Kotlin: use spring annotations like @Autowired in kotlin

Kotlin Programming

Question or issue of Kotlin Programming:

Is it possible to do something like following in Kotlin?

@Autowired
internal var mongoTemplate: MongoTemplate

@Autowired
internal var solrClient: SolrClient

How to solve this issue?

Solution no. 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

Solution no. 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) {
  // ...
}

Solution no. 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) {
    //...
}

Solution no. 4:

like that

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

Hope this helps!