Re: inject object into module

[Stephan Classen]

There are several points to mention. But don't worry we will get you where you want to be.

1. Guice modules do not get injected. Modules define what is binded in the injector. This means the modules are used to construct the injector. So they exist before the injector does. This makes it impossible for them to have anything injected into them.
PlayFramework does its own magic. It creates the injector for you. To do this you can pass it a list of module classes in the PlayFramework application.conf. If you read the documentation of PlayFramework [1] it states that it optionally will pass the Play Configuration to the constructor of the module. This is done by PlayFramework and has nothing to do with Guice. Note that there is not @Inject annotation on the constructor of the module.

2. Getting instances of module A into module B. This may sound logical in the beginning but it is not how Guice works. If the injector is constructed from multiple modules then it knows the union of the binding defined in all modules. This is also the reason why you cannot bind the same class twice. The injector simply wouldn't know which binding to use. So a single module may not know all bindings but the injector which is created in the end will.

3. Passing the DAO service to the Mail class. As you mention it in a comment this is the actual goal and this is the core competence of Guice. Your only misconception was that you try to do this in the binding. You need to do this in the Mail class. I recommend using constructor injection:

@Singleton
public class Mail {
    private final DAO dao;

    @Inject
    public Mail(DAO dao) {
        this.dao = dao;
    }
}

That's it. Technically you don't even need to bind the Mail class in your module. Guice will simply create a just in time binding. Never the less I would recommend creating an interface for the mail service and then specify which class implements the interface. This is especially useful for testing since you can replace the actual implementation with a mock or a stub for testing services depending on the mail service.

public class EmailModule extends AbstractModule {

    @Override
    protected void configure() {
         bind(Mail.class).to(MailImpl.class);
    }
}

Hope this gets you started and if you encounter more questions report back.


[1] https://www.playframework.com/documentation/2.5.x/JavaDependencyInjection#Providing-custom-bindings