Spring dependency inection or default value?-CodePudding

I have a class Car which has a dependency : Motor

My class Car is like this :

@Component
public class Car {

    private Motor motor;

    @Autowired
    public Car(Motor motor) {
        this.motor = motor;
    }

    public Car() {
        this.motor = new Motor("created by Car");
    }

I'm not sure how Spring manages what to do : using default/no-args constructor or constructor injection dependency ? Which motor instance should be used ?

CodePudding user response：

Based on Autowired documentation,

The constructor with the greatest number of dependencies that can be satisfied by matching beans in the Spring container will be chosen. If none of the candidates can be satisfied, then a primary/default constructor (if present) will be used.

Meaning, in your case, if Spring boot container finds bean of Motor during bootstrapping (may be via @Configuraton or @Component etc, ) then your Car bean will get instantiated with constructor based DI & in this case default constructor won't be used for creating Car bean.

If there is no Motor bean available during creation of Car bean, then default constructor is used.

CodePudding user response：

spring always call by default no arg constructor make sure you annotate motar as Component