# Language Support
# 1. Kotlin
Kotlin (opens new window) is a statically typed language that targets the JVM (and other platforms) which allows writing concise and elegant code while providing very good interoperability (opens new window)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. Most of the code samples of the reference documentation are provided in Kotlin in addition to Java.
The easiest way to build a Spring application with Kotlin is to leverage Spring Boot and its dedicated Kotlin support (opens new window).This comprehensive tutorial (opens new window)will teach you how to build Spring Boot applications with Kotlin using start.spring.io (opens new window).
Feel free to join the #spring channel of Kotlin Slack (opens new window)or ask a question with spring
and kotlin
as tags onStackoverflow (opens new window) if you need support.
# 1.1. Requirements
Spring Framework supports Kotlin 1.3+ and requireskotlin-stdlib
(opens new window)(or one of its variants, such as kotlin-stdlib-jdk8
(opens new window))
and kotlin-reflect
(opens new window)to be present on the classpath. They are provided by default if you bootstrap a Kotlin project onstart.spring.io (opens new window).
The Jackson Kotlin module (opens new window) is required for serializing or deserializing JSON data for Kotlin classes with Jackson, so make sure to add the com.fasterxml.jackson.module:jackson-module-kotlin dependency to your project if you have such need.It is automatically registered when found in the classpath. |
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# 1.2. Extensions
Kotlin extensions (opens new window) 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 Spring Framework KDoc API (opens new window) lists and documents all available Kotlin extensions and DSLs.
Keep in mind that Kotlin extensions need to be imported to be used. This means, for example, that the GenericApplicationContext.registerBean Kotlin extensionis 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. |
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For example, Kotlin reified type parameters (opens new window)provide a workaround for JVM generics type erasure (opens new window),
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.
Other libraries, such as Reactor and Spring Data, also provide Kotlin extensions for their APIs, thus giving a better Kotlin development experience overall. |
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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.
# 1.3. Null-safety
One of Kotlin’s key features is null-safety (opens new window),
which cleanly deals with null
values at compile time rather than bumping into the famousNullPointerException
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 thiscomprehensive guide to Kotlin null-safety (opens new window).)
Although Java does not let you express null-safety in its type-system, the Spring Framework
provides null-safety of the whole Spring Framework APIvia tooling-friendly annotations declared in the org.springframework.lang
package.
By default, types from Java APIs used in Kotlin are recognized asplatform types (opens new window),
for which null-checks are relaxed.Kotlin support for JSR-305 annotations (opens new window)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.
Libraries such as Reactor or Spring Data provide null-safe APIs to leverage this feature. |
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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.
Generic type arguments, varargs, and array elements nullability are not supported yet, but should be in an upcoming release. See this discussion (opens new window)for up-to-date information. |
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# 1.4. Classes and Interfaces
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.
You can declare configuration classes astop level or nested but not inner (opens new window), since the later requires a reference to the outer class.
# 1.5. Annotations
The Spring Framework also takes advantage of Kotlin null-safety (opens new window)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.
If you use bean validation on classes with properties or a primary constructor parameters, you may need to useannotation use-site targets (opens new window), such as @field:NotNull or @get:Size(min=5, max=15) , as described inthis Stack Overflow response (opens new window). |
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# 1.6. Bean Definition DSL
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 Kotlin bean definition DSL (opens new window)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 exampleWhen specifying
bean<Bar>()
orbean(::myRouter)
, parameters are autowired by typeThe
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 {
// ...
}
This DSL is programmatic, meaning it allows custom registration logic of beans through an if expression, a for loop, or any other Kotlin constructs. |
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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()
}
Spring Boot is based on JavaConfig anddoes not yet provide specific support for functional bean definition (opens new window), but you can experimentally use functional bean definitions through Spring Boot’s ApplicationContextInitializer support.See this Stack Overflow answer (opens new window)for more details and up-to-date information. See also the experimental Kofu DSL developed in Spring Fu incubator (opens new window). |
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# 1.7. Web
# 1.7.1. Router DSL
Spring Framework comes with a Kotlin router DSL available in 3 flavors:
WebMvc.fn DSL with router { } (opens new window)
WebFlux.fn Reactive DSL with router { } (opens new window)
WebFlux.fn Coroutines DSL with coRouter { } (opens new window)
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/"))
}
}
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 usefulwhen you need to register routes depending on dynamic data (for example, from a database). |
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See MiXiT project (opens new window) for a concrete example.
# 1.7.2. MockMvc DSL
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("$.name") { value("Lee") }
content { json("""{"someBoolean": false}""", false) }
}.andDo {
print()
}
# 1.7.3. Kotlin Script Templates
Spring Framework provides aScriptTemplateView
(opens new window)which supports JSR-223 (opens new window) to render templates by using script engines.
By leveraging scripting-jsr223
dependencies, it
is possible to use such feature to render Kotlin-based templates withkotlinx.html (opens new window) DSL or Kotlin multiline interpolated String
.
build.gradle.kts
dependencies {
runtime("org.jetbrains.kotlin:kotlin-scripting-jsr223:${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 (opens new window) example project for more details.
# 1.7.4. Kotlin multiplatform serialization
As of Spring Framework 5.3, Kotlin multiplatform serialization (opens new window) is supported in Spring MVC, Spring WebFlux and Spring Messaging (RSocket). The builtin support currently only targets JSON format.
To enable it, follow those instructions (opens new window) to add the related dependency and plugin.
With Spring MVC and WebFlux, both Kotlin serialization and Jackson will be configured by default if they are in the classpath since
Kotlin serialization is designed to serialize only Kotlin classes annotated with @Serializable
.
With Spring Messaging (RSocket), make sure that neither Jackson, GSON or JSONB are in the classpath if you want automatic configuration,
if Jackson is needed configure KotlinSerializationJsonMessageConverter
manually.
# 1.8. Coroutines
Kotlin Coroutines (opens new window) 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 sidekotlinx.coroutines (opens new window) provides functions likeasync { }
(opens new window)and types like Flow
(opens new window).
Spring Framework provides support for Coroutines on the following scope:
Deferred (opens new window) and Flow (opens new window) return values support in Spring MVC and WebFlux annotated
@Controller
Suspending function support in Spring MVC and WebFlux annotated
@Controller
Extensions for WebFlux client (opens new window) and server (opens new window) functional API.
WebFlux.fn coRouter { } (opens new window) DSL
Suspending function and
Flow
support in RSocket@MessageMapping
annotated methodsExtensions for
RSocketRequester
(opens new window)
# 1.8.1. Dependencies
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 1.4.0
and above are supported.
# 1.8.2. How Reactive translates to Coroutines?
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
(opens new window) 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 hybridBackpressure is implemented via suspending functions
Flow
has only a single suspendingcollect
method (opens new window) and operators are implemented as extensions (opens new window)Operators are easy to implement (opens new window) thanks to Coroutines
Extensions allow to add custom operators to
Flow
Collect operations are suspending functions
map
operator (opens new window) 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 (opens new window)for more details, including how to run code concurrently with Coroutines.
# 1.8.3. Controllers
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"
}
}
# 1.8.4. WebFlux.fn
Here is an example of Coroutines router defined via the coRouter { } (opens new window) 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>())
}
# 1.8.5. Transactions
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
}
}
# 1.9. Spring Projects in Kotlin
This section provides some specific hints and recommendations worth for developing Spring projects in Kotlin.
# 1.9.1. Final by Default
By default, all classes in Kotlin are final
(opens new window).
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 (opens new window) for more details. |
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Fortunately, Kotlin provides akotlin-spring
(opens new window)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 (opens new window) 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 specifyopen on the classes and their member functions. The samples are written for projectsusing the kotlin-allopen plugin, since this is the most commonly used setup. |
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# 1.9.2. 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 (opens new window)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 declarationcopy()
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” (opens new window),
since Kotlin provides a kotlin-jpa
(opens new window)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
(opens new window)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). |
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# 1.9.3. Injecting Dependencies
Our recommendation is to try to favor constructor injection with val
read-only (and
non-nullable when possible) properties (opens new window),
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 shownabove. |
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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
}
# 1.9.4. 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 forstring interpolation (opens new window).
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 (opens new window)instead of @Value annotations. |
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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()
# 1.9.5. Checked Exceptions
Java and Kotlin exception handling (opens new window)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
(opens new window)to specify explicitly the checked exceptions thrown (for example @Throws(IOException::class)
).
# 1.9.6. 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 theKotlin documentation (opens new window) 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 willbe matched, not only the GET method. |
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# 1.9.7. Testing
This section addresses testing with the combination of Kotlin and Spring Framework. The recommended testing framework is JUnit 5 (opens new window) along withMockk (opens new window) for mocking.
If you are using Spring Boot, seethis related documentation (opens new window). |
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# Constructor injection
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@TestConstructor(autowireMode = AutowireMode.ALL)
(opens new window)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
}
# PER_CLASS
Lifecycle
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()
}
}
# 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)
}
}
}
# WebTestClient
Type Inference Issue in Kotlin
Due to a type inference issue (opens new window), 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 (opens new window) issue.
# 1.10. Getting Started
The easiest way to learn how to build a Spring application with Kotlin is to followthe dedicated tutorial (opens new window).
# 1.10.1. start.spring.io
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 (opens new window).
# 1.10.2. Choosing the Web Flavor
Spring Framework now comes with two different web stacks: Spring MVC andSpring 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.
# 1.11. Resources
We recommend the following resources for people learning how to build applications with Kotlin and the Spring Framework:
Kotlin Slack (opens new window) (with a dedicated #spring channel)
Stackoverflow, with
spring
andkotlin
tags (opens new window)
# 1.11.1. Examples
The following Github projects offer examples that you can learn from and possibly even extend:
spring-boot-kotlin-demo (opens new window): Regular Spring Boot and Spring Data JPA project
mixit (opens new window): Spring Boot 2, WebFlux, and Reactive Spring Data MongoDB
spring-kotlin-functional (opens new window): Standalone WebFlux and functional bean definition DSL
spring-kotlin-fullstack (opens new window): WebFlux Kotlin fullstack example with Kotlin2js for frontend instead of JavaScript or TypeScript
spring-petclinic-kotlin (opens new window): Kotlin version of the Spring PetClinic Sample Application
spring-kotlin-deepdive (opens new window): A step-by-step migration guide for Boot 1.0 and Java to Boot 2.0 and Kotlin
spring-cloud-gcp-kotlin-app-sample (opens new window): Spring Boot with Google Cloud Platform Integrations
# 1.11.2. Issues
The following list categorizes the pending issues related to Spring and Kotlin support:
Spring Framework
Kotlin
Parent issue for Spring Framework support (opens new window)
Kotlin requires type inference where Java doesn’t (opens new window)
Impossible to pass not all SAM argument as function (opens new window)
Support JSR 223 bindings directly via script variables (opens new window)
Kotlin properties do not override Java-style getters and setters (opens new window)
# 2. Apache Groovy
Groovy is a powerful, optionally typed, and dynamic language, with static-typing and static compilation capabilities. It offers a concise syntax and integrates smoothly with any existing Java application.
The Spring Framework provides a dedicated ApplicationContext
that supports a Groovy-based
Bean Definition DSL. For more details, seeThe Groovy Bean Definition DSL.
Further support for Groovy, including beans written in Groovy, refreshable script beans, and more is available in Dynamic Language Support.
# 3. Dynamic Language Support
Spring provides comprehensive support for using classes and objects that have been defined by using a dynamic language (such as Groovy) with Spring. This support lets you write any number of classes in a supported dynamic language and have the Spring container transparently instantiate, configure, and dependency inject the resulting objects.
Spring’s scripting support primarily targets Groovy and BeanShell. Beyond those specifically supported languages, the JSR-223 scripting mechanism is supported for integration with any JSR-223 capable language provider (as of Spring 4.2), e.g. JRuby.
You can find fully working examples of where this dynamic language support can be immediately useful in Scenarios.
# 3.1. A First Example
The bulk of this chapter is concerned with describing the dynamic language support in detail. Before diving into all of the ins and outs of the dynamic language support, we look at a quick example of a bean defined in a dynamic language. The dynamic language for this first bean is Groovy. (The basis of this example was taken from the Spring test suite. If you want to see equivalent examples in any of the other supported languages, take a look at the source code).
The next example shows the Messenger
interface, which the Groovy bean is going to
implement. Note that this interface is defined in plain Java. Dependent objects that
are injected with a reference to the Messenger
do not know that the underlying
implementation is a Groovy script. The following listing shows the Messenger
interface:
package org.springframework.scripting;
public interface Messenger {
String getMessage();
}
The following example defines a class that has a dependency on the Messenger
interface:
package org.springframework.scripting;
public class DefaultBookingService implements BookingService {
private Messenger messenger;
public void setMessenger(Messenger messenger) {
this.messenger = messenger;
}
public void processBooking() {
// use the injected Messenger object...
}
}
The following example implements the Messenger
interface in Groovy:
// from the file 'Messenger.groovy'
package org.springframework.scripting.groovy;
// import the Messenger interface (written in Java) that is to be implemented
import org.springframework.scripting.Messenger
// define the implementation in Groovy
class GroovyMessenger implements Messenger {
String message
}
To use the custom dynamic language tags to define dynamic-language-backed beans, you need to have the XML Schema preamble at the top of your Spring XML configuration file. You also need to use a Spring ApplicationContext implementation as your IoCcontainer. Using the dynamic-language-backed beans with a plain BeanFactory implementation is supported, but you have to manage the plumbing of the Spring internalsto do so. For more information on schema-based configuration, see XML Schema-based Configuration. |
---|
Finally, the following example shows the bean definitions that effect the injection of the
Groovy-defined Messenger
implementation into an instance of theDefaultBookingService
class:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:lang="http://www.springframework.org/schema/lang"
xsi:schemaLocation="
http://www.springframework.org/schema/beans https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/lang https://www.springframework.org/schema/lang/spring-lang.xsd">
<!-- this is the bean definition for the Groovy-backed Messenger implementation -->
<lang:groovy id="messenger" script-source="classpath:Messenger.groovy">
<lang:property name="message" value="I Can Do The Frug" />
</lang:groovy>
<!-- an otherwise normal bean that will be injected by the Groovy-backed Messenger -->
<bean id="bookingService" class="x.y.DefaultBookingService">
<property name="messenger" ref="messenger" />
</bean>
</beans>
The bookingService
bean (a DefaultBookingService
) can now use its private messenger
member variable as normal, because the Messenger
instance that was injected into it is
a Messenger
instance. There is nothing special going on here — just plain Java and
plain Groovy.
Hopefully, the preceding XML snippet is self-explanatory, but do not worry unduly if it is not. Keep reading for the in-depth detail on the whys and wherefores of the preceding configuration.
# 3.2. Defining Beans that Are Backed by Dynamic Languages
This section describes exactly how you define Spring-managed beans in any of the supported dynamic languages.
Note that this chapter does not attempt to explain the syntax and idioms of the supported dynamic languages. For example, if you want to use Groovy to write certain of the classes in your application, we assume that you already know Groovy. If you need further details about the dynamic languages themselves, see Further Resources at the end of this chapter.
# 3.2.1. Common Concepts
The steps involved in using dynamic-language-backed beans are as follows:
Write the test for the dynamic language source code (naturally).
Then write the dynamic language source code itself.
Define your dynamic-language-backed beans by using the appropriate
<lang:language/>
element in the XML configuration (you can define such beans programmatically by using the Spring API, although you will have to consult the source code for directions on how to do this, as this chapter does not cover this type of advanced configuration). Note that this is an iterative step. You need at least one bean definition for each dynamic language source file (although multiple bean definitions can reference the same source file).
The first two steps (testing and writing your dynamic language source files) are beyond the scope of this chapter. See the language specification and reference manual for your chosen dynamic language and crack on with developing your dynamic language source files. You first want to read the rest of this chapter, though, as Spring’s dynamic language support does make some (small) assumptions about the contents of your dynamic language source files.
# The <lang:language/> element
The final step in the list in the preceding sectioninvolves defining dynamic-language-backed bean definitions, one for each bean that you
want to configure (this is no different from normal JavaBean configuration). However,
instead of specifying the fully qualified class name of the class that is to be
instantiated and configured by the container, you can use the <lang:language/>
element to define the dynamic language-backed bean.
Each of the supported languages has a corresponding <lang:language/>
element:
<lang:groovy/>
(Groovy)<lang:bsh/>
(BeanShell)<lang:std/>
(JSR-223, e.g. with JRuby)
The exact attributes and child elements that are available for configuration depends on exactly which language the bean has been defined in (the language-specific sections later in this chapter detail this).
# Refreshable Beans
One of the (and perhaps the single) most compelling value adds of the dynamic language support in Spring is the “refreshable bean” feature.
A refreshable bean is a dynamic-language-backed bean. With a small amount of configuration, a dynamic-language-backed bean can monitor changes in its underlying source file resource and then reload itself when the dynamic language source file is changed (for example, when you edit and save changes to the file on the file system).
This lets you deploy any number of dynamic language source files as part of an application, configure the Spring container to create beans backed by dynamic language source files (using the mechanisms described in this chapter), and (later, as requirements change or some other external factor comes into play) edit a dynamic language source file and have any change they make be reflected in the bean that is backed by the changed dynamic language source file. There is no need to shut down a running application (or redeploy in the case of a web application). The dynamic-language-backed bean so amended picks up the new state and logic from the changed dynamic language source file.
This feature is off by default. |
---|
Now we can take a look at an example to see how easy it is to start using refreshable
beans. To turn on the refreshable beans feature, you have to specify exactly one
additional attribute on the <lang:language/>
element of your bean definition. So,
if we stick with the example from earlier in
this chapter, the following example shows what we would change in the Spring XML
configuration to effect refreshable beans:
<beans>
<!-- this bean is now 'refreshable' due to the presence of the 'refresh-check-delay' attribute -->
<lang:groovy id="messenger"
refresh-check-delay="5000" <!-- switches refreshing on with 5 seconds between checks -->
script-source="classpath:Messenger.groovy">
<lang:property name="message" value="I Can Do The Frug" />
</lang:groovy>
<bean id="bookingService" class="x.y.DefaultBookingService">
<property name="messenger" ref="messenger" />
</bean>
</beans>
That really is all you have to do. The refresh-check-delay
attribute defined on themessenger
bean definition is the number of milliseconds after which the bean is
refreshed with any changes made to the underlying dynamic language source file.
You can turn off the refresh behavior by assigning a negative value to therefresh-check-delay
attribute. Remember that, by default, the refresh behavior is
disabled. If you do not want the refresh behavior, do not define the attribute.
If we then run the following application, we can exercise the refreshable feature.
(Please excuse the “jumping-through-hoops-to-pause-the-execution” shenanigans
in this next slice of code.) The System.in.read()
call is only there so that the
execution of the program pauses while you (the developer in this scenario) go off
and edit the underlying dynamic language source file so that the refresh triggers
on the dynamic-language-backed bean when the program resumes execution.
The following listing shows this sample application:
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import org.springframework.scripting.Messenger;
public final class Boot {
public static void main(final String[] args) throws Exception {
ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml");
Messenger messenger = (Messenger) ctx.getBean("messenger");
System.out.println(messenger.getMessage());
// pause execution while I go off and make changes to the source file...
System.in.read();
System.out.println(messenger.getMessage());
}
}
Assume then, for the purposes of this example, that all calls to the getMessage()
method of Messenger
implementations have to be changed such that the message is
surrounded by quotation marks. The following listing shows the changes that you
(the developer) should make to the Messenger.groovy
source file when the
execution of the program is paused:
package org.springframework.scripting
class GroovyMessenger implements Messenger {
private String message = "Bingo"
public String getMessage() {
// change the implementation to surround the message in quotes
return "'" + this.message + "'"
}
public void setMessage(String message) {
this.message = message
}
}
When the program runs, the output before the input pause will be I Can Do The Frug
.
After the change to the source file is made and saved and the program resumes execution,
the result of calling the getMessage()
method on the dynamic-language-backedMessenger
implementation is 'I Can Do The Frug'
(notice the inclusion of the
additional quotation marks).
Changes to a script do not trigger a refresh if the changes occur within the window of
the refresh-check-delay
value. Changes to the script are not actually picked up until
a method is called on the dynamic-language-backed bean. It is only when a method is
called on a dynamic-language-backed bean that it checks to see if its underlying script
source has changed. Any exceptions that relate to refreshing the script (such as
encountering a compilation error or finding that the script file has been deleted)
results in a fatal exception being propagated to the calling code.
The refreshable bean behavior described earlier does not apply to dynamic language
source files defined with the <lang:inline-script/>
element notation (seeInline Dynamic Language Source Files). Additionally, it applies only to beans where
changes to the underlying source file can actually be detected (for example, by code
that checks the last modified date of a dynamic language source file that exists on the
file system).
# Inline Dynamic Language Source Files
The dynamic language support can also cater to dynamic language source files that are
embedded directly in Spring bean definitions. More specifically, the<lang:inline-script/>
element lets you define dynamic language source immediately
inside a Spring configuration file. An example might clarify how the inline script
feature works:
<lang:groovy id="messenger">
<lang:inline-script>
package org.springframework.scripting.groovy;
import org.springframework.scripting.Messenger
class GroovyMessenger implements Messenger {
String message
}
</lang:inline-script>
<lang:property name="message" value="I Can Do The Frug" />
</lang:groovy>
If we put to one side the issues surrounding whether it is good practice to define
dynamic language source inside a Spring configuration file, the <lang:inline-script/>
element can be useful in some scenarios. For instance, we might want to quickly add a
Spring Validator
implementation to a Spring MVC Controller
. This is but a moment’s
work using inline source. (See Scripted Validators for such an
example.)
# Understanding Constructor Injection in the Context of Dynamic-language-backed Beans
There is one very important thing to be aware of with regard to Spring’s dynamic language support. Namely, you can not (currently) supply constructor arguments to dynamic-language-backed beans (and, hence, constructor-injection is not available for dynamic-language-backed beans). In the interests of making this special handling of constructors and properties 100% clear, the following mixture of code and configuration does not work:
An approach that cannot work
// from the file 'Messenger.groovy'
package org.springframework.scripting.groovy;
import org.springframework.scripting.Messenger
class GroovyMessenger implements Messenger {
GroovyMessenger() {}
// this constructor is not available for Constructor Injection
GroovyMessenger(String message) {
this.message = message;
}
String message
String anotherMessage
}
<lang:groovy id="badMessenger"
script-source="classpath:Messenger.groovy">
<!-- this next constructor argument will not be injected into the GroovyMessenger -->
<!-- in fact, this isn't even allowed according to the schema -->
<constructor-arg value="This will not work" />
<!-- only property values are injected into the dynamic-language-backed object -->
<lang:property name="anotherMessage" value="Passed straight through to the dynamic-language-backed object" />
</lang>
In practice this limitation is not as significant as it first appears, since setter injection is the injection style favored by the overwhelming majority of developers (we leave the discussion as to whether that is a good thing to another day).
# 3.2.2. Groovy Beans
This section describes how to use beans defined in Groovy in Spring.
The Groovy homepage includes the following description:
“Groovy is an agile dynamic language for the Java 2 Platform that has many of the features that people like so much in languages like Python, Ruby and Smalltalk, making them available to Java developers using a Java-like syntax.”
If you have read this chapter straight from the top, you have alreadyseen an example of a Groovy-dynamic-language-backed bean. Now consider another example (again using an example from the Spring test suite):
package org.springframework.scripting;
public interface Calculator {
int add(int x, int y);
}
The following example implements the Calculator
interface in Groovy:
// from the file 'calculator.groovy'
package org.springframework.scripting.groovy
class GroovyCalculator implements Calculator {
int add(int x, int y) {
x + y
}
}
The following bean definition uses the calculator defined in Groovy:
<!-- from the file 'beans.xml' -->
<beans>
<lang:groovy id="calculator" script-source="classpath:calculator.groovy"/>
</beans>
Finally, the following small application exercises the preceding configuration:
package org.springframework.scripting;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class Main {
public static void main(String[] args) {
ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml");
Calculator calc = ctx.getBean("calculator", Calculator.class);
System.out.println(calc.add(2, 8));
}
}
The resulting output from running the above program is (unsurprisingly) 10
.
(For more interesting examples, see the dynamic language showcase project for a more
complex example or see the examples Scenarios later in this chapter).
You must not define more than one class per Groovy source file. While this is perfectly legal in Groovy, it is (arguably) a bad practice. In the interests of a consistent approach, you should (in the opinion of the Spring team) respect the standard Java conventions of one (public) class per source file.
# Customizing Groovy Objects by Using a Callback
The GroovyObjectCustomizer
interface is a callback that lets you hook additional
creation logic into the process of creating a Groovy-backed bean. For example,
implementations of this interface could invoke any required initialization methods,
set some default property values, or specify a custom MetaClass
. The following listing
shows the GroovyObjectCustomizer
interface definition:
public interface GroovyObjectCustomizer {
void customize(GroovyObject goo);
}
The Spring Framework instantiates an instance of your Groovy-backed bean and then
passes the created GroovyObject
to the specified GroovyObjectCustomizer
(if one
has been defined). You can do whatever you like with the supplied GroovyObject
reference. We expect that most people want to set a custom MetaClass
with this
callback, and the following example shows how to do so:
public final class SimpleMethodTracingCustomizer implements GroovyObjectCustomizer {
public void customize(GroovyObject goo) {
DelegatingMetaClass metaClass = new DelegatingMetaClass(goo.getMetaClass()) {
public Object invokeMethod(Object object, String methodName, Object[] arguments) {
System.out.println("Invoking '" + methodName + "'.");
return super.invokeMethod(object, methodName, arguments);
}
};
metaClass.initialize();
goo.setMetaClass(metaClass);
}
}
A full discussion of meta-programming in Groovy is beyond the scope of the Spring
reference manual. See the relevant section of the Groovy reference manual or do a
search online. Plenty of articles address this topic. Actually, making use of aGroovyObjectCustomizer
is easy if you use the Spring namespace support, as the
following example shows:
<!-- define the GroovyObjectCustomizer just like any other bean -->
<bean id="tracingCustomizer" class="example.SimpleMethodTracingCustomizer"/>
<!-- ... and plug it into the desired Groovy bean via the 'customizer-ref' attribute -->
<lang:groovy id="calculator"
script-source="classpath:org/springframework/scripting/groovy/Calculator.groovy"
customizer-ref="tracingCustomizer"/>
If you do not use the Spring namespace support, you can still use theGroovyObjectCustomizer
functionality, as the following example shows:
<bean id="calculator" class="org.springframework.scripting.groovy.GroovyScriptFactory">
<constructor-arg value="classpath:org/springframework/scripting/groovy/Calculator.groovy"/>
<!-- define the GroovyObjectCustomizer (as an inner bean) -->
<constructor-arg>
<bean id="tracingCustomizer" class="example.SimpleMethodTracingCustomizer"/>
</constructor-arg>
</bean>
<bean class="org.springframework.scripting.support.ScriptFactoryPostProcessor"/>
You may also specify a Groovy CompilationCustomizer (such as an ImportCustomizer )or even a full Groovy CompilerConfiguration object in the same place as Spring’sGroovyObjectCustomizer . Furthermore, you may set a common GroovyClassLoader with customconfiguration for your beans at the ConfigurableApplicationContext.setClassLoader level;this also leads to shared GroovyClassLoader usage and is therefore recommendable in case ofa large number of scripted beans (avoiding an isolated GroovyClassLoader instance per bean). |
---|
# 3.2.3. BeanShell Beans
This section describes how to use BeanShell beans in Spring.
The BeanShell homepage (opens new window) includes the following description:
BeanShell is a small, free, embeddable Java source interpreter with dynamic language
features, written in Java. BeanShell dynamically runs standard Java syntax and
extends it with common scripting conveniences such as loose types, commands, and method
closures like those in Perl and JavaScript.
In contrast to Groovy, BeanShell-backed bean definitions require some (small) additional
configuration. The implementation of the BeanShell dynamic language support in Spring is
interesting, because Spring creates a JDK dynamic proxy that implements all of the
interfaces that are specified in the script-interfaces
attribute value of the<lang:bsh>
element (this is why you must supply at least one interface in the value
of the attribute, and, consequently, program to interfaces when you use BeanShell-backed
beans). This means that every method call on a BeanShell-backed object goes through the
JDK dynamic proxy invocation mechanism.
Now we can show a fully working example of using a BeanShell-based bean that implements
the Messenger
interface that was defined earlier in this chapter. We again show the
definition of the Messenger
interface:
package org.springframework.scripting;
public interface Messenger {
String getMessage();
}
The following example shows the BeanShell “implementation” (we use the term loosely here)
of the Messenger
interface:
String message;
String getMessage() {
return message;
}
void setMessage(String aMessage) {
message = aMessage;
}
The following example shows the Spring XML that defines an “instance” of the above “class” (again, we use these terms very loosely here):
<lang:bsh id="messageService" script-source="classpath:BshMessenger.bsh"
script-interfaces="org.springframework.scripting.Messenger">
<lang:property name="message" value="Hello World!" />
</lang:bsh>
See Scenarios for some scenarios where you might want to use BeanShell-based beans.
# 3.3. Scenarios
The possible scenarios where defining Spring managed beans in a scripting language would be beneficial are many and varied. This section describes two possible use cases for the dynamic language support in Spring.
# 3.3.1. Scripted Spring MVC Controllers
One group of classes that can benefit from using dynamic-language-backed beans is that of Spring MVC controllers. In pure Spring MVC applications, the navigational flow through a web application is, to a large extent, determined by code encapsulated within your Spring MVC controllers. As the navigational flow and other presentation layer logic of a web application needs to be updated to respond to support issues or changing business requirements, it may well be easier to effect any such required changes by editing one or more dynamic language source files and seeing those changes being immediately reflected in the state of a running application.
Remember that, in the lightweight architectural model espoused by projects such as Spring, you typically aim to have a really thin presentation layer, with all the meaty business logic of an application being contained in the domain and service layer classes. Developing Spring MVC controllers as dynamic-language-backed beans lets you change presentation layer logic by editing and saving text files. Any changes to such dynamic language source files is (depending on the configuration) automatically reflected in the beans that are backed by dynamic language source files.
To effect this automatic “pickup” of any changes to dynamic-language-backed beans, you have to enable the “refreshable beans” functionality. SeeRefreshable Beans for a full treatment of this feature. |
---|
The following example shows an org.springframework.web.servlet.mvc.Controller
implemented
by using the Groovy dynamic language:
// from the file '/WEB-INF/groovy/FortuneController.groovy'
package org.springframework.showcase.fortune.web
import org.springframework.showcase.fortune.service.FortuneService
import org.springframework.showcase.fortune.domain.Fortune
import org.springframework.web.servlet.ModelAndView
import org.springframework.web.servlet.mvc.Controller
import javax.servlet.http.HttpServletRequest
import javax.servlet.http.HttpServletResponse
class FortuneController implements Controller {
@Property FortuneService fortuneService
ModelAndView handleRequest(HttpServletRequest request,
HttpServletResponse httpServletResponse) {
return new ModelAndView("tell", "fortune", this.fortuneService.tellFortune())
}
}
<lang:groovy id="fortune"
refresh-check-delay="3000"
script-source="/WEB-INF/groovy/FortuneController.groovy">
<lang:property name="fortuneService" ref="fortuneService"/>
</lang:groovy>
# 3.3.2. Scripted Validators
Another area of application development with Spring that may benefit from the flexibility afforded by dynamic-language-backed beans is that of validation. It can be easier to express complex validation logic by using a loosely typed dynamic language (that may also have support for inline regular expressions) as opposed to regular Java.
Again, developing validators as dynamic-language-backed beans lets you change validation logic by editing and saving a simple text file. Any such changes is (depending on the configuration) automatically reflected in the execution of a running application and would not require the restart of an application.
To effect the automatic “pickup” of any changes to dynamic-language-backed beans, you have to enable the 'refreshable beans' feature. SeeRefreshable Beans for a full and detailed treatment of this feature. |
---|
The following example shows a Spring org.springframework.validation.Validator
implemented by using the Groovy dynamic language (see Validation using Spring’s Validator interface for a discussion of theValidator
interface):
import org.springframework.validation.Validator
import org.springframework.validation.Errors
import org.springframework.beans.TestBean
class TestBeanValidator implements Validator {
boolean supports(Class clazz) {
return TestBean.class.isAssignableFrom(clazz)
}
void validate(Object bean, Errors errors) {
if(bean.name?.trim()?.size() > 0) {
return
}
errors.reject("whitespace", "Cannot be composed wholly of whitespace.")
}
}
# 3.4. Additional Details
This last section contains some additional details related to the dynamic language support.
# 3.4.1. AOP — Advising Scripted Beans
You can use the Spring AOP framework to advise scripted beans. The Spring AOP framework actually is unaware that a bean that is being advised might be a scripted bean, so all of the AOP use cases and functionality that you use (or aim to use) work with scripted beans. When you advise scripted beans, you cannot use class-based proxies. You must use interface-based proxies.
You are not limited to advising scripted beans. You can also write aspects themselves in a supported dynamic language and use such beans to advise other Spring beans. This really would be an advanced use of the dynamic language support though.
# 3.4.2. Scoping
In case it is not immediately obvious, scripted beans can be scoped in the same way as
any other bean. The scope
attribute on the various <lang:language/>
elements lets
you control the scope of the underlying scripted bean, as it does with a regular
bean. (The default scope is singleton,
as it is with “regular” beans.)
The following example uses the scope
attribute to define a Groovy bean scoped as
a prototype:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:lang="http://www.springframework.org/schema/lang"
xsi:schemaLocation="
http://www.springframework.org/schema/beans https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/lang https://www.springframework.org/schema/lang/spring-lang.xsd">
<lang:groovy id="messenger" script-source="classpath:Messenger.groovy" scope="prototype">
<lang:property name="message" value="I Can Do The RoboCop" />
</lang:groovy>
<bean id="bookingService" class="x.y.DefaultBookingService">
<property name="messenger" ref="messenger" />
</bean>
</beans>
See Bean Scopes in The IoC Containerfor a full discussion of the scoping support in the Spring Framework.
# 3.4.3. The lang
XML schema
The lang
elements in Spring XML configuration deal with exposing objects that have been
written in a dynamic language (such as Groovy or BeanShell) as beans in the Spring container.
These elements (and the dynamic language support) are comprehensively covered inDynamic Language Support. See that section
for full details on this support and the lang
elements.
To use the elements in the lang
schema, you need to have the following preamble at the
top of your Spring XML configuration file. The text in the following snippet references
the correct schema so that the tags in the lang
namespace are available to you:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:lang="http://www.springframework.org/schema/lang"
xsi:schemaLocation="
http://www.springframework.org/schema/beans https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/lang https://www.springframework.org/schema/lang/spring-lang.xsd">
<!-- bean definitions here -->
</beans>
# 3.5. Further Resources
The following links go to further resources about the various dynamic languages referenced in this chapter:
The Groovy (opens new window) homepage
The BeanShell (opens new window) homepage
The JRuby (opens new window) homepage