Field injection is not recommended, because the list of required dependencies are unclear during instance creation. This makes testing more difficult and could lead to runtime exceptions when the bean is instantiated without spring.

To clearly state the required dependencies of the class, they should be added to the constructor that has the @Autowired annotation. This way, these dependencies can be provided more easily through Spring's dependency injection mechanism.

@Configuration
public class Config {

    @Bean
    public Engine engine() {
        return new Engine("v8");
    }

    @Bean
    public Transmission transmission() {
        return new Transmission("sliding");
    }
}

@Component
public class Car {

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

• Baeldung on Spring dependency injection