Injecting all implementations

Definition

Spring can inject every bean of a type at once, as a List or a Map keyed by bean name. That lets you add a new implementation without changing the code that uses them, for example a new payment method.

Example code

A list and a map of all discount rules

import java.util.List;
import java.util.Map;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.stereotype.Component;

interface DiscountRule { int apply(int price); }

@Component("student")
class StudentRule implements DiscountRule { public int apply(int price) { return price - 10; } }

@Component("festival")
class FestivalRule implements DiscountRule { public int apply(int price) { return price - 5; } }

@Component
class PriceCalculator {
    private final List<DiscountRule> rules;
    private final Map<String, DiscountRule> byName;

    PriceCalculator(List<DiscountRule> rules, Map<String, DiscountRule> byName) {   // every implementation
        this.rules = rules;
        this.byName = byName;
    }

    int afterAll(int price) {
        for (DiscountRule rule : rules) price = rule.apply(price);
        return price;
    }

    int afterOne(String name, int price) { return byName.get(name).apply(price); }
}

@ComponentScan
class Demo {
    public static void main(String[] args) {
        try (var context = new AnnotationConfigApplicationContext(Demo.class)) {
            PriceCalculator calculator = context.getBean(PriceCalculator.class);
            System.out.println(calculator.afterAll(100));              // both rules applied
            System.out.println(calculator.afterOne("student", 100));   // one rule picked by bean name
        }
    }
}

// Output:
// 85
// 90