Guice vs Spring - Injeksi Ketergantungan

1. Perkenalan

Google Guice dan Spring adalah dua framework kuat yang digunakan untuk injeksi dependensi. Kedua kerangka kerja mencakup semua gagasan injeksi ketergantungan, tetapi masing-masing memiliki cara sendiri untuk mengimplementasikannya.

Dalam tutorial ini, kita akan membahas perbedaan framework Guice dan Spring dalam konfigurasi dan implementasi.

2. Ketergantungan Maven

Mari kita mulai dengan menambahkan dependensi Guice dan Spring Maven ke dalam file pom.xml kita :

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

Kami selalu dapat mengakses konteks pegas atau dependensi guice terbaru dari Maven Central.

3. Konfigurasi Injeksi Ketergantungan

Injeksi ketergantungan adalah teknik pemrograman yang kami gunakan untuk membuat kelas kami tidak bergantung pada ketergantungannya.

Di bagian ini, kami akan merujuk ke beberapa fitur inti yang berbeda antara Spring dan Guice dalam cara mengonfigurasi injeksi ketergantungan.

3.1. Kabel Musim Semi

Spring mendeklarasikan konfigurasi injeksi dependensi dalam kelas konfigurasi khusus. Kelas ini harus dianotasi dengan anotasi @Configuration . Kontainer Spring menggunakan kelas ini sebagai sumber definisi kacang.

Kelas yang dikelola oleh Spring disebut kacang musim semi.

Spring menggunakan anotasi @Autowired untuk menghubungkan dependensi secara otomatis . @Autowired adalah bagian dari anotasi inti bawaan Spring. Kita dapat menggunakan @Autowired pada variabel anggota, metode penyetel, dan konstruktor.

Spring juga mendukung @Inject. @Inject adalah bagian dari Java CDI (Contexts and Dependency Injection) yang mendefinisikan standar untuk injeksi ketergantungan.

Katakanlah kita ingin secara otomatis mentransfer ketergantungan ke variabel anggota. Kami cukup menambahkan anotasi dengan @Autowired :

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

Kedua, mari buat kelas konfigurasi untuk digunakan sebagai sumber kacang saat memuat konteks aplikasi kita:

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

Perhatikan bahwa kami juga telah dijelaskan UserService dan AccountServiceImpl dengan @Component untuk mendaftarkan mereka sebagai kacang. Ini adalah anotasi @ComponentScan yang akan memberi tahu Spring tempat mencari komponen beranotasi.

Meskipun kami telah membuat anotasi AccountServiceImpl , Spring dapat memetakannya ke AccountService karena mengimplementasikan AccountService .

Kemudian, kita perlu mendefinisikan konteks aplikasi untuk mengakses kacang. Perhatikan bahwa kami akan merujuk ke konteks ini di semua pengujian unit Spring kami:

ApplicationContext context = new AnnotationConfigApplicationContext(SpringMainConfig.class);

Sekarang saat runtime, kita bisa mengambil instance A ccountService dari kacang UserService kita :

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

3.2. Guice Binding

Guice mengelola dependensinya dalam kelas khusus yang disebut modul. Modul Guice harus memperluas kelas AbstractModule dan mengganti metode configure () -nya .

Guice menggunakan penjilidan yang setara dengan pengabelan di Spring. Sederhananya, binding memungkinkan kita untuk menentukan bagaimana dependensi akan dimasukkan ke dalam kelas . Binding guice dideklarasikan dalam metode konfigurasi () modul kita .

Sebagai ganti @Autowired , Guice menggunakan anotasi @Inject untuk memasukkan dependensi.

Mari buat contoh Guice yang setara:

public class GuiceUserService { @Inject private AccountService accountService; }

Kedua, kita akan membuat kelas modul yang merupakan sumber definisi pengikatan kita:

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

Biasanya, kami mengharapkan Guice untuk membuat instance setiap objek dependensi dari konstruktor defaultnya jika tidak ada pengikatan yang didefinisikan secara eksplisit dalam metode configure () . Tetapi karena antarmuka tidak dapat dibuat secara langsung, kita perlu mendefinisikan binding untuk memberi tahu Guice antarmuka mana yang akan dipasangkan dengan implementasi mana.

Kemudian, kita perlu mendefinisikan Injector menggunakan GuiceModule untuk mendapatkan instance dari kelas kita. Mari kita perhatikan bahwa semua tes Guice kami akan menggunakan Injector ini :

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

Terakhir, pada waktu proses kami mengambil instance GuiceUserService dengan dependensi accountService bukan null :

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

3.3. Anotasi @Bean Spring

Spring juga menyediakan anotasi level metode @Bean untuk mendaftarkan bean sebagai alternatif dari anotasi level kelasnya seperti @Component . Nilai kembalian dari metode beranotasi @Bean terdaftar sebagai kacang dalam wadah.

Katakanlah kita memiliki instance BookServiceImpl yang ingin kita sediakan untuk injeksi. Kita bisa menggunakan @Bean untuk mendaftarkan instance kita:

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

Dan sekarang kita bisa mendapatkan kacang BookService :

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

3.4. Anotasi @Provides 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

Kami sudah melihat bagaimana menerapkan injeksi lapangan baik untuk Spring dan Guice di semua contoh kami. Jadi, ini bukan konsep baru bagi kami. Tapi mari kita daftar lagi untuk kelengkapannya.

Dalam kasus injeksi ketergantungan berbasis lapangan, kami memasukkan ketergantungan dengan menandainya dengan @Autowired atau @Inject .

4.6. Injeksi Lapangan di Guice

Seperti yang kami sebutkan di bagian atas, kami sudah membahas injeksi lapangan untuk Guice menggunakan @Inject .

5. Kesimpulan

Dalam tutorial ini, kami menjelajahi beberapa perbedaan inti antara kerangka kerja Guice dan Spring dalam cara mereka mengimplementasikan injeksi ketergantungan. Seperti biasa, contoh kode Guice dan Spring sudah berakhir di GitHub.