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Spring Dependency Injection

Published on August 3, 2022
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By Pankaj
Developer and author at DigitalOcean.
Spring Dependency Injection

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Today we will look into Spring Dependency Injection. Spring Framework core concepts are “Dependency Injection” and “Aspect Oriented Programming”. I have written earlier about Java Dependency Injection and how we can use Google Guice framework to automate this process in our applications.

Spring Dependency Injection

spring dependency injection, spring DI, spring @inject This tutorial is aimed to provide details about Spring Dependency Injection example with both annotation based configuration and XML file based configuration. I will also provide JUnit test case example for the application, since easy testability is one of the major benefits of dependency injection. I have created spring-dependency-injection maven project whose structure looks like below image. Spring Dependency Injection Let’s look at each of the components one by one.

Spring Dependency Injection - Maven Dependencies

I have added Spring and JUnit maven dependencies in pom.xml file, final pom.xml code is below.

<project xmlns="https://maven.apache.org/POM/4.0.0" xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="https://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<groupId>com.journaldev.spring</groupId>
	<artifactId>spring-dependency-injection</artifactId>
	<version>0.0.1-SNAPSHOT</version>

	<dependencies>
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-context</artifactId>
			<version>4.0.0.RELEASE</version>
		</dependency>
		<dependency>
			<groupId>junit</groupId>
			<artifactId>junit</artifactId>
			<version>4.8.1</version>
			<scope>test</scope>
		</dependency>
	</dependencies>

</project>

Current stable version of Spring Framework is 4.0.0.RELEASE and JUnit current version is 4.8.1, if you are using any other versions then there might be a small chance that the project will need some change. If you will build the project, you will notice some other jars are also added to maven dependencies because of transitive dependencies, just like above image.

Spring Dependency Injection - Service Classes

Let’s say we want to send email message and twitter message to the users. For dependency injection, we need to have a base class for the services. So I have MessageService interface with single method declaration for sending message.

package com.journaldev.spring.di.services;

public interface MessageService {

	boolean sendMessage(String msg, String rec);
}

Now we will have actual implementation classes to send email and twitter message.

package com.journaldev.spring.di.services;

public class EmailService implements MessageService {

	public boolean sendMessage(String msg, String rec) {
		System.out.println("Email Sent to "+rec+ " with Message="+msg);
		return true;
	}

}
package com.journaldev.spring.di.services;

public class TwitterService implements MessageService {

	public boolean sendMessage(String msg, String rec) {
		System.out.println("Twitter message Sent to "+rec+ " with Message="+msg);
		return true;
	}

}

Now that our services are ready, we can move on to Component classes that will consume the service.

Spring Dependency Injection - Component Classes

Let’s write a consumer class for above services. We will have two consumer classes - one with Spring annotations for autowiring and another without annotation and wiring configuration will be provided in the XML configuration file.

package com.journaldev.spring.di.consumer;

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

import com.journaldev.spring.di.services.MessageService;

@Component
public class MyApplication {

	//field-based dependency injection
	//@Autowired
	private MessageService service;
	
//	constructor-based dependency injection	
//	@Autowired
//	public MyApplication(MessageService svc){
//		this.service=svc;
//	}
	
	@Autowired
	public void setService(MessageService svc){
		this.service=svc;
	}
	
	public boolean processMessage(String msg, String rec){
		//some magic like validation, logging etc
		return this.service.sendMessage(msg, rec);
	}
}

Few important points about MyApplication class:

  • @Component annotation is added to the class, so that when Spring framework will scan for the components, this class will be treated as component. @Component annotation can be applied only to the class and it’s retention policy is Runtime. If you are not not familiar with Annotations retention policy, I would suggest you to read java annotations tutorial.
  • @Autowired annotation is used to let Spring know that autowiring is required. This can be applied to field, constructor and methods. This annotation allows us to implement constructor-based, field-based or method-based dependency injection in our components.
  • For our example, I am using method-based dependency injection. You can uncomment the constructor method to switch to constructor based dependency injection.

Now let’s write similar class without annotations.

package com.journaldev.spring.di.consumer;

import com.journaldev.spring.di.services.MessageService;

public class MyXMLApplication {

	private MessageService service;

	//constructor-based dependency injection
//	public MyXMLApplication(MessageService svc) {
//		this.service = svc;
//	}
	
	//setter-based dependency injection
	public void setService(MessageService svc){
		this.service=svc;
	}

	public boolean processMessage(String msg, String rec) {
		// some magic like validation, logging etc
		return this.service.sendMessage(msg, rec);
	}
}

A simple application class consuming the service. For XML based configuration, we can use implement either constructor-based spring dependency injection or method-based spring dependency injection. Note that method-based and setter-based injection approaches are same, it’s just that some prefer calling it setter-based and some call it method-based.

Spring Dependency Injection Configuration with Annotations

For annotation based configuration, we need to write a Configurator class that will be used to inject the actual implementation bean to the component property.

package com.journaldev.spring.di.configuration;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

import com.journaldev.spring.di.services.EmailService;
import com.journaldev.spring.di.services.MessageService;

@Configuration
@ComponentScan(value={"com.journaldev.spring.di.consumer"})
public class DIConfiguration {

	@Bean
	public MessageService getMessageService(){
		return new EmailService();
	}
}

Some important points related to above class are:

  • @Configuration annotation is used to let Spring know that it’s a Configuration class.
  • @ComponentScan annotation is used with @Configuration annotation to specify the packages to look for Component classes.
  • @Bean annotation is used to let Spring framework know that this method should be used to get the bean implementation to inject in Component classes.

Let’s write a simple program to test our annotation based Spring Dependency Injection example.

package com.journaldev.spring.di.test;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

import com.journaldev.spring.di.configuration.DIConfiguration;
import com.journaldev.spring.di.consumer.MyApplication;

public class ClientApplication {

	public static void main(String[] args) {
		AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(DIConfiguration.class);
		MyApplication app = context.getBean(MyApplication.class);
		
		app.processMessage("Hi Pankaj", "pankaj@abc.com");
		
		//close the context
		context.close();
	}

}

AnnotationConfigApplicationContext is the implementation of AbstractApplicationContext abstract class and it’s used for autowiring the services to components when annotations are used. AnnotationConfigApplicationContext constructor takes Class as argument that will be used to get the bean implementation to inject in component classes. getBean(Class) method returns the Component object and uses the configuration for autowiring the objects. Context objects are resource intensive, so we should close them when we are done with it. When we run above program, we get below output.

Dec 16, 2013 11:49:20 PM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@3067ed13: startup date [Mon Dec 16 23:49:20 PST 2013]; root of context hierarchy
Email Sent to pankaj@abc.com with Message=Hi Pankaj
Dec 16, 2013 11:49:20 PM org.springframework.context.support.AbstractApplicationContext doClose
INFO: Closing org.springframework.context.annotation.AnnotationConfigApplicationContext@3067ed13: startup date [Mon Dec 16 23:49:20 PST 2013]; root of context hierarchy

Spring Dependency Injection XML Based Configuration

We will create Spring configuration file with below data, file name can be anything. applicationContext.xml code:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="https://www.springframework.org/schema/beans"
	xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="
https://www.springframework.org/schema/beans https://www.springframework.org/schema/beans/spring-beans-4.0.xsd">

<!-- 
	<bean id="MyXMLApp" class="com.journaldev.spring.di.consumer.MyXMLApplication">
		<constructor-arg>
			<bean class="com.journaldev.spring.di.services.TwitterService" />
		</constructor-arg>
	</bean>
-->
	<bean id="twitter" class="com.journaldev.spring.di.services.TwitterService"></bean>
	<bean id="MyXMLApp" class="com.journaldev.spring.di.consumer.MyXMLApplication">
		<property name="service" ref="twitter"></property>
	</bean>
</beans>

Notice that above XML contains configuration for both constructor-based and setter-based spring dependency injection. Since MyXMLApplication is using setter method for injection, the bean configuration contains property element for injection. For constructor based injection, we have to use constructor-arg element. The configuration XML file is placed in the source directory, so it will be in the classes directory after build. Let’s see how to use XML based configuration with a simple program.

package com.journaldev.spring.di.test;

import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.journaldev.spring.di.consumer.MyXMLApplication;

public class ClientXMLApplication {

	public static void main(String[] args) {
		ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext(
				"applicationContext.xml");
		MyXMLApplication app = context.getBean(MyXMLApplication.class);

		app.processMessage("Hi Pankaj", "pankaj@abc.com");

		// close the context
		context.close();
	}

}

ClassPathXmlApplicationContext is used to get the ApplicationContext object by providing the configuration files location. It has multiple overloaded constructors and we can provide multiple config files also. Rest of the code is similar to annotation based configuration test program, the only difference is the way we get the ApplicationContext object based on our configuration choice. When we run above program, we get following output.

Dec 17, 2013 12:01:23 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@4eeaabad: startup date [Tue Dec 17 00:01:23 PST 2013]; root of context hierarchy
Dec 17, 2013 12:01:23 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [applicationContext.xml]
Twitter message Sent to pankaj@abc.com with Message=Hi Pankaj
Dec 17, 2013 12:01:23 AM org.springframework.context.support.AbstractApplicationContext doClose
INFO: Closing org.springframework.context.support.ClassPathXmlApplicationContext@4eeaabad: startup date [Tue Dec 17 00:01:23 PST 2013]; root of context hierarchy

Notice that some of the output is written by Spring Framework. Since Spring Framework uses log4j for logging purpose and I have not configured it, the output is getting written to console.

Spring Dependency Injection JUnit Test Case

One of the major benefit of dependency injection in spring is the ease of having mock service classes rather than using actual services. So I have combined all of the learning from above and written everything in a single JUnit 4 test class for dependency injection in spring.

package com.journaldev.spring.di.test;

import org.junit.Assert;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

import com.journaldev.spring.di.consumer.MyApplication;
import com.journaldev.spring.di.services.MessageService;

@Configuration
@ComponentScan(value="com.journaldev.spring.di.consumer")
public class MyApplicationTest {
	
	private AnnotationConfigApplicationContext context = null;

	@Bean
	public MessageService getMessageService() {
		return new MessageService(){

			public boolean sendMessage(String msg, String rec) {
				System.out.println("Mock Service");
				return true;
			}
			
		};
	}

	@Before
	public void setUp() throws Exception {
		context = new AnnotationConfigApplicationContext(MyApplicationTest.class);
	}
	
	@After
	public void tearDown() throws Exception {
		context.close();
	}

	@Test
	public void test() {
		MyApplication app = context.getBean(MyApplication.class);
		Assert.assertTrue(app.processMessage("Hi Pankaj", "pankaj@abc.com"));
	}

}

The class is annotated with @Configuration and @ComponentScan annotation because getMessageService() method returns the MessageService mock implementation. That’s why getMessageService() is annotated with @Bean annotation. Since I am testing MyApplication class that is configured with annotation, I am using AnnotationConfigApplicationContext and creating it’s object in the setUp() method. The context is getting closed in tearDown() method. test() method code is just getting the component object from context and testing it. Do you wonder how Spring Framework does the autowiring and calling the methods that are unknown to Spring Framework. It’s done with the heavy use of Java Reflection that we can use to analyze and modify the behaviors of the classes at runtime.

Download Spring Dependency Injection Project

Download the sample Spring Dependency Injection (DI) project from above URL and play around with it to learn more.


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About the authors
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Pankaj

author

Developer and author at DigitalOcean.

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Was this helpful?

Great Article. I am new to Spring. Thoroughly understood this part. in my implementation, forgot to add @Autowired annotation and getting null pointer exception. Thanks for going in so intricate details regarding annotations and their uses in spring project.

- Ashish

    No words to say, nice tutorial…

    - MJ

      Nice explanation of the concept good work!

      - nitish khadelwal

        Great blog Pankaj, I can only support you by turning off the my Ads-Blocker, Thank a lot.

        - aduckdev

          Hello! I have downloaded the project and created it in eclipse and all the maven repository dependencies have been resolved except for org.junit. The jar actually shows up under the Maven Dependencies folder but it is ‘grayed out’. Also, the MyApplicationTest.java file has the exception that it cannot find org.junit. Any ideas? Thanks!

          - Daniel Grindstaff

            Hi , Pankaj here is another way to test your application by Mockito framework . public class MyApplicationTest { @Test public void test() { MyApplication app =new MyApplication app(); MessageService mockito= mock(MessageService.class); when(mockito.sendMessage(“Hi Pankaj”, “pankaj@abc.com”)).thenReturn(true); app.setService(mockito); Assert.assertTrue(app.processMessage(“Hi Pankaj”, “pankaj@abc.com”)); } } pom.xml modification org.mockito mockito-core 2.0.73-beta

            - Raj Gopal

              Can you also please provide the code for Spring Dependency Injection JUnit Test Case in for XML Based Configuration

              - RajGopal

                In class DIConfiguration EmailService() instance is returned . Please also return the instance of TwitterService and use @Qualifier annotation to perform autowiring . Qualifier annotation https://dzone.com/articles/a-guide-to-spring-framework-annotations

                - RajGopal

                  Super Explanation . Great tutorial .

                  - Raj Gopal

                    Hi, I’m using the annotation based configuration and what if I want to send message by Email then by Twitter?

                    - Triet