Question

I started exploring Google Guice today to do dependency injection in my application.

I noticed I am not using @Inject annotation anywhere. But it’s working. Can not understand this concept. In this example, where @Inject is the best fit in my scenerio? If any one can point me out.

public interface Tweeter {
    public void sendTweet(String message);
}

public class SmsTweeter implements Tweeter {
    @Override
    public void sendTweet(String message) {
        System.out.println("You SMS tweet: "+message);
    }
}

import com.google.inject.AbstractModule;

public class TweetModule extends AbstractModule{

    @Override
    protected void configure() {
        bind(Tweeter.class).to(SmsTweeter.class);       
    }
}

import com.google.inject.Guice;
import com.google.inject.Injector;

public class TestTweetClient {
    public static void main(String[] args) {
        Injector injector = Guice.createInjector(new TweetModule());
        Tweeter tweeter = injector.getInstance(Tweeter.class);
        tweeter.sendTweet("Hi there");
    }
}

It prints (the hidden implementation works):
You SMS tweet: Hi there

Answer 1

There is no best fit for @Inject in your example. The class SmsTweeter has an implicit zero-args constructor. You could make it explicit and add @Inject there but it is not necessary.

public class SmsTweeter implements Tweeter {

    @Inject
    SmsTweeter() {
        // nothing to do
    }

    @Override
    public void sendTweet(String message) {
        System.out.println("You SMS tweet: "+message);
    }
}