Guice vs Spring - Dependency Injection

1. Introduzione

Google Guice e Spring sono due robusti framework utilizzati per l'inserimento delle dipendenze. Entrambi i framework coprono tutte le nozioni di dependency injection, ma ognuno ha il proprio modo di implementarle.

In questo tutorial, discuteremo in che modo i framework Guice e Spring differiscono per configurazione e implementazione.

2. Dipendenze di Maven

Iniziamo aggiungendo le dipendenze Guice e Spring Maven nel nostro file pom.xml :

 org.springframework spring-context 5.1.4.RELEASE   com.google.inject guice 4.2.2 

Possiamo sempre accedere alle ultime dipendenze spring-context o guice da Maven Central.

3. Configurazione dell'iniezione delle dipendenze

L'inserimento delle dipendenze è una tecnica di programmazione che usiamo per rendere le nostre classi indipendenti dalle loro dipendenze.

In questa sezione, faremo riferimento a diverse funzionalità principali che differiscono tra Spring e Guice nel modo in cui configurano l'inserimento delle dipendenze.

3.1. Cablaggio a molla

Spring dichiara le configurazioni di inserimento delle dipendenze in una classe di configurazione speciale. Questa classe deve essere annotata dall'annotazione @Configuration . Il contenitore Spring utilizza questa classe come origine delle definizioni dei bean.

Le classi gestite da Spring sono chiamate Spring Beans.

Spring utilizza l' annotazione @Autowired per collegare automaticamente le dipendenze . @Autowired fa parte delle annotazioni principali integrate di Spring. Possiamo usare @Autowired su variabili membro, metodi setter e costruttori.

Spring supporta anche @Inject. @Inject fa parte di Java CDI (Contexts and Dependency Injection) che definisce uno standard per l' inserimento delle dipendenze.

Supponiamo di voler collegare automaticamente una dipendenza a una variabile membro. Possiamo semplicemente annotarlo con @Autowired :

@Component public class UserService { @Autowired private AccountService accountService; }
@Component public class AccountServiceImpl implements AccountService { }

In secondo luogo, creiamo una classe di configurazione da utilizzare come origine dei bean durante il caricamento del contesto dell'applicazione:

@Configuration @ComponentScan("com.baeldung.di.spring") public class SpringMainConfig { }

Si noti che abbiamo anche annotato UserService e AccountServiceImpl con @Component registrarli come fagioli. È l' annotazione @ComponentScan che dirà a Spring dove cercare i componenti annotati.

Anche se abbiamo annotato AccountServiceImpl , Spring può mapparlo ad AccountService poiché implementa AccountService .

Quindi, dobbiamo definire un contesto dell'applicazione per accedere ai bean. Notiamo solo che faremo riferimento a questo contesto in tutti i nostri test unitari di primavera:

ApplicationContext context = new AnnotationConfigApplicationContext(SpringMainConfig.class);

Ora in fase di esecuzione, possiamo recuperare l' istanza A ccountService dal nostro bean UserService :

UserService userService = context.getBean(UserService.class); assertNotNull(userService.getAccountService());

3.2. Guice Binding

Guice gestisce le sue dipendenze in una classe speciale chiamata modulo. Un modulo Guice deve estendere la classe AbstractModule e sovrascrivere il suo metodo configure () .

Guice usa l'associazione come l'equivalente del cablaggio in primavera. In poche parole, i collegamenti ci consentono di definire come le dipendenze verranno iniettate in una classe . I collegamenti Guice sono dichiarati nel metodo configure () del nostro modulo .

Invece di @Autowired , Guice usa l' annotazione @Inject per iniettare le dipendenze.

Creiamo un esempio Guice equivalente:

public class GuiceUserService { @Inject private AccountService accountService; }

In secondo luogo, creeremo la classe del modulo che è una fonte delle nostre definizioni di binding:

public class GuiceModule extends AbstractModule { @Override protected void configure() { bind(AccountService.class).to(AccountServiceImpl.class); } }

Normalmente, ci aspettiamo che Guice installi ogni oggetto di dipendenza dai loro costruttori predefiniti se non c'è alcun legame definito esplicitamente nel metodo configure () . Ma poiché le interfacce non possono essere istanziate direttamente, è necessario definire le associazioni per dire a Guice quale interfaccia sarà accoppiata con quale implementazione.

Quindi, dobbiamo definire un iniettore usando GuiceModule per ottenere istanze delle nostre classi. Notiamo solo che tutti i nostri test Guice useranno questo iniettore :

Injector injector = Guice.createInjector(new GuiceModule());

Infine, in fase di esecuzione recuperiamo un GuiceUserService esempio con un non nulla AccountService dipendenza:

GuiceUserService guiceUserService = injector.getInstance(GuiceUserService.class); assertNotNull(guiceUserService.getAccountService());

3.3. Annotazione @Bean di primavera

Spring fornisce anche un'annotazione a livello di metodo @Bean per registrare i bean in alternativa alle sue annotazioni a livello di classe come @Component . Il valore di ritorno di un metodo annotato @Bean viene registrato come bean nel contenitore.

Supponiamo di avere un'istanza di BookServiceImpl che vogliamo rendere disponibile per l'iniezione. Potremmo usare @Bean per registrare la nostra istanza:

@Bean public BookService bookServiceGenerator() { return new BookServiceImpl(); }

E ora possiamo ottenere un bean BookService :

BookService bookService = context.getBean(BookService.class); assertNotNull(bookService);

3.4. @Provides Annotation di Guice

As an equivalent of Spring's @Bean annotation, Guice has a built-in annotation @Provides to do the same job. Like @Bean, @Provides is only applied to the methods.

Now let's implement the previous Spring bean example with Guice. All we need to do is to add the following code into our module class:

@Provides public BookService bookServiceGenerator() { return new BookServiceImpl(); }

And now, we can retrieve an instance of BookService:

BookService bookService = injector.getInstance(BookService.class); assertNotNull(bookService);

3.5. Classpath Component Scanning in Spring

Spring provides a @ComponentScan annotation detects and instantiates annotated components automatically by scanning pre-defined packages.

The @ComponentScan annotation tells Spring which packages will be scanned for annotated components. It is used with @Configuration annotation.

3.6. Classpath Component Scanning in Guice

Unlike Spring, Guice doesn't have such a component scanning feature. But it's not difficult to simulate it. There are some plugins like Governator that can bring this feature into Guice.

3.7. Object Recognition in Spring

Spring recognizes objects by their names. Spring holds the objects in a structure which is roughly like a Map. This means that we cannot have two objects with the same name.

Bean collision due to having multiple beans of the same name is one common problem Spring developers hit. For example, let's consider the following bean declarations:

@Configuration @Import({SpringBeansConfig.class}) @ComponentScan("com.baeldung.di.spring") public class SpringMainConfig { @Bean public BookService bookServiceGenerator() { return new BookServiceImpl(); } }
@Configuration public class SpringBeansConfig { @Bean public AudioBookService bookServiceGenerator() { return new AudioBookServiceImpl(); } }

As we remember, we already had a bean definition for BookService in SpringMainConfig class.

To create a bean collision here, we need to declare the bean methods with the same name. But we are not allowed to have two different methods with the same name in one class. For that reason, we declared the AudioBookService bean in another configuration class.

Now, let's refer these beans in a unit test:

BookService bookService = context.getBean(BookService.class); assertNotNull(bookService); AudioBookService audioBookService = context.getBean(AudioBookService.class); assertNotNull(audioBookService);

The unit test will fail with:

org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'AudioBookService' available

First, Spring registered the AudioBookService bean with “bookServiceGenerator” name in its bean map. Then, it had to override it by the bean definition for BookService due to the “no duplicate names allowed” nature of the HashMap data structure.

Lastly, we can overcome this issue by making bean method names unique or setting the name attribute to a unique name for each @Bean.

3.8. Object Recognition in Guice

Unlike Spring, Guice basically has a Map structure . This means that we cannot have multiple bindings to the same type without using additional metadata.

Guice provides binding annotations to enable defining multiple bindings for the same type. Let's see what happens if we have two different bindings for the same type in Guice.

public class Person { }

Now, let's declare two different binding for the Person class:

bind(Person.class).toConstructor(Person.class.getConstructor()); bind(Person.class).toProvider(new Provider() { public Person get() { Person p = new Person(); return p; } });

And here is how we can get an instance of Person class:

Person person = injector.getInstance(Person.class); assertNotNull(person);

This will fail with:

com.google.inject.CreationException: A binding to Person was already configured at GuiceModule.configure()

We can overcome this issue by just simply discarding one of the bindings for the Person class.

3.9. Optional Dependencies in Spring

Optional dependencies are dependencies which are not required when autowiring or injecting beans.

For a field that has been annotated with @Autowired, if a bean with matching data type is not found in the context, Spring will throw NoSuchBeanDefinitionException.

However, sometimes we may want to skip autowiring for some dependencies and leave them as nullwithout throwing an exception:

Now let's take a look at the following example:

@Component public class BookServiceImpl implements BookService { @Autowired private AuthorService authorService; }
public class AuthorServiceImpl implements AuthorService { }

As we can see from the code above, AuthorServiceImpl class hasn't been annotated as a component. And we'll assume that there isn't a bean declaration method for it in our configuration files.

Now, let's run the following test to see what happens:

BookService bookService = context.getBean(BookService.class); assertNotNull(bookService);

Not surprisingly, it will fail with:

org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'AuthorService' available

We can make authorService dependency optional by using Java 8's Optional type to avoid this exception.

public class BookServiceImpl implements BookService { @Autowired private Optional authorService; }

Now, our authorService dependency is more like a container that may or may not contain a bean of AuthorService type. Even though there isn't a bean for AuthorService in our application context, our authorService field will still be non-null empty container. Hence, Spring won't have any reason to throw NoSuchBeanDefinitionException.

As an alternative to Optional, we can use @Autowired‘s required attribute, which is set to true by default, to make a dependency optional. We can set the required attribute to false to make a dependency optional for autowiring.

Hence, Spring will skip injecting the dependency if a bean for its data type is not available in the context. The dependency will remain set to null:

@Component public class BookServiceImpl implements BookService { @Autowired(required = false) private AuthorService authorService; }

Sometimes marking dependencies optional can be useful since not all the dependencies are always required.

With this in mind, we should remember that we'll need to use extra caution and null-checks during development to avoid any NullPointerException due to the null dependencies.

3.10. Optional Dependencies in Guice

Just like Spring, Guice can also use Java 8's Optional type to make a dependency optional.

Let's say that we want to create a class and with a Foo dependency:

public class FooProcessor { @Inject private Foo foo; }

Now, let's define a binding for the Foo class:

bind(Foo.class).toProvider(new Provider() { public Foo get() { return null; } });

Now let's try to get an instance of FooProcessor in a unit test:

FooProcessor fooProcessor = injector.getInstance(FooProcessor.class); assertNotNull(fooProcessor);

Our unit test will fail with:

com.google.inject.ProvisionException: null returned by binding at GuiceModule.configure(..) but the 1st parameter of FooProcessor.[...] is not @Nullable

In order to skip this exception, we can make the foo dependency optional with a simple update:

public class FooProcessor { @Inject private Optional foo; }

@Inject doesn't have a required attribute to mark the dependency optional. An alternative approach to make a dependency optional in Guice is to use the @Nullable annotation.

Guice tolerates injecting null values in case of using @Nullable as expressed in the exception message above. Let's apply the @Nullable annotation:

public class FooProcessor { @Inject @Nullable private Foo foo; }

4. Implementations of Dependency Injection Types

In this section, we'll take a look at the dependency injection types and compare the implementations provided by Spring and Guice by going through several examples.

4.1. Constructor Injection in Spring

In constructor-based dependency injection, we pass the required dependencies into a class at the time of instantiation.

Let's say that we want to have a Spring component and we want to add dependencies through its constructor. We can annotate that constructor with @Autowired:

@Component public class SpringPersonService { private PersonDao personDao; @Autowired public SpringPersonService(PersonDao personDao) { this.personDao = personDao; } }

Starting with Spring 4, the @Autowired dependency is not required for this type of injection if the class has only one constructor.

Let's retrieve a SpringPersonService bean in a test:

SpringPersonService personService = context.getBean(SpringPersonService.class); assertNotNull(personService);

4.2. Constructor Injection in Guice

We can rearrange the previous example to implement constructor injection in Guice. Note that Guice uses @Inject instead of @Autowired.

public class GuicePersonService { private PersonDao personDao; @Inject public GuicePersonService(PersonDao personDao) { this.personDao = personDao; } }

Here is how we can get an instance of GuicePersonService class from the injector in a test:

GuicePersonService personService = injector.getInstance(GuicePersonService.class); assertNotNull(personService);

4.3. Setter or Method Injection in Spring

In setter-based dependency injection, the container will call setter methods of the class, after invoking the constructor to instantiate the component.

Let's say that we want Spring to autowire a dependency using a setter method. We can annotate that setter method with @Autowired:

@Component public class SpringPersonService { private PersonDao personDao; @Autowired public void setPersonDao(PersonDao personDao) { this.personDao = personDao; } }

Whenever we need an instance of SpringPersonService class, Spring will autowire the personDao field by invoking the setPersonDao() method.

We can get a SpringPersonService bean and access its personDao field in a test as below:

SpringPersonService personService = context.getBean(SpringPersonService.class); assertNotNull(personService); assertNotNull(personService.getPersonDao());

4.4. Setter or Method Injection in Guice

We'll simply change our example a bit to achieve setter injection in Guice.

public class GuicePersonService { private PersonDao personDao; @Inject public void setPersonDao(PersonDao personDao) { this.personDao = personDao; } }

Every time we get an instance of GuicePersonService class from the injector, we'll have the personDao field passed to the setter method above.

Here is how we can create an instance of GuicePersonService class and access its personDao fieldin a test:

GuicePersonService personService = injector.getInstance(GuicePersonService.class); assertNotNull(personService); assertNotNull(personService.getPersonDao());

4.5. Field Injection in Spring

Abbiamo già visto come applicare l'iniezione sul campo sia per Spring che per Guice in tutti i nostri esempi. Quindi, non è un nuovo concetto per noi. Ma cerchiamo di elencarlo di nuovo per completezza.

Nel caso di inserimento delle dipendenze basato sul campo, iniettiamo le dipendenze contrassegnandole con @Autowired o @Inject .

4.6. Iniezione sul campo a Guice

Come accennato nella sezione precedente, abbiamo già coperto l' iniezione di campo per Guice usando @Inject .

5. conclusione

In questo tutorial, abbiamo esplorato le diverse differenze principali tra i framework Guice e Spring nei loro modi di implementare l'inserimento delle dipendenze. Come sempre, gli esempi di codice Guice e Spring sono finiti su GitHub.