Guice Tutorial - 01 - The simplest injection
Introduction
Guice is very powerful and simple dependency injection library from Google. There is no need to create xml configuration or have container to use it. Moreover, it gives us objects already casted to desired types. In this post I will show how to start working with guice and make the simplest injection. In this case, domain will be adding persons. All sources are available
here.
Main
First, we have to add maven dependency to guice and optionally to javax.inject:
<dependency>
<groupId>com.google.inject</groupId>
<artifactId>guice</artifactId>
<version>3.0</version>
</dependency>
<dependency>
<groupId>javax.inject</groupId>
<artifactId>javax.inject</artifactId>
<version>1</version>
</dependency>
Guice has own annotations to describe injections, but I like to use javax.inject annotation, which could also be used with Guice and are part of JavaEE.
Person is described by given class:
public class Person {
private String firstName;
private String lastName;
private Integer age;
public Person(final String firstName, final String lastName,
final Integer age) {
this.firstName = firstName;
this.lastName = lastName;
this.age = age;
}
//...
}We also need PersonDao and PersonValidator:
public class PersonDao {
public void persist(final Person user) {
// ...
}
}public class PersonValidator {
void validate(final Person user) throws PersonValidationException {
// ...
}
}Now we have all dependencies so let's create controller, which will use this classes:
public class PersonController {
private final PersonValidator userValidator;
private final PersonDao userDao;
@Inject
public PersonController(final PersonValidator userValidator,
final PersonDao userDao){
this.userValidator = userValidator;
this.userDao = userDao;
}
public void add(final Person user) throws PersonValidationException{
userValidator.validate(user);
userDao.persist(user);
}
}The magic is in annotation
Inject. In Guice, when we want to use class with not default, no argument constructor, we have to add this annotation to one of constructor.
Testing
It is time to test it. I will use
JUnit.
@Test
public void shouldGetPersonControllerFromInjectorAndPersistPerson() {
//given
Injector injector = Guice.createInjector();
Person user = new Person("John", "Smith", 25);
//when
PersonController userController = injector.getInstance(PersonController.class);
//then
userController.add(user);
}
All classes could be obtained from Guice via Injector. Injector collects all classes in classpath and allows to get their instances if they have default constructors or constructors annotated with Inject (from Guice or javax).
Method add of userController pass without exception so Guice has injected PersonDao and Person Validator classes.
Conclusion
Guice is very simple and in basic cases we need only to use Inject annotation on class with dependecies. We only have to call getInstance and tell Guice which class (or interface) we want.
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