hk2: why bind(X.class).to(X.class)

Question

I am learning Java, but found the following piece of code. I am confused. What is bind(X.class).to(X.class); for?

import org.glassfish.hk2.utilities.binding.AbstractBinder;
public class ApplicationBinder extends AbstractBinder {
    @Override
    protected void configure() {
        bind(X.class).to(X.class);
    }
}

Thanks

Paul Samsotha · Accepted Answer · 2018-09-06T06:20:39.897

You're configuring how you want your services to be discovered in the DI (dependency injection) system. bind(Service).to(Contract) is basically saying that you want to provide the Service as an injectable service, and want to "advertise" it as Contract. By "advertise", I mean what you want to be able to inject it as. For instance Service can be UserRepositoryImpl, while Contract can be UserRepository (interface). With this you would only be able @Inject UserRepository as that's what you advertise. The benefit of this is all the benefits that come with programming to an interface.

Example

interface UserRepository {
    List<User> findAll();
}

class UserRepositoryImpl implements UserRepository {
    @Override
    public List<User> findAll() {
        return Arrays.asList(new User("username"));
    }
}

@Path("users")
class UserResource {
    @Inject
    private UserRepository repository;

    @GET
    public List<User> getUsers() {
        return repository.findAll();
    }
}

class JerseyApp extends ResourceConfig {
    public JerseyApp() {
        register(UserResource.class);
        register(new AbstractBinder() {
            @Override
            public void configure() {
                bind(UserRepositoryImpl.class)
                    .to(UserRepository.class);
            }
        });
    }
}

Here the UserRepository is injected into the UserResource. When the DI system injects it, it will actually be the UserRepositoryImpl instance.

but as for my post, it is better to import that class or do DI on that class? — BAE, Oct 25 '17 at 18:45
Not sure what you mean by "import or DI". Do you mean manually instantiating vs DI? For that, you can read any article on DI and you will see the benefits. — Paul Samsotha, Oct 25 '17 at 18:48
is there any way to inject in manually? Anything like `appBinder.get(UserResource.class)`? — dmitryvim, Mar 27 '22 at 20:25

score 1 · Answer 2 · answered May 17 '19 at 19:00

1

By doing that you are actually binding a new contract to a service.

bind(Service.class).to(Contract.class);

OR (binding a new contract to a service in Singleton)

bind(Service.class).to(Contract.class)..in(Singleton.class);

answered May 17 '19 at 19:00

Waqas Ahmed

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hk2: why bind(X.class).to(X.class)

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