How to use spring annotations like @Autowired in kotlin?

Question

Is it possible to do something like following in Kotlin?

@Autowired
internal var mongoTemplate: MongoTemplate

@Autowired
internal var solrClient: SolrClient

Answer 1

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:

1. Documentation on lateinit
2. Documentation on constructors
3. Developing Spring Boot applications with Kotlin

Answer 2

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) {
  // ...
}

Answer 3

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) {
    //...
}

Answer 4

像那样

@Component class Girl( @Autowired var outfit: Outfit)

Answer 5

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")

}