Guice vs Spring - Abhängigkeitsinjektion

1. Einleitung

Google Guice und Spring sind zwei robuste Frameworks, die für die Abhängigkeitsinjektion verwendet werden. Beide Frameworks decken alle Begriffe der Abhängigkeitsinjektion ab, aber jedes hat seine eigene Art, sie zu implementieren.

In diesem Tutorial werden wir diskutieren, wie sich die Guice- und Spring-Frameworks in Konfiguration und Implementierung unterscheiden.

2. Maven-Abhängigkeiten

Beginnen wir mit dem Hinzufügen der Abhängigkeiten Guice und Spring Maven zu unserer Datei pom.xml :

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

Von Maven Central aus können wir immer auf die neuesten Frühlingskontext- oder Guice- Abhängigkeiten zugreifen .

3. Konfiguration der Abhängigkeitsinjektion

Die Abhängigkeitsinjektion ist eine Programmiertechnik, mit der wir unsere Klassen von ihren Abhängigkeiten unabhängig machen.

In diesem Abschnitt werden wir auf einige Kernfunktionen verweisen, die sich zwischen Spring und Guice in der Konfiguration der Abhängigkeitsinjektion unterscheiden.

3.1. Federverdrahtung

Spring deklariert die Abhängigkeitsinjektionskonfigurationen in einer speziellen Konfigurationsklasse. Diese Klasse muss mit der Annotation @Configuration versehen werden . Der Spring-Container verwendet diese Klasse als Quelle für Bean-Definitionen.

Von Spring verwaltete Klassen werden Spring Beans genannt.

Spring verwendet die Annotation @Autowired, um die Abhängigkeiten automatisch zu verkabeln . @Autowired ist Teil der in Spring integrierten Kernanmerkungen . Wir können @Autowired für Mitgliedsvariablen, Setter-Methoden und Konstruktoren verwenden.

Spring unterstützt auch @Inject. @Inject ist Teil der Java-CDI (Contexts and Dependency Injection), die einen Standard für die Abhängigkeitsinjektion definiert.

Angenommen, wir möchten eine Abhängigkeit automatisch mit einer Mitgliedsvariablen verbinden. Wir können es einfach mit @Autowired kommentieren :

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

Zweitens erstellen wir eine Konfigurationsklasse, die als Quelle für Beans beim Laden unseres Anwendungskontexts verwendet wird:

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

Beachten Sie, dass wir UserService und AccountServiceImpl auch mit @Component versehen haben , um sie als Beans zu registrieren. Es ist die @ ComponentScan- Annotation, die Spring mitteilt , wo nach annotierten Komponenten gesucht werden soll.

Obwohl wir AccountServiceImpl mit Anmerkungen versehen haben , kann Spring es dem AccountService zuordnen, da es AccountService implementiert .

Dann müssen wir einen Anwendungskontext definieren, um auf die Beans zugreifen zu können. Beachten Sie nur, dass wir in allen unseren Spring-Unit-Tests auf diesen Kontext verweisen:

ApplicationContext context = new AnnotationConfigApplicationContext(SpringMainConfig.class);

Jetzt zur Laufzeit können wir die A ccountService- Instanz aus unserer UserService- Bean abrufen :

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

3.2. Guice Bindung

Guice verwaltet seine Abhängigkeiten in einer speziellen Klasse, die als Modul bezeichnet wird. Ein Guice-Modul muss die AbstractModule- Klasse erweitern und ihre configure () -Methode überschreiben .

Guice verwendet die Bindung als Äquivalent zur Verkabelung im Frühjahr. Einfach ausgedrückt, können wir mithilfe von Bindungen definieren, wie Abhängigkeiten in eine Klasse eingefügt werden sollen . Guice-Bindungen werden in der configure () -Methode unseres Moduls deklariert .

Anstelle von @Autowired verwendet Guice die Annotation @Inject , um die Abhängigkeiten einzufügen .

Erstellen wir ein gleichwertiges Guice-Beispiel:

public class GuiceUserService { @Inject private AccountService accountService; }

Zweitens erstellen wir die Modulklasse, aus der unsere Bindungsdefinitionen stammen:

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

Normally, we expect Guice to instantiate each dependency object from their default constructors if there isn't any binding defined explicitly in the configure() method. But since interfaces can't be instantiated directly, we need to define bindings to tell Guice which interface will be paired with which implementation.

Then, we need to define an Injector using GuiceModule to get instances of our classes. Let's just note that all of our Guice tests will use this Injector:

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

Finally, at runtime we retrieve a GuiceUserService instance with a non-null accountService dependency:

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

3.3. Spring's @Bean Annotation

Spring also provides a method level annotation @Bean to register beans as an alternative to its class level annotations like @Component. The return value of a @Bean annotated method is registered as a bean in the container.

Let's say that we have an instance of BookServiceImpl that we want to make available for injection. We could use @Bean to register our instance:

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

And now we can get a BookService bean:

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

3.4. Guice's @Provides Annotation

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

In all unseren Beispielen haben wir bereits gesehen, wie die Feldinjektion sowohl für Spring als auch für Guice angewendet wird. Es ist also kein neues Konzept für uns. Aber lassen Sie es uns der Vollständigkeit halber noch einmal auflisten.

Bei der feldbasierten Abhängigkeitsinjektion fügen wir die Abhängigkeiten ein, indem wir sie mit @Autowired oder @Inject markieren .

4.6. Feldinjektion in Guice

Wie oben erwähnt, haben wir die Feldinjektion für Guice bereits mit @Inject behandelt .

5. Schlussfolgerung

In diesem Tutorial haben wir die verschiedenen Kernunterschiede zwischen Guice- und Spring-Frameworks bei der Implementierung der Abhängigkeitsinjektion untersucht. Wie immer sind Guice- und Spring-Codebeispiele auf GitHub vorbei.