example of a Coroutines @RestController
.
@RestController
class CoroutinesRestController(client: WebClient, banner: Banner) {
@GetMapping("/suspend")
suspend fun suspendingEndpoint(): Banner {
delay(10)
return banner
}
@GetMapping("/flow")
fun flowEndpoint() = flow {
delay(10)
emit(banner)
delay(10)
emit(banner)
}
@GetMapping("/deferred")
fun deferredEndpoint() = GlobalScope.async {
delay(10)
banner
}
@GetMapping("/sequential")
suspend fun sequential(): List<Banner> {
val banner1 = client
.get()
.uri("/suspend")
.accept(MediaType.APPLICATION_JSON)
.awaitExchange()
.awaitBody<Banner>()
val banner2 = client
.get()
.uri("/suspend")
.accept(MediaType.APPLICATION_JSON)
.awaitExchange()
.awaitBody<Banner>()
return listOf(banner1, banner2)
}
@GetMapping("/parallel")
suspend fun parallel(): List<Banner> = coroutineScope {
val deferredBanner1: Deferred<Banner> = async {
client
.get()
.uri("/suspend")
.accept(MediaType.APPLICATION_JSON)
.awaitExchange()
.awaitBody<Banner>()
}
val deferredBanner2: Deferred<Banner> = async {
client
.get()
.uri("/suspend")
.accept(MediaType.APPLICATION_JSON)
.awaitExchange()
.awaitBody<Banner>()
}
listOf(deferredBanner1.await(), deferredBanner2.await())
}
@GetMapping("/error")
suspend fun error() {
throw IllegalStateException()
}
@GetMapping("/cancel")
suspend fun cancel() {
throw CancellationException()
}
}
View rendering with a @Controller
is also supported.
@Controller
class CoroutinesViewController(banner: Banner) {
@GetMapping("/")
suspend fun render(model: Model): String {
delay(10)
model["banner"] = banner
return "index"
}
}
Here is an example of Coroutines router defined via the {doc-root}/spring-framework/docs/{spring-version}/kdoc-api/spring-framework/org.springframework.web.reactive.function.server/co-router.html[coRouter { }] DSL and related handlers.
@Configuration
class RouterConfiguration {
@Bean
fun mainRouter(userHandler: UserHandler) = coRouter {
GET("/", userHandler::listView)
GET("/api/user", userHandler::listApi)
}
}
class UserHandler(builder: WebClient.Builder) {
private val client = builder.baseUrl("...").build()
suspend fun listView(request: ServerRequest): ServerResponse =
ServerResponse.ok().renderAndAwait("users", mapOf("users" to
client.get().uri("...").awaitExchange().awaitBody<User>()))
suspend fun listApi(request: ServerRequest): ServerResponse =
ServerResponse.ok().contentType(MediaType.APPLICATION_JSON).bodyAndAwait(
client.get().uri("...").awaitExchange().awaitBody<User>())
}
Transactions on Coroutines are supported via the programmatic variant of the Reactive transaction management provided as of Spring Framework 5.2.
For suspending functions, a TransactionalOperator.executeAndAwait
extension is provided.
import org.springframework.transaction.reactive.executeAndAwait
class PersonRepository(private val operator: TransactionalOperator) {
suspend fun initDatabase() = operator.executeAndAwait {
insertPerson1()
insertPerson2()
}
private suspend fun insertPerson1() {
// INSERT SQL statement
}
private suspend fun insertPerson2() {
// INSERT SQL statement
}
}
For Kotlin Flow
, a Flow<T>.transactional
extension is provided.
import org.springframework.transaction.reactive.transactional
class PersonRepository(private val operator: TransactionalOperator) {
fun updatePeople() = findPeople().map(::updatePerson).transactional(operator)
private fun findPeople(): Flow<Person> {
// SELECT SQL statement
}
private suspend fun updatePerson(person: Person): Person {
// UPDATE SQL statement
}
}
This section provides some specific hints and recommendations worth for developing Spring projects in Kotlin.
By default, all classes in Kotlin are final
.
The open
modifier on a class is the opposite of Java’s final
: It allows others to inherit from this
class. This also applies to member functions, in that they need to be marked as open
to be overridden.
While Kotlin’s JVM-friendly design is generally frictionless with Spring, this specific Kotlin feature
can prevent the application from starting, if this fact is not taken into consideration. This is because
Spring beans (such as @Configuration
annotated classes which by default need to be extended at runtime for technical
reasons) are normally proxied by CGLIB. The workaround is to add an open
keyword on each class and
member function of Spring beans that are proxied by CGLIB, which can
quickly become painful and is against the Kotlin principle of keeping code concise and predictable.
Note
|
It is also possible to avoid CGLIB proxies for configuration classes by using @Configuration(proxyBeanMethods = false) .
See {api-spring-framework}/context/annotation/Configuration.html#proxyBeanMethods--[proxyBeanMethods Javadoc] for more details.
|
Fortunately, Kotlin provides a
kotlin-spring
plugin (a preconfigured version of the kotlin-allopen
plugin) that automatically opens classes
and their member functions for types that are annotated or meta-annotated with one of the following
annotations:
-
@Component
-
@Async
-
@Transactional
-
@Cacheable
Meta-annotation support means that types annotated with @Configuration
, @Controller
,
@RestController
, @Service
, or @Repository
are automatically opened since these
annotations are meta-annotated with @Component
.
start.spring.io enables
the kotlin-spring
plugin by default. So, in practice, you can write your Kotlin beans
without any additional open
keyword, as in Java.
Note
|
The Kotlin code samples in Spring Framework documentation do not explicitly specify
open on the classes and their member functions. The samples are written for projects
using the kotlin-allopen plugin, since this is the most commonly used setup.
|
In Kotlin, it is convenient and considered to be a best practice to declare read-only properties within the primary constructor, as in the following example:
class Person(val name: String, val age: Int)
You can optionally add the data
keyword
to make the compiler automatically derive the following members from all properties declared
in the primary constructor:
-
equals()
andhashCode()
-
toString()
of the form"User(name=John, age=42)"
-
componentN()
functions that correspond to the properties in their order of declaration -
copy()
function
As the following example shows, this allows for easy changes to individual properties, even if Person
properties are read-only:
data class Person(val name: String, val age: Int)
val jack = Person(name = "Jack", age = 1)
val olderJack = jack.copy(age = 2)
Common persistence technologies (such as JPA) require a default constructor, preventing this
kind of design. Fortunately, there is a workaround for this
“default constructor hell”,
since Kotlin provides a kotlin-jpa
plugin that generates synthetic no-arg constructor for classes annotated with JPA annotations.
If you need to leverage this kind of mechanism for other persistence technologies, you can configure
the kotlin-noarg
plugin.
Note
|
As of the Kay release train, Spring Data supports Kotlin immutable class instances and
does not require the kotlin-noarg plugin if the module uses Spring Data object mappings
(such as MongoDB, Redis, Cassandra, and others).
|
Our recommendation is to try to favor constructor injection with val
read-only (and
non-nullable when possible) properties,
as the following example shows:
@Component
class YourBean(
private val mongoTemplate: MongoTemplate,
private val solrClient: SolrClient
)
Note
|
Classes with a single constructor have their parameters automatically autowired.
That’s why there is no need for an explicit @Autowired constructor in the example shown
above.
|
If you really need to use field injection, you can use the lateinit var
construct,
as the following example shows:
@Component
class YourBean {
@Autowired
lateinit var mongoTemplate: MongoTemplate
@Autowired
lateinit var solrClient: SolrClient
}
In Java, you can inject configuration properties by using annotations (such as @Value("${property}")
).
However, in Kotlin, $
is a reserved character that is used for
string interpolation.
Therefore, if you wish to use the @Value
annotation in Kotlin, you need to escape the $
character by writing @Value("\${property}")
.
Note
|
If you use Spring Boot, you should probably use
@ConfigurationProperties
instead of @Value annotations.
|
As an alternative, you can customize the property placeholder prefix by declaring the following configuration beans:
@Bean
fun propertyConfigurer() = PropertySourcesPlaceholderConfigurer().apply {
setPlaceholderPrefix("%{")
}
You can customize existing code (such as Spring Boot actuators or @LocalServerPort
)
that uses the ${…}
syntax, with configuration beans, as the following example shows:
@Bean
fun kotlinPropertyConfigurer() = PropertySourcesPlaceholderConfigurer().apply {
setPlaceholderPrefix("%{")
setIgnoreUnresolvablePlaceholders(true)
}
@Bean
fun defaultPropertyConfigurer() = PropertySourcesPlaceholderConfigurer()
Java and Kotlin exception handling
are pretty close, with the main difference being that Kotlin treats all exceptions as
unchecked exceptions. However, when using proxied objects (for example classes or methods
annotated with @Transactional
), checked exceptions thrown will be wrapped by default in
an UndeclaredThrowableException
.
To get the original exception thrown like in Java, methods should be annotated with
@Throws
to specify explicitly the checked exceptions thrown (for example @Throws(IOException::class)
).
Kotlin annotations are mostly similar to Java annotations, but array attributes (which are
extensively used in Spring) behave differently. As explained in the
Kotlin documentation you can omit
the value
attribute name, unlike other attributes, and specify it as a vararg
parameter.
To understand what that means, consider @RequestMapping
(which is one of the most widely
used Spring annotations) as an example. This Java annotation is declared as follows:
public @interface RequestMapping {
@AliasFor("path")
String[] value() default {};
@AliasFor("value")
String[] path() default {};
RequestMethod[] method() default {};
// ...
}
The typical use case for @RequestMapping
is to map a handler method to a specific path
and method. In Java, you can specify a single value for the annotation array attribute,
and it is automatically converted to an array.
That is why one can write
@RequestMapping(value = "/toys", method = RequestMethod.GET)
or
@RequestMapping(path = "/toys", method = RequestMethod.GET)
.
However, in Kotlin, you must write @RequestMapping("/toys", method = [RequestMethod.GET])
or @RequestMapping(path = ["/toys"], method = [RequestMethod.GET])
(square brackets need
to be specified with named array attributes).
An alternative for this specific method
attribute (the most common one) is to
use a shortcut annotation, such as @GetMapping
, @PostMapping
, and others.
Note
|
If the @RequestMapping method attribute is not specified, all HTTP methods will
be matched, not only the GET method.
|
This section addresses testing with the combination of Kotlin and Spring Framework. The recommended testing framework is JUnit 5 along with Mockk for mocking.
Note
|
If you are using Spring Boot, see this related documentation. |
As described in the dedicated section,
JUnit 5 allows constructor injection of beans which is pretty useful with Kotlin
in order to use val
instead of lateinit var
. You can use
{api-spring-framework}/test/context/TestConstructor.html[@TestConstructor(autowireMode = AutowireMode.ALL)
]
to enable autowiring for all parameters.
@SpringJUnitConfig(TestConfig::class)
@TestConstructor(autowireMode = AutowireMode.ALL)
class OrderServiceIntegrationTests(val orderService: OrderService,
val customerService: CustomerService) {
// tests that use the injected OrderService and CustomerService
}
Kotlin lets you specify meaningful test function names between backticks (`
).
As of JUnit 5, Kotlin test classes can use the @TestInstance(TestInstance.Lifecycle.PER_CLASS)
annotation to enable single instantiation of test classes, which allows the use of @BeforeAll
and @AfterAll
annotations on non-static methods, which is a good fit for Kotlin.
You can also change the default behavior to PER_CLASS
thanks to a junit-platform.properties
file with a junit.jupiter.testinstance.lifecycle.default = per_class
property.
The following example demonstrates @BeforeAll
and @AfterAll
annotations on non-static methods:
@TestInstance(TestInstance.Lifecycle.PER_CLASS)
class IntegrationTests {
val application = Application(8181)
val client = WebClient.create("http://localhost:8181")
@BeforeAll
fun beforeAll() {
application.start()
}
@Test
fun `Find all users on HTML page`() {
client.get().uri("/users")
.accept(TEXT_HTML)
.retrieve()
.bodyToMono<String>()
.test()
.expectNextMatches { it.contains("Foo") }
.verifyComplete()
}
@AfterAll
fun afterAll() {
application.stop()
}
}
You can create specification-like tests with JUnit 5 and Kotlin. The following example shows how to do so:
class SpecificationLikeTests {
@Nested
@DisplayName("a calculator")
inner class Calculator {
val calculator = SampleCalculator()
@Test
fun `should return the result of adding the first number to the second number`() {
val sum = calculator.sum(2, 4)
assertEquals(6, sum)
}
@Test
fun `should return the result of subtracting the second number from the first number`() {
val subtract = calculator.subtract(4, 2)
assertEquals(2, subtract)
}
}
}
Due to a type inference issue, you must
use the Kotlin expectBody
extension (such as .expectBody<String>().isEqualTo("toys")
),
since it provides a workaround for the Kotlin issue with the Java API.
See also the related SPR-16057 issue.
The easiest way to learn how to build a Spring application with Kotlin is to follow the dedicated tutorial.
The easiest way to start a new Spring Framework project in Kotlin is to create a new Spring Boot 2 project on start.spring.io.
Spring Framework now comes with two different web stacks: Spring MVC and Spring WebFlux.
Spring WebFlux is recommended if you want to create applications that will deal with latency, long-lived connections, streaming scenarios or if you want to use the web functional Kotlin DSL.
For other use cases, especially if you are using blocking technologies such as JPA, Spring MVC and its annotation-based programming model is the recommended choice.
We recommend the following resources for people learning how to build applications with Kotlin and the Spring Framework:
-
Kotlin Slack (with a dedicated #spring channel)
The following Github projects offer examples that you can learn from and possibly even extend:
-
spring-boot-kotlin-demo: Regular Spring Boot and Spring Data JPA project
-
mixit: Spring Boot 2, WebFlux, and Reactive Spring Data MongoDB
-
spring-kotlin-functional: Standalone WebFlux and functional bean definition DSL
-
spring-kotlin-fullstack: WebFlux Kotlin fullstack example with Kotlin2js for frontend instead of JavaScript or TypeScript
-
spring-petclinic-kotlin: Kotlin version of the Spring PetClinic Sample Application
-
spring-kotlin-deepdive: A step-by-step migration guide for Boot 1.0 and Java to Boot 2.0 and Kotlin
-
spring-cloud-gcp-kotlin-app-sample: Spring Boot with Google Cloud Platform Integrations
The following list categorizes the pending issues related to Spring and Kotlin support:
-
Spring Framework
-
Kotlin