Constructor injection

Definition

Constructor injection means a class lists what it needs as constructor parameters and Spring supplies them. It is the preferred style: the dependencies are visible, they can be final, and the class is easy to create in a plain unit test.

Example code

A service that receives its dependency through the constructor

import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.stereotype.Component;
import org.springframework.stereotype.Service;

@Component
class PriceRepository {
    int priceOf(String item) { return item.length() * 10; }
}

@Service
class OrderService {
    private final PriceRepository prices;                       // final: it can never be missing or changed

    OrderService(PriceRepository prices) {                      // Spring passes the bean in
        this.prices = prices;
    }

    int total(String... items) {
        int sum = 0;
        for (String item : items) sum += prices.priceOf(item);
        return sum;
    }
}

@ComponentScan
class Demo {
    public static void main(String[] args) {
        try (var context = new AnnotationConfigApplicationContext(Demo.class)) {
            System.out.println(context.getBean(OrderService.class).total("pen", "book"));
        }
        // in a unit test you can skip Spring: new OrderService(new PriceRepository())
    }
}

// Output:
// 70