Spring 3: Type safe dependency injection

Before jumping to Type Safe Dependency Injection from Spring, I would like to to discuss the way we have been doing it earlier. We have been using dependency injection by type with the help of Autowired annotation from Spring.

Something like this would inject the spring bean.

@Autowired
private StudentDao studentDao; // Autowires by type. Injects the instance whose type is StudentDao

But if we have more than one spring bean of one type then we use Qualifier Annotation along with Autowired which in facts injects the spring bean by name.

An application context with :

<bean id="studentDao1" class="StudentDao" />
<bean id="studentDao2" class="StudentDao" />

So now if we have two instances of StudentDao (studentDao1 and studentDao2), we can inject spring bean by name.

@Autowired
@Qualifier("studentDao1")
private StudentDao studentDao1;

@Autowired
@Qualifier("studentDao2")
private StudentDao studentDao2;

Same thing can be achieved with Resource annotaion specified by JSR-250. So we can inject a bean into a field or single parameter method with this annotation. Autowired is little more flexible than Resource since it can be used with multi parameter method as well as constructors.

We can inject bean by name with Resource annotation in the following manner.

@Resource
private StudentDao studentDao1;

Type safe dependency injection in Spring 3

Define a custom annotation using @Qualifier

To identify the injected bean without specifying the name, we need to create a custom annotation. This is an equivalent procedure to the use of JSR 330 annotations(Inject) in CDI.

@Target({ElementType.Field, ElementType.Parameter})
@Retention(RetentionPolicy.RUNTIME)
@Qualifier
public @Interface Student  {
}

Now assign this custom annotation to implementation of EntityDao Interface

@Component
@Student
public class StudentDao implements EntityDao   {
}

@Component tells Spring that this a bean definition. @Student annotation is used by Spring IoC to identify StudentDao as EntityDao’s implementation whenever reference of EntityDao is used.

Inject the bean using @Autowired and custom qualifier

Something like this.

@Autowired
@Student
private EntityDao studentDao; // So the spring injects the instance of StudentDao here.

This makes less use of String-names, which can be misspelled and are harder to maintain.


Reference:
How to use Type safe dependency injection in Spring 3? from our
JCG partner Saurab Parakh at the
Coding is Cool blog.

Source : http://www.javacodegeeks.com/2012/05/spring-3-type-safe-dependency-injection.html

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