Android Dagger Dependency Injection fails on private Fields

Question

I'm new to dagger (though I have experience with DI from working on Java EE WebApps using Weld).

What I'm trying to do is to inject a dependency into a class. The field is private.

Dagger then throws an exception stating it can't inject into a private field.

What's the reason for that?

After all it is possible to write to private fields using reflections, even on android..

If I set the visibility of the field to something other than private the injection seems to work.

Answer 1

Making a private field 'package visible' may not always be what you want. The Dagger documentation suggests the following:

Injecting final fields and private members. For best performance Dagger generates code. Work around this by using constructor injection.

Here's an example:

private ItemFactoryImpl itemFactory;
private BuildingFactory buildingFactory;

@Inject
public World(ItemFactoryImpl itemFactory, BuildingFactory buildingFactory) {
    this.itemFactory = itemFactory;
    this.buildingFactory = buildingFactory;
}

Answer 2

Dagger cannot support private fields and still support code-generated adapters (to avoid reflection). The way systems like Guice support private fields is they change the access to the field reflectively before accessing them. Since dagger generates an InjectAdapter in the same package as the class to be injected, it can access package-friendly, protected, or public fields. It cannot access private fields.

One of Dagger's advantages IS that it avoids reflection, so using reflection to bypass field visibility is not a desirable feature.

Answer 3

Just the remove the private to set the visibility of your field to package friendly.

Dagger doesn't support injection on private fields.