A noob question: When using Spring’s `@Autowired` ...

# spring

[JP]

A noob question: When using Spring’s @Autowired with Kotlin,
I’m confused whether this

@Autowired
private lateinit var userService: UserService

is a field injection or a setter injection.

In the documentation, the Kotlin equivalent of both field and setter injection of Java were all in this fashion.

I read that field injection should be avoided, and I’ll try to stick with constructor injection if possible, but I’m just trying them out to learn.

If there is actually like a more stricter equivalents, I’d like to know how to write them in Kotlin.

Can anyone shed some light on this to me?

[thanksforallthefish]

that is setter injection and is not the preferred way. a var in kotlin generates setter and getter and since your user service is not part of the constructor setter injection will be used

[thanksforallthefish]

@Component
class MyBean(private val userService: UserService)

will use constructor injection (and a final user service). in modern spring, when using constructor injection and there is only one constructor, you don’t even have to use @Autowired, that just to show how much constructor injection is preferred

[JP]

As I stated, I read about the constructor injection being preferred and the implicit @Autowired for newer Spring versions, it was more of a theoretical question.

[JP]

Then would this be seen as BOTH field and setter injection when using Kotlin? Or is it only one of the two?

[thanksforallthefish]

field injection happens when the setter is not used. I am not sure how to do it in kotlin honestly. maybe @field:Autowired would bypass the setter

[thanksforallthefish]

afaik, the main point is that in java you declare a variable and then define setter and getter, in kotlin that is done for you based on val or var . when you do val you need to initialize at construction time, so with spring it will always be construction injection. when you do var a setter is always (and here is where I am not sure) is always created for you and you can use lateinit thus enabling setter injection

[JP]

This part of the documentation was what got me confused.
https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#beans-autowired-annotation

You can also apply the @Autowired annotation to traditional setter methods, as the following example shows:

class SimpleMovieLister {

    @Autowired
    lateinit var movieFinder: MovieFinder

    // ...

}

You can apply @Autowired to fields as well and even mix it with constructors, as the following example shows:

class MovieRecommender @Autowired constructor(
    private val customerPreferenceDao: CustomerPreferenceDao) {

    @Autowired
    private lateinit var movieCatalog: MovieCatalog

    // ...
}

[Ed M]

note the private in the second example - kotlin doesn’t generate a setter for private vars

[thanksforallthefish]

good catch, I did not see it was private. eh eh, sometimes eyes can gloss over words in a pretty significant way. I am anyway cross posting this to the #spring channel, they are mixing public and private fields without a note highlighting the difference and even though in hindsight it is obvious I think one extra line might prevent such confusion in the future

[thanksforallthefish]

actually we are in the spring channel, early friday morning, apparently am totally awake yet