Can I use Dagger 2's field injection in Kotlin

问题:

I posted a question (Dagger 2 does not generate the component class (Android, Kotlin)), and after some experiments, it seems that the problem might be due to that Kotlin hides the field.

class CoffeeShop {
    @Inject
    var theCoffee: Coffee? = null
}

The error message is,

:app:kaptDebugKotline: ...\CoffeeShop.java:7:
error: Dagger does not support injection into private fields
e:     private ....Coffee theCoffee;

theCoffee was not private in my source code. But I think Kotlin may be translating

class CoffeeShop {
    @Inject
    var theCoffee: Coffee? = null
}

into Java code of

class CoffeeShop {
    @Inject
    private Coffee theCoffee = null;
    public Coffee getTheCoffee();
    public void setTheCoffee();
}

Can I use field injection in Kotlin?

回答1:

I think Kotlin may be translating [...] into Java code of [...]

And you would be correct, that's exactly what happens.

Typically in Kotlin you wouldn't write

@Inject var coffee: Coffee? = null

because when you're going to access coffee, it will never be null. In other words you will always inject the object before accessing it's fields. That makes the operators !! redundant and ? unnecessary. Kotlin has lateinit property modifier to express this.

@Inject lateinit var coffee: Coffee

When you use lateinit the generated field has the same visibility as its getter and setter, in this case public. This makes it work with Dagger.

You can see the result by viewing generated Kotlin bytecode.

Main menu > Tools > Kotlin > Show Kotlin Bytecode

However, even better approach would be injecting the class constructor:

class CoffeeShop @Inject constructor(val coffee: Coffee) {
    //...
}

In this case coffee is not var and can't be reassigned.

Injecting constructor is not an option when the instance is created for you by a framework, Android activity is a good example.

---

Note: When using qualifiers you have to specify field annotation target on them:

@Inject @field:Named("Arabica") @field:Arabica
lateinit var coffee: Coffee

---

Can I use field injection in Kotlin?

Yes you can. As explained above, field injection is actually applied for lateinit properties.

But you were probably interested in generating and injecting fields without getter/setter in Kotlin.

@JvmField @Inject
var coffee: Coffee? = null