What is the Spring Dependency Injection?
Today we will look into Spring Dependency Injection. Spring Framework core concepts are “Dependency Injection” and “Aspect Oriented Programming”.
Spring Dependency Injection
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.
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.
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.
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.
- @Autowired annotation is used to let Spring know that autowiring is required. This can be applied to field, constructor and methods.
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.
Testing Annotation Based Spring Dependency Injection
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…
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…
Testing XML Based Configuration
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…
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 its 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.
