Play Store Application link – Spring Framework in 9 steps – Apps on Google Play
Understanding Inversion of Control (IoC) in Spring
IoC is a design principle where the control of creating objects is transferred from the application code to a configuration file (either XML or annotations). This helps to decouple the code and manage dependencies more effectively.
Dependency Injection (DI)
In Spring, IoC is achieved through Dependency Injection (DI). Instead of instantiating objects directly, you configure how they should be created and wired together in a separate configuration file.
Example Program Without IoC
Here’s how you might implement a simple text editor without IoC:
public class Main {
public static void main(String[] args) {
// Without IoC: Creating dependencies manually (tight coupling)
SpellChecker spellChecker = new SimpleSpellChecker();
TextEditor textEditor = new TextEditor(spellChecker);
textEditor.spellCheck();
}
}
public class TextEditor {
private SpellChecker spellChecker;
public TextEditor(SpellChecker spellChecker) {
this.spellChecker = spellChecker;
}
public void spellCheck() {
spellChecker.checkSpelling();
}
}
public interface SpellChecker {
void checkSpelling();
}
public class SimpleSpellChecker implements SpellChecker {
@Override
public void checkSpelling() {
System.out.println("Checking spelling using SimpleSpellChecker...");
}
}
public class AdvancedSpellChecker implements SpellChecker {
@Override
public void checkSpelling() {
System.out.println("Checking spelling using AdvancedSpellChecker...");
}
}
Example Program With IoC
Now, let’s see how we can achieve the same functionality using Spring IoC with XML configuration:
applicationContext.xml
<beans>
<bean id="textEditor" class="com.example.TextEditor">
<constructor-arg ref="advancedSpellChecker"/>
</bean>
<bean id="simpleSpellChecker" class="com.example.SimpleSpellChecker"/>
<bean id="advancedSpellChecker" class="com.example.AdvancedSpellChecker"/>
</beans>
Main.java
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class Main {
public static void main(String[] args) {
// With IoC: Beans are created and wired by Spring
ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
TextEditor textEditor = context.getBean("textEditor", TextEditor.class);
textEditor.spellCheck();
}
}
Benefits of IoC
Using IoC makes managing dependencies easier, especially in large applications with many classes. It promotes loose coupling and simplifies testing.
Types of IoC Containers
- BeanFactory: Basic container for managing beans. It uses lazy initialization (beans are created on demand).
- ApplicationContext: A more advanced container that supports features like event propagation and internationalization. It uses eager initialization (all beans are created at startup).
Types of Dependency Injection
- Constructor-Based DI:
public class MyBean {
private final Dependency dependency;
public MyBean(Dependency dependency) {
this.dependency = dependency;
}
}
XML Configuration:
<bean id="myBean" class="com.example.MyBean">
<constructor-arg ref="dependency"/>
</bean>
- Setter-Based DI:
public class MyBean {
private Dependency dependency;
public void setDependency(Dependency dependency) {
this.dependency = dependency;
}
}
XML Configuration:
<bean id="myBean" class="com.example.MyBean">
<property name="dependency" ref="dependency"/>
</bean>
Github project link – https://github.com/kuldeep101990/SpringIOC_DependencyInjection
Types of AutoWiring –
| Autowire Mode | Description | Matching Criteria | When to Use |
|---|
| No /Default | No automatic wiring / Uses global container setting | Explicitly wired by the developer / Depends on container configuration | For full manual control over wiring. / For consistency with global settings. |
byName | Matches bean ID with property name | Property name = Bean ID | When bean IDs match property names. |
byType | Matches type of bean | Single matching bean type | When only one bean of a type exists. |
constructor | Matches constructor parameters | Constructor arguments = Bean types/values | For mandatory dependencies via constructor. |
1. Default Autowiring
XML Based: Github project link – https://github.com/kuldeep101990/SpringDefaultAutoWiringXml
Annotation Based: Github project link – https://github.com/kuldeep101990/SpringDefaultAutoWiringAnnotation
2. Autowiring by Name
XML way: Github project link – https://github.com/kuldeep101990/SpringAutoWiringByNameXml
Annotation way: Github project link – https://github.com/kuldeep101990/SpringAutoWiringByNameAnnotation
3. Autowiring by Type
XML way: Github project link – https://github.com/kuldeep101990/SpringAutoWiringByTypeXml
Annotation way: Github project link – https://github.com/kuldeep101990/SpringAutoWiringByTypeAnnotation
4. Autowiring by Constructor
XML way: Github project link – https://github.com/kuldeep101990/SpringAutoWiringByConstructorXml
Annotation way: Github project link – https://github.com/kuldeep101990/SpringAutoWiringByConstructorAnnotation