Dependency Injection in Java is easy - Part 2 - Google Guice

Part 2 - Google Guice

This post is part of a multiple-part tutorial. As the heading suggests, this part will focus on Dependency Injection using Google Guice.

You can check out the code for this tutorial part in Github.

Background

It's advised to start with Part 1 - A Design Pattern, which this part requires. The next Part 3 - Spring Context doesn't require this one.

If you have read [Part 2][2], you can skip to the incorporating guice section.

Dependency Injection Frameworks

DI Frameworks concept is pretty straightforward; the framework creates and manages our dependencies for us.
On one end, we provide the framework instructions for creating our dependencies; on the other, we ask the framework for instances of the dependencies.

Comparing multiple frameworks, we'll notice different features, component names, default behaviors, and probably different implementations under the hood. But the gist will be the same: a container that holds our dependencies. Let's explore some similarities between various frameworks:

Spring's Autowired and Guice's Inject, both mark a dependency for injection.
Spring's Bean and Guice's Provides, both incorporate logic for creating a dependency.

Not a believer yet ❔ Alright, let's throw C# in the mix ❕

Spring's Configuration, Guice's Modules, and Autofac's Modules, can be used to configure dependencies.
Guice's Binding and Autofac's Registration are used to configure dependencies with the framework's DSL.

We can go on. But the point is made.
😎

Under the hood, DI Frameworks builds factories providing dependencies based on multiple criteria, such as type and name.

At the base level, there are three typical scopes for dependencies living in a DI Framework container:

Eager Singleton: one instance of the dependency will be created immediately upon the framework's instantiation; the same instance will be provided for every request.
Lazy Singleton: one instance of the dependency will be created only when requested. Upon its instantiation, the same instance will be provided for every request.
Non-Singleton: A new instance of the dependency will be provided for every request.

There are more scopes, but these are the three most commonly used. Some frameworks offer different scopes than others. Such as per session, per request, per dependency, etc.

Example App

Heads up: The example app next is based on Part 1 - A Design Pattern; you can skip to the incorporating guice section.

Mail Collector App

Let's build an app pulling emails from both Gmail and Microsoft leveraging the Dependency Injection pattern with Google Guice.

Contracts

An Enum called MailSource for categorizing the email source:

public enum MailSource {
  GMAIL,
  MICROSOFT;
}

An abstract class Mail for contracting mail objects.

public abstract class Mail {
  public abstract String from();

  public abstract String subject();

  public abstract MailSource source();

  @Override
  public String toString() {
    return String.format("Got mail by %s, from %s, with the subject %s", source(), from(), subject());
  }
}

An interface for contracting services responsible for pulling Mail from suppliers, the MailService.

public interface MailService {
  List<Mail> getMail();
}

And last, an interface for contracting an engine responsible for collecting Mail from multiple services, the MailEngine.

public interface MailEngine {
  List<Mail> getAllMail();
}

Implementations

Mail

The concrete Mail implementations were designed with a builder pattern for convenience and immutability.
The Gmail Mail implementation, GmailImpl:

public final class GmailImpl extends Mail {
  private final String setFrom;
  private final String setSubject;

  private GmailImpl(final String from, final String subject) {
    setFrom = from;
    setSubject = subject;
  }

  @Override
  public String from() {
    return setFrom;
  }

  @Override
  public String subject() {
    return setSubject;
  }

  @Override
  public MailSource source() {
    return MailSource.GMAIL;
  }

  public static GmailImpl.Builder builder() {
    return new GmailImpl.Builder();
  }

  public static final class Builder {
    private String prepFrom;
    private String prepSubject;

    public Builder from(final String setFrom) {
      prepFrom = setFrom;
      return this;
    }

    public Builder subject(final String setSubject) {
      prepSubject = setSubject;
      return this;
    }

    public GmailImpl build() {
      requireNonNull(emptyToNull(prepFrom), "from cannot be empty or null");
      requireNonNull(emptyToNull(prepSubject), "subject cannot be empty or null");

      return new GmailImpl(prepFrom, prepSubject);
    }
  }
}

The Micsorosft Mail implementation, MicrosoftImpl:

public final class MicrosoftImpl extends Mail {
  private final String setFrom;
  private final String setSubject;

  private MicrosoftImpl(final String from, final String subject) {
    setFrom = from;
    setSubject = subject;
  }

  @Override
  public String from() {
    return setFrom;
  }

  @Override
  public String subject() {
    return setSubject;
  }

  @Override
  public MailSource source() {
    return MailSource.MICROSOFT;
  }

  public static MicrosoftImpl.Builder builder() {
    return new MicrosoftImpl.Builder();
  }

  public static final class Builder {
    private String prepFrom;
    private String prepSubject;

    public Builder from(final String setFrom) {
      prepFrom = setFrom;
      return this;
    }

    public Builder subject(final String setSubject) {
      prepSubject = setSubject;
      return this;
    }

    public MicrosoftImpl build() {
      requireNonNull(emptyToNull(prepFrom), "from cannot be empty or null");
      requireNonNull(emptyToNull(prepSubject), "subject cannot be empty or null");

      return new MicrosoftImpl(prepFrom, prepSubject);
    }
  }
}

Mail Services

The Gmail MailService implementation:

public final class GmailService implements MailService {
  @Override
  public List<Mail> getMail() {
    //This is where the actual Gmail API access goes.
    //We'll fake a couple of emails instead.
    var firstFakeMail =
        GmailImpl.builder()
            .from("a.cool.friend@gmail.com")
            .subject("wanna get together and write some code?")
            .build();

    var secondFakeMail =
        GmailImpl.builder()
            .from("an.annoying.salesman@some.company.com")
            .subject("wanna buy some stuff?")
            .build();

    return List.of(firstFakeMail, secondFakeMail);
  }
}

The Microsoft MailService implementation:

public final class MicrosoftService implements MailService {
  @Override
  public List<Mail> getMail() {
    //This is where the actual Microsoft API access goes.
    //We'll fake a couple of emails instead.
    var firstFakeMail =
        MicrosoftImpl.builder()
            .from("my.boss@work.info")
            .subject("stop writing tutorials and get back to work!")
            .build();

    var secondFakeMail =
        MicrosoftImpl.builder()
            .from("next.door.neighbor@kibutz.org")
            .subject("do you have philips screwdriver?")
            .build();

    return List.of(firstFakeMail, secondFakeMail);
  }
}

Mail Engine

public final class RobustMailEngine implements MailEngine {
  private final Set<MailService> mailServices;

  public RobustMailEngine(final Set<MailService> setMailSerices) {
    mailServices = setMailSerices;
  }

  @Override
  public List<Mail> getAllMail() {
    return mailServices.stream().map(MailService::getMail).flatMap(List::stream).collect(toList());
  }
}

The Main App

This is the app itself, the MailCollectorApp:

public final class MailCollectorApp {
  private MailEngine engine;

  public MailCollectorApp(final MailEngine setEngine) {
    engine = setEngine;
  }

  public String getMail() {
    var ret = "No mail found.";
    if (!engine.getAllMail().isEmpty()) {
      ret = Joiner.on(System.lineSeparator()).join(engine.getAllMail());
    }
    return ret;
  }

  public static void main(final String... args) {
    var gmailService = new GmailService();
    var microsoftService = new MicrosoftService();

    var engine = new RobustMailEngine(Set.of(gmailService, microsoftService));

    var app = new MailCollectorApp(engine);

    System.out.println(app.getMail());
  }
}

Executing the main method will print:

Got Mail by GMAIL, from a.cool.friend@gmail.com, with the subject wanna get together and write some code?
Got Mail by GMAIL, from an.annoying.salesman@some.company.com, with the subject wanna buy some stuff?
Got Mail by MICROSOFT, from my.boss@work.info, with the subject stop writing tutorials and get back to work!
Got Mail by MICROSOFT, from next.door.neighbor@kibutz.org, with the subject do you have a star screwdriver?

This application uses the dependency injection design pattern. The dependencies are currently controlled by the main method, so it should be easy to incorporate Google Guice.

Incorporating Spring Context

Include maven dependency

First, let's add this to our pom.xml in the dependencies section:
Note that this version was the latest when this tutorial was written.

<dependency>
  <groupId>com.google.inject</groupId>
  <artifactId>guice</artifactId>
  <version>4.2.3</version>
</dependency>

Mark Inject

We need to tell Guice about the dependencies we want to be injected. Let's do this for the concrete engine class:

public final class RobustMailEngine implements MailEngine {
  private final Set<MailService> mailServices;

  @Inject
  public RobustMailEngine(final Set<MailService> setMailSerices) {
    mailServices = setMailSerices;
  }

  @Override
  public List<Mail> getAllMail() {
    return mailServices.stream().map(MailService::getMail).flatMap(List::stream).collect(toList());
  }
}

And for the app class:

public final class MailCollectorApp {
  private MailEngine engine;

  @Inject
  public MailCollectorApp(final MailEngine setEngine) {
    engine = setEngine;
  }

  public String getMail() {
    var ret = "No mail found.";
    if (!engine.getAllMail().isEmpty()) {
      ret = Joiner.on(System.lineSeparator()).join(engine.getAllMail());
    }
    return ret;
  }

  //...
}

Now, we need to instruct Guice on providing those dependencies.

Provide Dependencies

Let's create a Guice Module for configuring the bindings; multiple approaches exist to achieve that. For instance, using Guice's Provides Methods:

public final class DIModule extends AbstractModule {
  @Provides
  static Set<MailService> getServices() {
    return Set.of(new GmailService(), new MicrosoftService());
  }

  @Provides
  @Singleton
  static MailEngine getEngine(final Set<MailService> services) {
    return new RobustMailEngine(services);
  }
}

Another approach to configuring Guice bindings is using Guice's bind DSL:

public final class DIModule extends AbstractModule {
  @Override
  public void configure() {
    var listBinder = newSetBinder(binder(), MailService.class);
    listBinder.addBinding().toInstance(new GmailService());
    listBinder.addBinding().toInstance(new MicrosoftService());

    bind(MailEngine.class).to(RobustMailEngine.class).in(Scopes.SINGLETON);
  }
}

Both options will produce the same dependencies.

Guice's default scope is Non-Singleton. This means that the Set of GmailService and MicrosoftService will be created as new instances for every object that needs them.

On the other end, the RobustMailEngine will be bound as a Singelton.
Based on Guice's docs, that means that in Production stage it will be an Eager Singleton, and in Development stage it will be a Lazy Singleton.
Either way, it's going to be a Singleton, meaning our app will have only one instance of RobustMailEngine.

The default stage for Guice is Development stage, so we can expect a Lazy Singleton.

Note, RobustMailEngine is a dependency; it will be injected to whoever requests it, but as we configured earlier, it also needs dependencies for itself (the Set of MailService).

Guice will catch that and inject the set of mail services while instantiating the engine.

Update the app to use Guice

Getting back to our app. Let's update it to work with Guice:

public final class MailCollectorApp {
  private MailEngine engine;

  @Inject
  public MailCollectorApp(final MailEngine setEngine) {
    engine = setEngine;
  }

  public String getMail() {
    var ret = "No mail found.";
    if (!engine.getAllMail().isEmpty()) {
      ret = Joiner.on(System.lineSeparator()).join(engine.getAllMail());
    }
    return ret;
  }

  public static void main(final String... args) {
    var injector = Guice.createInjector(new DIModule());

    var app = injector.getInstance(MailCollectorApp.class);

    System.out.println(app.getMail());
  }
}

Let's analyze what's going on here...

Running the main method will create a Guice's injector, which is the context for all dependencies. As we create it with an instance of our DIModule class, the injector's context will have the following dependencies configured in it:

A Set of two MailService objects (GmailService and Microsoft Service).
A Singleton instance of MailEngine (RobustMailEngine).

As stated, in the Development stage the MainEngine is a Lazy Singleton, plus, we configured the Set of MailService as Non-Singleton. That means that there's nothing instantiated in the injector's context at this point.

The next step, asking the injector for an instance of MailCollectorApp, will accomplish the following:

Guice will pick up the constructor in MailCollectorApp, as it's the only constructor.
As the constructor is marked with @Inject, Guice will look in its context for a dependency of type MailEngine.
It will find the RobustMailEngine configured, which is a Lazy Singleton.
While trying to instantiate it, it will pick up its constructor, which is also marked with @Inject. And look for a suitable dependency with the type Set of MailService.
It will find the Set of GoogleService and MicrosoftService, which is a Non-Singleton.

After preparing the dependency graph, Guice will:

Create the set after instantiating both GmailService and MicrosoftService.
Instantiate the RobustMailEngine injecting the Set.
Instantiate the MailCollectorApp injecting the RobustMailEngine.

We then get our instance MailCollectorApp with everything we need, from which we invoke getMail to get all our Mail.

That's it, Guice in a nutshell.
😆

Now, Let's test the code.

Unit Tests

I will start by saying that when it comes to unit tests, if possible, I always prefer not to use DI Frameworks.

Unit tests are about testing small parts, units, of our application. We can avoid the overhead of creating the DI Context. We'll be better off simply instantiating the subject under test manually.

On the other end, if we're writing integration, acceptance tests, or any different situation when we might need to test our application end-to-end, well, in that case, a suitable DI Framework could be our best friend.

Let's move on to unit tests with a DI Framework for demonstration purposes only.

public final class MailCollectorAppTest extends AbstractModule {
  private MailService gmailServiceMock;
  private MailService microsoftServiceMock;
  private MailService thirdServiceMock;

  private MailCollectorApp sut;

  private Faker faker;

  @Override
  public void configure() {
    var listBinder = newSetBinder(binder(), MailService.class);
    listBinder.addBinding().toInstance(gmailServiceMock);
    listBinder.addBinding().toInstance(microsoftServiceMock);
    listBinder.addBinding().toInstance(thirdServiceMock);

    bind(MailEngine.class).to(RobustMailEngine.class).in(Scopes.SINGLETON);
  }

  @BeforeEach
  public void initialize() {
    faker = new Faker();

    gmailServiceMock = mock(MailService.class);
    microsoftServiceMock = mock(MailService.class);
    thirdServiceMock = mock(MailService.class);

    var injector = Guice.createInjector(this);

    sut = injector.getInstance(MailCollectorApp.class);
  }

  @Test
  @DisplayName(
      "make the services mocks return no mail and validate the return string as 'No mail found'")
  public void getMail_noMailExists_returnsNoMailFound() {
    willReturn(emptyList()).given(gmailServiceMock).getMail();
    willReturn(emptyList()).given(microsoftServiceMock).getMail();
    willReturn(emptyList()).given(thirdServiceMock).getMail();

    then(sut.getMail()).isEqualTo("No mail found.");
  }

  @Test
  @DisplayName(
      "make the services return legitimate mail and validate the return string as expected")
  public void getMail_foundMail_returnsExpectedString() {
    var mail1 =
        GmailImpl.builder()
            .from(faker.internet().emailAddress())
            .subject(faker.lorem().sentence())
            .build();
    var mail2 =
        MicrosoftImpl.builder()
            .from(faker.internet().emailAddress())
            .subject(faker.lorem().sentence())
            .build();
    var mail3 =
        MicrosoftImpl.builder()
            .from(faker.internet().emailAddress())
            .subject(faker.lorem().sentence())
            .build();

    willReturn(List.of(mail1)).given(gmailServiceMock).getMail();
    willReturn(List.of(mail2, mail3)).given(microsoftServiceMock).getMail();
    willReturn(emptyList()).given(thirdServiceMock).getMail();

    then(sut.getMail().split(System.lineSeparator()))
        .containsOnly(mail1.toString(), mail2.toString(), mail3.toString());
  }
}

We created the injector with a different module than the one we've used for the app (DIModule).

In this case, we extended the test class to be a module in itself. Injecting our mocks instead of the real GmailService and MicrosoftService. We also added a third mail service, demonstrating how easy it is. 😁

The RobustMailEngine was not mocked because there was no real reason to do so, but it could have been easily replaced with a mock or a spy.

The test class behaved exactly like the main app, except for the services being mocks instead of real objects.

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