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javaspring
La Carbonell
La Carbonell

Reputation: 2066

Constructor Dependency Injection in Spring Framework

As far as I know constructors injection enforces mandatory dependencies and setters injection allow optional dependencies, but then...

Wouldn't be possible then this approach ???

@Component
public class Car {

    @Autowired(required=false)
    public Car(Engine engine, Transmission transmission) {
        this.engine = engine;
        this.transmission = transmission;
    }
}

Upvotes: 3

Views: 930

Answers (1)

Sergii Bishyr
Sergii Bishyr

Reputation: 8651

Your approach won't work as Spring will not inject null if no bean of given type is found. If you set @Autowired(required=false) on the setter method, in case there is no such bean this setter won't be called. It is not possible with the constructor.

For Spring version 4.1+ you can use Java 8 Optional for declaring optional dependencies:

@Component
public class Car {

    @Autowired
    public Car(Engine engine, Optional<Transmission> transmission) {
        this.engine = engine;
        this.transmission = transmission.orElse(null);
    }
}

In this case Spring will understand that Engine is required and but Transmission is optional. So if no bean of type Transmission is found then Optional.empty() is injected.

Upvotes: 6

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