How to use spring annotations like @Autowired in kotlin?
SpringKotlinSpring Problem Overview
Is it possible to do something like following in Kotlin?
@Autowired
internal var mongoTemplate: MongoTemplate
@Autowired
internal var solrClient: SolrClient
Spring Solutions
Solution 1 - Spring
Recommended approach to do Dependency Injection in Spring is constructor injection:
@Component
class YourBean(
private val mongoTemplate: MongoTemplate,
private val solrClient: SolrClient
) {
// code
}
Prior to Spring 4.3 constructor should be explicitly annotated with Autowired
:
@Component
class YourBean @Autowired constructor(
private val mongoTemplate: MongoTemplate,
private val solrClient: SolrClient
) {
// code
}
In rare cases, you might like to use field injection, and you can do it with the help of lateinit
:
@Component
class YourBean {
@Autowired
private lateinit var mongoTemplate: MongoTemplate
@Autowired
private lateinit var solrClient: SolrClient
}
Constructor injection checks all dependencies at bean creation time and all injected fields is val
, at other hand lateinit injected fields can be only var
, and have little runtime overhead. And to test class with constructor, you don't need reflection.
Links:
Solution 2 - Spring
Yes, java annotations are supported in Kotlin mostly as in Java. One gotcha is annotations on the primary constructor requires the explicit 'constructor' keyword:
From https://kotlinlang.org/docs/reference/annotations.html > If you need to annotate the primary constructor of a class, you need to add the constructor keyword to the constructor declaration, and add the annotations before it:
class Foo @Inject constructor(dependency: MyDependency) {
// ...
}
Solution 3 - Spring
You can also autowire dependencies through the constructor. Remember to annotate your dependencies with @Configuration, @Component, @Service
etc
import org.springframework.stereotype.Component
@Component
class Foo (private val dependency: MyDependency) {
//...
}
Solution 4 - Spring
like that
@Component class Girl( @Autowired var outfit: Outfit)
Solution 5 - Spring
If you want property injection but don't like lateinit var
, here is my solution using property delegate:
private lateinit var ctx: ApplicationContext
@Component
private class CtxVarConfigurer : ApplicationContextAware {
override fun setApplicationContext(context: ApplicationContext) {
ctx = context
}
}
inline fun <reified T : Any> autowired(name: String? = null) = Autowired(T::class.java, name)
class Autowired<T : Any>(private val javaType: Class<T>, private val name: String?) {
private val value by lazy {
if (name == null) {
ctx.getBean(javaType)
} else {
ctx.getBean(name, javaType)
}
}
operator fun getValue(thisRef: Any?, property: KProperty<*>): T = value
}
Then you can use the much better by
delegate syntax:
@Service
class MyService {
private val serviceToBeInjected: ServiceA by autowired()
private val ambiguousBean: AmbiguousService by autowired("qualifier")
}