Spring - Understanding Inversion of Control with Example
Spring IoC (Inversion of Control) Container is the core of the Spring Framework. It creates objects (beans), configures them, injects dependencies, and manages their life cycles. The container uses Dependency Injection (DI) to manage application components. It retrieves object configuration from XML files, Java-based configuration, annotations, or POJOs. Since the Spring IoC container, not the developer, controls the object lifecycle and dependency management, this concept is called Inversion of Control (IoC).
Spring IoC Container
The Spring IoC Container is the core of the Spring Framework. It is responsible for managing the lifecycle of beans and their dependencies.
There are two types of IoC containers in Spring:
1. BeanFactory
- The BeanFactory is the most basic version of the IoC container.
- It provides basic support for dependency injection and bean lifecycle management.
- It is suitable for lightweight applications where advanced features are not required.
2. ApplicationContext
- The ApplicationContext is an advancement of the BeanFactory and provides additional features like event propagation, internationalization, and more.
- It is the preferred choice for most Spring applications due to its advanced capabilities.
Key Features of IoC Container
The key features of IoC Container are listed below
- Dependency Injection: Automatically injects dependencies into our classes.
- Lifecycle Management: Manages the lifecycle of beans, including instantiation, initialization, and deletion.
- Configuration Flexibility: Supports both XML-based and annotation-based configurations.
- Loose Coupling: Promotes loose coupling by decoupling the implementation of objects from their usage.
Understanding IoC in Spring with a Practical Example
Step 1: Create the Sim Interface
So now let's understand what is IoC in Spring with an example. Suppose we have one interface named Sim and it has some abstract methods calling() and data().
// Java Program to Illustrate Sim Interface
public interface Sim
{
void calling();
void data();
}
Step 2: Implement the Sim interface
Now we have created another two classes, Airtel and Jio which implement the Sim interface and override its methods..
public class Airtel implements Sim {
@Override
public void calling() {
System.out.println("Airtel Calling");
}
@Override
public void data() {
System.out.println("Airtel Data");
}
}
public class Jio implements Sim {
@Override
public void calling() {
System.out.println("Jio Calling");
}
@Override
public void data() {
System.out.println("Jio Data");
}
}
Step 3: Calling Methods Without Spring IoC
Without Spring IoC, we would manually create instances of the Sim implementation in the main method. For example:
// Java Program to Illustrate Mobile Class
public class Mobile {
// Main driver method
public static void main(String[] args)
{
// Manually creating an instance of Jio
Sim sim = new Jio();
// Calling methods
sim.calling();
sim.data();
}
}
This approach tightly couples the Mobile class to the Jio implementation. If we want to switch to Airtel, we need to modify the source code.
Step 4: Using Spring IoC with XML Configuration
To avoid tight coupling, we can use the Spring IoC container. First, we create an XML configuration file (beans.xml) to define the beans.
Example: beans.xml File
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
https://www.springframework.org/schema/beans/spring-beans.xsd">
<!-- Define the Jio bean -->
<bean id="sim" class="Jio"></bean>
</beans>
Explanation: In the beans.xml file, we have created beans. So inside the id, we have to pass the unique id and inside the class, we have to pass the Class name for which you want to create the bean. Later on, inside the main method, we can tweek it out that will be described in the upcoming program.
Bean Definition: In Spring, the objects that form the backbone of your application and that are managed by the Spring IoC container are called beans. A bean is an object that is instantiated, assembled, and otherwise managed by a Spring IoC container.
Step 5: Run the Code
In the Mobile class, we use the ApplicationContext to retrieve the bean:
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class Mobile {
public static void main(String[] args) {
// Using ApplicationContext tom implement Spring IoC
ApplicationContext applicationContext
= new ClassPathXmlApplicationContext("beans.xml");
// Get the bean
Sim sim = applicationContext.getBean("sim", Sim.class);
// Calling the methods
sim.calling();
sim.data();
}
}
Output:
Jio Calling
Jio Data
And now if you want to use the Airtel sim so you have to change only inside the beans.xml file. The main method is going to be the same.
<bean id="sim" class="Airtel"></bean>
Implementation:
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class Mobile {
public static void main(String[] args) {
// Using ApplicationContext tom implement Spring IoC
ApplicationContext applicationContext
= new ClassPathXmlApplicationContext("beans.xml");
// Get the bean
Sim sim = applicationContext.getBean("sim", Sim.class);
// Calling the methods
sim.calling();
sim.data();
}
}
Output:
Airtel Calling
Airtel Data
Step 6: Using Java-Based Configuration
Modern Spring applications often use Java-based configuration instead of XML. Let’s see how to configure the same example using Java-based configuration.
1. Create a Configuration Class: Define a configuration class using the @Configuration annotation. Use the @Bean annotation to define beans.
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
@Configuration
public class AppConfig {
@Bean
public Sim sim() {
return new Jio(); // Change to new Airtel() to switch implementations
}
}
2. Use the Configuration in the Mobile Class: Update the Mobile class to use the Java-based configuration.
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
public class Mobile {
public static void main(String[] args) {
// Load the Spring IoC container
// using Java-based configuration
ApplicationContext context
= new AnnotationConfigApplicationContext(AppConfig.class);
// Retrieve the bean
Sim sim = context.getBean("sim", Sim.class);
// Call methods
sim.calling();
sim.data();
}
}
Output:
Jio Calling
Jio Data
To switch to Airtel, simply update the AppConfig class:
@Bean
public Sim sim() {
return new Airtel();
}
Step 6: Using Annotations for Dependency Injection
Spring also supports annotation-based configuration, which is widely used in modern applications. Let’s update the example to use annotations.
1. Enable Component Scanning: Add the @ComponentScan annotation to the configuration class to enable component scanning.
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
@Configuration
@ComponentScan(basePackages = "com.example")
public class AppConfig {
}
2. Annotate Classes with @Component: Annotate the Airtel and Jio classes with @Component.
import org.springframework.stereotype.Component;
@Component
public class Airtel implements Sim {
@Override
public void calling() {
System.out.println("Airtel Calling");
}
@Override
public void data() {
System.out.println("Airtel Data");
}
}
@Component
public class Jio implements Sim {
@Override
public void calling() {
System.out.println("Jio Calling");
}
@Override
public void data() {
System.out.println("Jio Data");
}
}
3. Inject the Dependency Using @Autowired: Update the Mobile class to use @Autowired for dependency injection.
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.stereotype.Component;
@Component
public class Mobile {
@Autowired
private Sim sim;
public void useSim() {
sim.calling();
sim.data();
}
public static void main(String[] args) {
ApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
Mobile mobile = context.getBean(Mobile.class);
mobile.useSim();
}
}
Output:
Jio Calling
Jio Data
Note: When we run the above code, Spring will throw a NoUniqueBeanDefinitionException because it finds two beans (Airtel and Jio) of type Sim. To resolve this, we need to specify which bean to inject.
Fixing The NoUniqueBeanDefinitionException
1. Use @Primary Annotation
Mark one of the beans as the primary bean using the @Primary annotation.
@Component
@Primary
public class Airtel implements Sim {
// Methods implementation
}
2. Use @Qualifier Annotation
Use the @Qualifier annotation to specify which bean to inject
@Component
public class Mobile {
@Autowired
@Qualifier("jio") // Specify the bean name
private Sim sim;
public void useSim() {
sim.calling();
sim.data();
}
public static void main(String[] args) {
ApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
Mobile mobile = context.getBean(Mobile.class);
mobile.useSim();
}
}
3. Explicit Bean Names
Explicitly name the beans and use those names in the @Qualifier annotation.
@Component("airtelBean")
public class Airtel implements Sim {
// Methods implementation
}
@Component("jioBean")
public class Jio implements Sim {
// Methods implementation
}
Then in the Mobile class,
@Component
public class Mobile {
@Autowired
@Qualifier("jioBean") // Use the explicit bean name
private Sim sim;
public void useSim() {
sim.calling();
sim.data();
}
public static void main(String[] args) {
ApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
Mobile mobile = context.getBean(Mobile.class);
mobile.useSim();
}
}
Conclusion
In this article, we explored the concept of Inversion of Control (IoC) in Spring and demonstrated its implementation using:
- XML Configuration
- Java-Based Configuration
- Annotation-Based Configuration
