Guice Tutorial - 04 - Default providers and scopes

Introduction

This time I want to show you, what Guice will do, when we ask for provider for class, but we have not defined any provider and when we obtain new objects from injector and when already used objects. All sources are available here.

Default providers

First, let's create a small class:

public class UserSession {}

We do not define any profider, but when we ask for provider, than provider will be given to us.

@Test
public void shouldGetEachTimeNewSessionFromProvider(){
    //given
    Injector injector = Guice.createInjector();
    Provider<UserSession> sut = injector.getProvider(UserSession.class);
    //when
    UserSession userSession = sut.get();
    //then
    assertNotNull(userSession);
}

Scopes

For each request to injector or default provider we obtain new object. This is default scope of dependencies maintained by Guice. For example:

@Test
public void shouldGetEachTimeNewSession(){
    //given
    Injector sut = Guice.createInjector();
    //when
    UserSession userSession1 = sut.getInstance(UserSession.class);
    UserSession userSession2 = sut.getInstance(UserSession.class);
    //then
    assertNotEquals(userSession1, userSession2);
}

But suppose we should have only one instance of class, for example generator.

@Singleton
public class SequenceGenerator {
    private int counter = 0;

    public int next(){
        return counter++;
    }
}

The Singleton annotation tells Guice that we want only one instance of this class, so if we get instance of this class from injector or default provider, than we obtain the same instance.

@Test
public void shouldGetTheSameSequenceGeneratorEachTime(){
    //given
    Injector injector = Guice.createInjector();
    //when
    SequenceGenerator sequenceGenerator1 = injector.getInstance(SequenceGenerator.class);
    SequenceGenerator sequenceGenerator2 = injector.getInstance(SequenceGenerator.class);
    //then
    assertEquals(sequenceGenerator1, sequenceGenerator2);
}

@Test
public void shouldGetTheSameSequenceGeneratorEachTimeFromProvider(){
    //given
    Injector injector = Guice.createInjector();
    Provider<SequenceGenerator> sut = injector.getProvider(SequenceGenerator.class);
    //when
    SequenceGenerator sequenceGenerator1 = sut.get();
    SequenceGenerator sequenceGenerator2 = sut.get();
    //then
    assertEquals(sequenceGenerator1, sequenceGenerator2);
}

Of course dependency is singleton in Guice context, so you could create new object of this class outside from Guice and no one could stop you.

Conclusion

Guice generates for us default providers in default scope. Providers give us new objects for each request to their. We could also change scope of class to Singleton and then for each request we obtain the same object.