Guice: Using @Named to create object

Question

Using Guice, if I have

@Inject @Named("light")
Color light;

I can use

bind(Color.class).annotatedWith(Names.named("light"))
    .toInstance(new Color("white"));

to bind it to white color if Color constructor only requires color name.

But how can I do it if Color constructor also requires objects which are created by Guice? For example:

@Inject
public Color(ColorSet colorset, String colorName) {...}

where colorset is create at runtime by Guice and in not available in configure(){...}.

I would like to do it without:

• creating a class/subclass for each color that I need to inject (like WhiteColor)
• using reflection directly in my code
• changing my classes to use ColorFactoy factory; factory.get("light"); instead of @Named("light") Color light;

Solution 1

The best way to do this is not with a factory but with @Provides methods. My company uses Guice very, very extensively, and requestInjection is always considered a bad idea because it can easily set up a very fragile graph of implicit dependencies.

Here's what it should look like:

public class FooModule extends AbstractModule {
  protected void configure() {
    // do configuration
  }

  @Provides
  @Named("white")
  Color provideWhiteColor(ColorSet colorSet) {
    return colorSet.white(); // or whatever
  }

  @Provides
  @Named("black")
  Color provideBlackColor(ColorSet colorSet) {
    return colorSet.black(); // or whatever
  }

  // etc
}

Solution 2

You could setup a factory within the module, and request injection on it to fill in the ColorSet.

Module:

ColorFactory colorFactory = new ColorFactory();

requestInjection(colorFactory);

bind(Color.class).annotatedWith(Names.named("light")).toInstance(colorFactory.buildColor("white"));
bind(Color.class).annotatedWith(Names.named("dark")).toInstance(colorFactory.buildColor("black"));

ColorFactory:

public class ColorFactory {

    private ColorSet colorSet;

    public Color buildColor(String color){
        return new Color(colorSet, color);
    }

    @Inject
    public void setColorSet(ColorSet colorSet) {
        this.colorSet = colorSet;
    }
}

Solution 3

There are few code bases where I have seen the use of injector directly in order fetch certain object.

injector.getInstance(Color.class);

In this case, you can use the following:

injector.getInstance(Key.get(Color.class, Names.named("light")));

Comments

[John Ericksen]

Oh, I read that as you didn't want a subclass of each color (ie: WhiteColor). Are you looking to reference other colors as well?

[John Ericksen]

Can you clarify your usage of ColorSet?

[Ali Shakiba]

That's just a simplified example of the real situation. There is not a colorset, there is a graph of objects created and connected by guice.

[John Ericksen]

Are your colors considered immutable?

[Ali Shakiba]

No, but does it help if I make them immutable?

[John Ericksen]

Im not sure... just trying to figure out your architecture. I'll think about this usage of guice some more and let you know if I come up with anything.

[Ali Shakiba]

I doubt that, but your idea may still be helpful.

[Ali Shakiba]

Thanks! They doesn't need to be immutable this way. I can bind named color to injected providers (instead of color instances).

[John Ericksen]

Cool. Good luck on your project.

[Ali Shakiba]

This is great idea, probably I can also mix it with Modules.override().with() to reuse them.

[Galz]

That's helpful - any idea how you can use a Guice injector to get either of the instances? Looking for something like Guice.createInjector(FooModule).getInstance(Color.class) but with a specification of "white" or "black"..

[smjZPkYjps]

I'm not sure how you would do it with @Named, but you should be using binding annotations (github.com/google/guice/wiki/BindingAnnotations) anyway. Then you could say injector.getInstance(Key.get(Color.class, Black.class)) (though I may be missing a generic type specification in there somewhere). getInstance(...) is overloaded to also take a Key (google.github.io/guice/api-docs/latest/javadoc/index.html?c‌​om/…), not just a class.