The document explains the concept of Inversion of Control (IoC) in Spring and its advantages over traditional approaches, emphasizing how IoC promotes loose coupling and easier code maintenance. It details how the Spring container manages beans through dependency injection by reading configuration metadata, which can be represented in various formats. Additionally, the document provides sample code for developing Spring applications using XML configuration and demonstrates instantiation of Spring beans.

1. Advantage of Spring IOC over traditional approach & Developing
spring application sample
The Core concept of spring is IOC[ Inversion of control ] . IoC or Inversion of Control, is also
known as Dependency Injection. It is used to create loosely coupled components. The below
example best describes the traditional approach vs loose coupling model .
Example :
Suppose there exists a class known as ImageFileWriter that creates a file and writes the Image
bytes to the File . The ImageFileWriter gets the image data from from another class known as
ImageWriter and in response it returns the instance of the file to which it writes the data.
For this situation the ImageWriter need to call the 'writeToFile' method on the instance of
ImageFileWriter class. The scenario in this case will be like the following :
The code in the ImageWriter class for the above scenario will be :-
I ImageFilewriter imagefileWriter = new ImageFileWriter();
imageFileWriter.writeToFile(byteBuffer);
Here if we are going to take a close look , then we can find that there remains a tight
dependency between the ImageWriter and ImageFileWriter class. ImageWriter class is
dependent on the ImageFileWriter class. If in future for some reason the “ImageFileWriter”
class will be changed to ImagePdfFileWriter” , then we need to change the source code of
the “ImageWriter” class as well.
Advantage of IOC here :
IoC solves the above problem of tight couple and helps to makes the software component
loosely coupled, so that we can maintain our code pretty much easily . We can modify the code
any instance of time without hampering the other classes which referring to it .
Inversion of Control is a design pattern that provides instances of a bean or class to
another when they needed. No need to use new keyword to create instance of dependent class.
Only just create a reference of dependent class and IoC pattern will set the instance to the
reference when needed .
Now using IOC concept the code inside ImageWriter class will be :
ImageFileWriter imageFileWriter = null;
public void setImageFileWriter(ImageFileWriter imageFileWriter) {

2. this.imgaeFileWriter = imagFileWriter;
}
IoC container will call setter method to set the instance of ImageFileWriter class. We have to
configure the dependency IoC configuration file. IOC configuration file indicates the IoC
container when and how to inject the dependences.
Creating a completely independent model :
Here we will find how to use IoC to create effective independent components. Considering the
last ImageWriter class. There exists an interface named “GenericBufferedWriter” which
contains following definition :
public interface GenericBufferedWriter {
public File writeToFile(byte buffer);
}
Any class implementing the above interface need to have the “writeToFile” as per it'd own way.
Now the code of the ImageWriter class will be as follows.
private GenericBufferedWriter genericBufferedwriter = null;
public void setGenericBuffer(GenericBufferedwriter
ImageWriter) {
this.genericBufferWriter = ImageWriter;
}
// Some codes
genericBufferedWriter.writeToFile(bytes);
Suppose we want to types of output . For the 1st scenario the image bytes need to be written into
a JPEG file and in the second situation the image need to be generated as a PNG file. In this
situation we can create two classes which JPEGBufferedWriter as well as PNGBufferedWriter
and both the classes need to implement the GenericBufferedwriter interface. Here we can
configure the IOC container in such a manner that ioc container will provide the instance of J
PEGBufferedWriter and PNGBufferedWriter as per the requirement. This one is an generalized
structure and this is an advantage of spring IOC design pattern.
Spring container overview :
Spring container is the core of Spring Framework. The core container manages the The
container objects till the end of there life cycle. It is responsible for creating, wiring them
together and configuring them, The Spring container uses dependency injection (DI) to
manage the components that make up an application. The container gets its instructions on what
objects to instantiate, configure, and assemble by reading configuration metadata provided. The
configuration metadata can be represented either by XML, Java annotations, or Java code. We
can see over here that it does not forces the developer to use any particular type of

3. configuration file like most of the frame work supports xml file configuration type. It also helps
to avoid the singleton programming.
The following diagram shows the spring Container architecture :
Initializing spring Core container and accessing Spring Beans:
The Spring core container can be instantiated by creating an object of BeanFactory or
ApplicationContext implementation classes supplying the spring bean configurations. The
different types of BeanFactory are classified based on the spring bean configuration format they
used to understand and the way they locate the configurations. Here is simple code snippet
describes how to instantiate the the spring container.
Using BeanFactory to instantiate spring core container :
BeanFactory beans = new XmlBeanFactory(new FileSystemResources
("demobeans.xml"));
In the above code snippet we are using the XMLBeanFactory implementation for the
instantiating the the spring container with “demobeans.xml” file aa a configuration file. We can
even use application context to instantiate the container.
Using ApplicationContext to instantiate spring core container :
ApplicationContext context = new
ClassPathXmlApplicationContext("demobeans.xml");
In this case classPathApplicationContext is used to instantiate the spring container with
“demobeans.xml” . The ClassPathApplicationContext locates the the XML configuration file in
the classpath . Once after initializing the container we can use BeanFactory object method to
access the spring beans.

4. Developing sample application :-
From the above discussion now we are having overview on Basic IOC concept along with the
spring container . Based on the above basic concept below a simple example is present
demonstrating the basic XML configuration and instantiating the spring core container. This
example contains the following parts .
DemoService.java :
This is a simple plain java class which can be used anywhere without any modification
demobeans.xml :
This is the XML configuration file for this example.
DemoServiceTest.java :
This class having the main method that instantiates the spring core container and access
the bean defined in the spring bean XML configuration file.
Note : download the following jar files and use them in the classpath of the application
spring-beans-3.0.0.RELEASE.jar , spring-context-3.0.2.RELEASE.jar,
spring-core-3.0.0.RELEASE.jar
DemoService.java
package com.spring.demo;
/**
* Service class. This is simply a POJO class
*
* @author sunilm
*
*/
public class DemoService {
String message ;
/**
* No argument constructor
*/
public DemoService( ) {
message = "This is the default message" ;
}
/**
* Parameterized constructor
* @param msg message

5. */
public DemoService(String msg) {
message = msg ;
}
/**
* Get the input message
*
* @return the message
*/
public String getMessage( ) {
return message ;
}
}
demobeans.xml :
<?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
http://www.springframework.org/schema/beans/spring-beans.xsd">
<bean id = "demoService1" class = "com.spring.demo.DemoService" />
<bean id = "demoService2" class = "com.spring.demo.DemoService">
<constructor-arg>
<value>
demonstration of xml configuration file
</value>
</constructor-arg>
</bean>
</beans>
Description :
The <beans> tag is the root element of the spring beans xml configuration document. The bean
tag defines the spring bean that is the Plain Old Java Object to initialize and manage by the
spring ore container. The xml document of the above example includes two bean definition. Out
of which one initializes the no argument constructor , which is referred with the name
'demoService1' and the other initializes the DemoService class using one argument constructor
passing the message 'demonstration of xml configuration file' as a value where the definition is
referred to with the name demoService2.
DemoServiceTest.java
package com.spring.demo;

6. import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.xml.XmlBeanFactory;
import org.springframework.core.io.FileSystemResource;
/**
* Demonstration of instantiating the bean factory and accessing the
spring beans
*
* @author sunilm
*
*/
public class DemoServiceTest {
public static void main (String args[]) {
BeanFactory beans = new XmlBeanFactory(new
FileSystemResource("demobeans.xml"));
// get the instance of no argument constructor[demoservice1]
DemoService demoService1 = (DemoService)beans.getBean
("demoService1");
System.out.println("Mesage from the demoService1 bean :");
System.out.println(demoService1.getMessage());
// get the instance of parameterized constructor[demoservice2]
DemoService demoService2 = (DemoService)beans
.getBean("demoService2");
System.out.println("Mesage from the demoService2 bean :");
System.out.println(demoService2.getMessage());
}
}
The output of the above application will be as follows :-
Mesage from the demoService1 bean :
This is the default message
Mesage from the demoService2 bean :
demonstration of xml configuration file

7. I hope the above session will provide an overall idea on the spring IOC and it's
advantages . In the next session I will go more details on the various aspects of spring IOC.
Thanks