Spring Constructor Injection

This is one of the simplest and cleanest types of Dependency Injection in the Spring Framework. In this post we show how to use it with help of Spring annotations.

Spring bean to inject as dependency

Let’s specify an interface for dependency:

package com.farenda.spring.tutorial.injection.constructor;

public interface BookRepository {
    String titleById(int id);
}

And the actual Spring Bean implementation that will be injected:

package com.farenda.spring.tutorial.injection.constructor;

import org.springframework.stereotype.Repository;

import java.util.HashMap;
import java.util.Map;

@Repository
public class InMemoryBookRepository implements BookRepository {

    // It's our local database ;-)
    private Map<Integer, String> books = new HashMap<>();

    {
        books.put(1, "Effective Java, 2nd edition");
        books.put(2, "Java Concurrency in Practice");
        books.put(3, "Spring in Action");
    }

    @Override
    public String titleById(int id) {
        return books.get(id);
    }
}

Spring component with dependency injected by constructor

Here we define another Spring Bean, but this time we are going to inject BookRepository through constructor, using @Autowired annotation:

package com.farenda.spring.tutorial.injection.constructor;

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

@Component
public class Library {

    private BookRepository bookRepository;

    @Autowired
    public Library(BookRepository bookRepository) {
        this.bookRepository = bookRepository;
    }

    public String findBook(int id) {
        return bookRepository.titleById(id);
    }
}

Spring resolves dependencies using argument’s type. If it finds a bean matching the type, then the bean is instantiated and injected. Else exception is thrown.

Example App using Spring Boot

Here’s the simplest Spring Boot application to run the above code:

package com.farenda.spring.tutorial.injection.constructor;

import org.springframework.boot.SpringApplication;
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

@Configuration
@ComponentScan // scan this package for Spring beans
public class ConstructorInjection {

    public static void main(String[] args) {
        ApplicationContext context = SpringApplication
                .run(ConstructorInjection.class, args);

        // Get bean from the Spring Application Context:
        Library library = context.getBean(Library.class);

        System.out.println("Title 1: " + library.findBook(1));
        System.out.println("Title 2: " + library.findBook(2));
    }
}

The above code produces the following output:

Title 1: Effective Java, 2nd edition
Title 2: Java Concurrency in Practice

References:

• Check out Spring Boot “Hello, world” if you are new to Spring Boot