Spring Field Injection

This is the form of Dependency Injection that requires almost no boilerplate code, contrary to setter or constructor injections. Let’s see how it works!

Spring bean to inject as dependency

To start with, we’ll specify an interface for the dependency:

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

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

And the actual Spring Bean implementation that will be injected:

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

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, 3rd 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 using field injection

Here we define another Spring Bean, but we are going to inject /BookRepository/ using field injection. To do that we have to annotate appropriate fields using @Autowired (or @Inject or @Resource) annotation:

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

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

@Component
public class Library {

    @Autowired
    private BookRepository bookRepository;

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

It works without any setter, because underneath Spring is using reflection to inject dependencies.

Constructor or field injection

Although constructor injection allows to create beans as immutable objects (fields can be marked using final modifier) and makes dependencies explicit it requires to write some boilerplate code. Because of that, in practice, field injection is used.

Example App using Spring Boot

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

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

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

@Configuration
@ComponentScan
public class FieldInjection {

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

        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, 3rd edition
Title 2: Java Concurrency in Practice

References:

• Spring Reference Documentation
• Spring Constructor Injection
• Spring Setter Injection