Guice: differences between Singleton.class and @Singleton

Question

In Guice, what's the difference between:

// Inside your AbstractModule subclass:
@Override
public void configure() {
    bind(Service.class).to(ServiceImpl.class).in(Singleton.class);
}

And:

@Override
public void configure() {
    bind(Service.class).to(ServiceImpl.class);
}

@Provides @Singleton
public ServiceImpl providesService() {
    return new ServiceImpl();
}

Are they both the same? When would you use one versus the other? Thanks in advance.

Answer 1

They are nearly identical. The @Singleton syntax is useful for annotating @Provides methods, or annotating the class itself (though I prefer to keep my scoping annotations inside modules).

The difference lies in which key is marked Singleton, which has less to do with @Singleton versus Singleton.class (or Scopes.SINGLETON, asEagerSingleton, @Singleton class annotations, or toInstance implicit singletons) and more to do with what the default syntax makes easy. For example:

public class MyModule extends AbstractModule {
  @Override public void configure() {
    bind(A.class).to(AImpl.class).in(Singleton.class);

    bind(B.class).to(BImpl.class);
    bind(BImpl.class).in(Singleton.class);
  }

  @Provides @Singleton C provideC() { return new CImpl(); }

  @Provides @Singleton D provideD(DImpl dImpl) { return dImpl; }

  @Provides E provideE(EImpl eImpl) { return eImpl; }
  @Provides @Singleton EImpl provideEImpl() { return new EImpl(); }
}

Above we've bound interface A to class AImpl, and interface B to class BImpl, but the behavior is different:

• Injecting A will retrieve the same AImpl instance every time.
• Injecting AImpl will retrieve a different AImpl every time, all of which are different than A's instance.
• Injecting B will retrieve the same BImpl instance every time.
• Injecting BImpl will also retrieve that same BImpl instance that B injects.

As you can see, each key is different, and Guice will allow multiple implementation instances if only the interface is bound with Singleton. If you only ever inject A and B interfaces, the behavior looks identical, but if you inject both interfaces and implementations from the same Injector, you may see differing behavior.

Similar logic goes for @Provides methods:

• Injecting C will always return the same CImpl instance.
• Injecting CImpl will create a new CImpl every time, unless CImpl has no injectable public zero-arg constructor—then the injection will fail.
• Injecting D will always return the same DImpl instance.
• Injecting DImpl will return a new instance every time, and each will be different than the one returned by D.
• Injecting E will return the same EImpl instance every time.
• Injecting EImpl will also retrieve that same instance E injects.

This provides some flexibility. Imagine a hypothetical Cache that keeps a certain number of most-recently-retrieved objects, where you want to have @User Cache and @Product Cache both injectable. If you bind(Cache.class).in(Singleton.class), you will have one Cache shared between the objects (and any bare Cache injections), whereas if you bind(Cache.class).annotatedWith(User.class).to(Cache.class).in(Singleton.class) then the annotated key is kept in singleton scope and each object type will have its own cache.