Dependency Injection Basics

A simple Spring Boot project demonstrating the basics of Dependency Injection (DI) using Person and Pet (Cat and Dog) examples.

Step 1: Setup your Project

Create a new Spring Boot project using Spring Initializr with:

• Spring Web dependency
• Spring Boot Devtools dependency (allows hot reloading)
• Java version 17 or higher
• Maven project

Step 2: Initial Implementation

Create the following initial classes and interfaces:

Pet.java

public interface Pet {
    void feed();
}

Cat.java

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.stereotype.Component;

@Component
public class Cat implements Pet {
    private static final Logger logger = LoggerFactory.getLogger(Cat.class);

    @Override
    public void feed() {
        logger.info("Feeding the cat... ");
    }
}

Person.java

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.stereotype.Component;

@Component
public class Person {
    private static final Logger logger = LoggerFactory.getLogger(Person.class);
    private final Pet pet;

    public Person(Pet pet) {
        this.pet = pet;
    }

    public void feedPet() {
        logger.info("Feeding the pet...");
        pet.feed();
    }
}

Step 3: Run Your Application

Update your main application to use CommandLineRunner:

import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;

@SpringBootApplication
public class LabOneDemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(LabOneDemoApplication.class, args);
    }

    @Bean
    public CommandLineRunner demo(Person person) {
        return args -> {
            person.feedPet();
        };
    }
}

Run your application:

./mvnw spring-boot:run

Lab Extension Task: Adding a Dog

Task Instructions:

• Add a new class Dog that implements the Pet interface.
• Annotate it correctly so Spring recognizes it.
• Rerun your application. Observe what happens.

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.stereotype.Component;

@Component
public class Dog implements Pet {
    private static final Logger logger = LoggerFactory.getLogger(Dog.class);

    @Override
    public void feed() {
        logger.info("Feeding the dog...");
    }
}

Issue and Task: Resolving Bean Ambiguity

When you run the application after adding Dog, you will encounter an issue:

Field pet in Person required a single bean, but 2 were found: cat, dog.

Task:

• Research and apply the correct annotation (@Qualifier) to explicitly define which pet implementation (Cat or Dog) should be injected.
• Run the application again and verify that the chosen pet feeds correctly.

@Component
public class Person {
    private static final Logger logger = LoggerFactory.getLogger(Person.class);
    private final Pet pet;

    public Person(@Qualifier("dog") Pet pet) { // Use "cat" to inject Cat instead
        this.pet = pet;
    }

    public void feedPet() {
        logger.info("Feeding the pet...");
        pet.feed();
    }
}

Optional Advanced Task: Dynamic Pet Selection Service

Task:

• Create a new service class (PetService) that returns either the Cat or Dog implementation based on a command-line argument.
• Update your application to dynamically inject the chosen pet at runtime based on input arguments.

@Component
public class Person {
    private static final Logger logger = LoggerFactory.getLogger(Person.class);
    private final Pet pet;

    public Person(Pet pet) { // Remove the qualifier for Pet
        this.pet = pet;
    }

    public void feedPet() {
        logger.info("Feeding the pet...");
        pet.feed();
    }
}

@SpringBootApplication
public class LabOneDemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(LabOneDemoApplication.class, args);
    }

    @Bean
    public CommandLineRunner demo(ApplicationContext context) {
        return args -> {
            String petType = args.length > 0 ? args[0] : "dog";
            Pet pet = context.getBean(petType, Pet.class);
            Person person = new Person(pet);
            person.feedPet();
        };
    }
}

Example run command:

./mvnw spring-boot:run -Dspring-boot.run.arguments=dog

Verify the output dynamically changes depending on the provided argument.