Spring Fundamentals: Wiring & Dependency Injection, thesarfo

To bootstrap a plain Spring app (no Boot), create a main package with a Main/App class — that’s where the application context gets created, from an AnnotationConfigApplicationContext pointed at a @Configuration class.

package dev.thesarfo;

import dev.thesarfo.beans.Owner;
import dev.thesarfo.config.ProjectConfig;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class App {
    public static void main(String[] args) {
        try (AnnotationConfigApplicationContext c = new AnnotationConfigApplicationContext(ProjectConfig.class)) {
            Owner o = c.getBean(Owner.class);
            System.out.println(o);
        }
    }
}

Disambiguating multiple beans of the same type

Say we have a Cat bean and an Owner that depends on one:

package dev.thesarfo.beans;

public class Cat {

    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Cat{name='" + name + "'}";
    }
}

If the config defines two Cat beans, Spring has no way to know which one to inject — you need @Qualifier on both the bean definitions and the injection point, naming which one you want:

package dev.thesarfo.config;

import dev.thesarfo.beans.Cat;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

@Configuration
@ComponentScan(basePackages = "dev.thesarfo.beans")
public class ProjectConfig {

    @Bean
    @Qualifier("cat1") // the name this bean will be stored under in the context
    public Cat cat1() {
        Cat c = new Cat();
        c.setName("Tom");
        return c;
    }

    @Bean
    @Qualifier("cat2")
    // @Primary // alternative: mark one bean as the default used when no qualifier is given
    public Cat cat2() {
        Cat c = new Cat();
        c.setName("Leo");
        return c;
    }
}

package dev.thesarfo.beans;

import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.stereotype.Component;

@Component
public class Owner {

    @Qualifier("cat2") // tell the context which Cat bean to use
    private Cat cat;

    public Cat getCat() {
        return cat;
    }

    public void setCat(Cat cat) {
        this.cat = cat;
    }

    @Override
    public String toString() {
        return "Owner{cat=" + cat + "}";
    }
}

Owner resolves to the cat2 bean (“Leo”) specifically. The alternative to @Qualifier at every injection point is @Primary on one of the candidate bean definitions, marking it as the default whenever no explicit qualifier is given.