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Spring Projects in Kotlin

This section provides some specific hints and recommendations worth for developing Spring projects in Kotlin.

Final by Default

By default, all classes and member functions 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.

It is also possible to avoid CGLIB proxies for configuration classes by using @Configuration(proxyBeanMethods = false). See 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.

Some use cases involving proxies and automatic generation of final methods by the Kotlin compiler require extra care. For example, a Kotlin class with properties will generate related final getters and setters. In order to be able to proxy related methods, a type level @Component annotation should be preferred to method level @Bean in order to have those methods opened by the kotlin-spring plugin. A typical use case is @Scope and its popular @RequestScope specialization.

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.

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.

Using Immutable Class Instances for Persistence

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() and hashCode()

  • 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.

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).

Injecting Dependencies

Favor constructor injection

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
)
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
}

Internal functions name mangling

Kotlin functions with the internal visibility modifier have their names mangled when compiled to JVM bytecode, which has a side effect when injecting dependencies by name.

For example, this Kotlin class:

@Configuration
class SampleConfiguration {

	@Bean
	internal fun sampleBean() = SampleBean()
}

Translates to this Java representation of the compiled JVM bytecode:

@Configuration
@Metadata(/* ... */)
public class SampleConfiguration {

	@Bean
	@NotNull
	public SampleBean sampleBean$demo_kotlin_internal_test() {
		return new SampleBean();
	}
}

As a consequence, the related bean name represented as a Kotlin string is "sampleBean\$demo_kotlin_internal_test", instead of "sampleBean" for the regular public function use-case. Make sure to use the mangled name when injecting such bean by name, or add @JvmName("sampleBean") to disable name mangling.

Injecting Configuration Properties

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}").

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()

Checked Exceptions

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)).

Annotation Array Attributes

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.

If the @RequestMapping method attribute is not specified, all HTTP methods will be matched, not only the GET method.

Declaration-site variance

Dealing with generic types in Spring applications written in Kotlin may require, for some use cases, to understand Kotlin declaration-site variance which allows to define the variance when declaring a type, which is not possible in Java which supports only use-site variance.

For example, declaring List<Foo> in Kotlin is conceptually equivalent to java.util.List<? extends Foo> because kotlin.collections.List is declared as interface List<out E> : kotlin.collections.Collection<E>.

This needs to be taken into account by using the out Kotlin keyword on generic types when using Java classes, for example when writing a org.springframework.core.convert.converter.Converter from a Kotlin type to a Java type.

class ListOfFooConverter : Converter<List<Foo>, CustomJavaList<out Foo>> {
    // ...
}

When converting any kind of objects, star projection with * can be used instead of out Any.

class ListOfAnyConverter : Converter<List<*>, CustomJavaList<*>> {
    // ...
}
Spring Framework does not leverage yet declaration-site variance type information for injecting beans, subscribe to spring-framework#22313 to track related progresses.

Testing

This section addresses testing with the combination of Kotlin and Spring Framework. The recommended testing framework is JUnit 5 along with Mockk for mocking.

If you are using Spring Boot, see this related documentation.

Constructor injection

As described in the dedicated section, JUnit Jupiter (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 @TestConstructor(autowireMode = AutowireMode.ALL) to enable autowiring for all parameters.

You can also change the default behavior to ALL in a junit-platform.properties file with a spring.test.constructor.autowire.mode = all property. 
@SpringJUnitConfig(TestConfig::class)
@TestConstructor(autowireMode = AutowireMode.ALL)
class OrderServiceIntegrationTests(val orderService: OrderService,
                                   val customerService: CustomerService) {

    // tests that use the injected OrderService and CustomerService
}

PER_CLASS Lifecycle

Kotlin lets you specify meaningful test function names between backticks (`). With JUnit Jupiter (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 in 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()
  }
}

Specification-like Tests

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)
     }
  }
}