This Spring tutorial helps you understand how to use Java annotations to configure dependency injection for classes in an application. Besides using XML for dependency injection configuration, Spring also allows programmers to embed some special annotations into Java classes to do the same thing.

When the application is being loaded, the Spring IoC (Inversion of Control) container scans the classes and if Spring-annotated classes are found, it creates instances of these classes and wires them together according to the annotations used - hence dependency injection is made.

NOTE: If you want to understand the core concepts of dependency injection, please read What is Dependency Injection with Java Code Example tutorial.

 

1. Understand Spring Annotations for Dependency Injection

The first annotation you can use to mark a class as a managed bean in Spring is the @Component annotation. For example:

import org.springframework.stereotype.Component;

@Component("client1")
public class MyClientImpl implements MyClient {
…
}

Here, the class MyClientImpl is marked with the @Component annotation so Spring will create an instance of this class and manage it as a bean with the name client1 in the container.

To tell Spring to inject an instance of another class into this class, declare an instance field with the @Autowired annotation, for example:

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

@Component("client1")
public class MyClientImpl implements MyClient {

	@Autowired
	private MyService service;

	...
}

Here, Spring will find an instance of MyServicewith the name serviceand inject into the instance client1of MyClientImpl class. Therefore, an implementation class of MyService should be annotated like this:

import org.springframework.stereotype.Component;

@Component("service")
public class MyServiceImpl2 implements MyService {
…
}

You see, this MyServiceImpl2class is annotated with the @Component annotation with the name service which matches the name of the corresponding field in the MyClientImplclass:

@Autowired
private MyService service;

Also the MyServiceImpl2class must implements the MyService interface to match the type of the autowired field in the MyClientImplclass. You see, using the @Autowired annotation on a field is simpler than setter and constructor injection.

 

NOTES:

- There cannot have two beans in the IoC container with the same name, so the name you specify in the @Component must be unique.

- You can also mark a class with @Service and @Repository annotations. These annotations have same technical purpose as the @Component annotation. They have different names to mark classes in different layers of the application.

- The @Autowired annotation can be also applied on setter method and constructor.

Now, let’s see how to create sample project in Eclipse IDE to demonstrate dependency injection with Spring framework.

 

2. Create Maven Project in Eclipse

To create a simple Maven project in Eclipse, click menu File > New > Maven Project. Then check the option Create a simple project (skip archetype selection). Enter project’s Group Id and Artifact Id, and then add the following XML in the pom.xml file:

<dependencies>
  <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-context</artifactId>
      <version>5.0.7.RELEASE</version>
  </dependency>
</dependencies>

This dependency spring-context is the minimum requirement to use dependency injection with Spring.

Next, write code for Java classes in the package net.codejava under the src/main/java folder as below:

Code of the MyClient interface:

package net.codejava;

public interface MyClient {
	void doSomething();
}

 

Code of the ClientImpl class:

package net.codejava;

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

@Component("client1")
public class MyClientImpl implements MyClient {

	@Autowired
	private MyService service;

	@Override
	public void doSomething() {
		String info = service.getInfo();
		System.out.println(info);
	}
	
}

 

Code of the MyService interface:

package net.codejava;

public interface MyService {
	String getInfo();
}

 

An implementation of MyService interface:

package net.codejava;

import org.springframework.stereotype.Component;

@Component("service1")
public class MyServiceImpl1 implements MyService {

	@Override
	public String getInfo() {
		return "Service 1's Info";
	}

}

 

Another implementation of MyService interface:

package net.codejava;

import org.springframework.stereotype.Component;

@Component("service")
public class MyServiceImpl2 implements MyService {

	@Override
	public String getInfo() {
		return "Service 2's Info";
	}

}

 

3. Test Spring Dependency Injection with Annotations

Create a class with main method as shown below:

package net.codejava;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class SpringDependencyInjectionExample {

	public static void main(String[] args) {
		AnnotationConfigApplicationContext appContext = new AnnotationConfigApplicationContext(); 
		appContext.scan("net.codejava");
		appContext.refresh();
		
		MyClient client = (MyClient) appContext.getBean("client1");
		client.doSomething();
	}

} 

Here, you see an instance of AnnotationConfigApplicationContextis created to scan a Java package to instantiate Spring-annotated classes as managed beans in the container:

appContext.scan("net.codejava");

Then we need to refresh the application context to update the changes after scanning:

appContext.refresh();

Then we get the bean named client1 in the container, cast it to MyClient type and invoke its method: 

MyClient client = (MyClient) appContext.getBean("client1");
client.doSomething();

The program prints the following output:

Service 2's Info

That’s basically how to use Spring annotations @Component and @Autowired to configure dependency injection in an application. We hope you found this tutorial helpful in terms of helping you getting started with Spring - one of the most popular frameworks for developing enterprise Java applications.

 

References:

• Annotation-based configuration (Spring docs)

 

Related Spring Dependency Injection Tutorials: 

• Spring Dependency Injection Example with XML Configuration
• Spring Dependency Injection Example with Java Config

 

Other Spring Tutorials:

• Understand the core of Spring framework
• Understand Spring MVC
• Understand Spring AOP
• Spring MVC beginner tutorial with Spring Tool Suite IDE
• Spring MVC Form Handling Tutorial
• Spring MVC Form Validation Tutorial
• 14 Tips for Writing Spring MVC Controller
• Spring Web MVC Security Basic Example (XML Configuration)
• Understand Spring Data JPA with Simple Example