Getting multiple guice singletons of the same type

Question

can you get 2 singleton instances of the same underlying type?

this is obviously trivial in spring as it is based on named instances to which you attach a scope but I can't see the equivalent in guice which is about binding types to implementation classes. Note that I don't want to have to bind to the instance as the instances in question are injected with other dependencies by guice.

Answer 1

It's easy in Guice too! Create two biding annotations, say @One and @Two and then

bind(MySingleton.class).annotatedWith(One.class).toInstance(new MySingleton());
bind(MySingleton.class).annotatedWith(Two.class).toInstance(new MySingleton());

and then

@Inject
public SomethingThatDependsOnSingletons(@One MySingleton s1,
    @Two MySingleton t2) { ... }

Answer 2

I'd like to complement Marcin's response, by adding that you don't have to limit yourself to using toInstance() or provider methods in such a situation.

The following will work just as well:

bind(Person.class).annotatedWith(Driver.class).to(MartyMcFly.class).in(Singleton.class);
bind(Person.class).annotatedWith(Inventor.class).to(DocBrown.class).in(Singleton.class);

[...]

@Inject
public BackToTheFuture(@Driver Person marty, @Inventor Person doc) { ... }

Guice will inject the dependencies as usual when instantiating the MartyMcFly and DocBrown classes.

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Note that it also works when you want to bind multiple singletons of the same type:

bind(Person.class).annotatedWith(Driver.class).to(Person.class).in(Singleton.class);
bind(Person.class).annotatedWith(Inventor.class).to(Person.class).in(Singleton.class);

For this to work, you must make sure that Person is not bound in the Singleton scope, either explicitely in the Guice module, or with the @Singleton annotation. More details in this Gist.

Edit: The sample code I give as an example comes from a Guice Grapher Test. Looking at the Guice tests is a great way to better understand how to use the API (which also applies to every project with good unit tests).