Spring's IoC container is built as the core module of the Spring architecture. IoC is also known as DI. It is a design pattern which eliminates the dependency of the code to provide ease in managing and testing the application. In DI, the objects themselves characterize their dependencies with the other objects they work, just through constructor arguments, arguments to a factory method, or properties that are set on the object instance after it is created or returned from a factory method. 

The container is then responsible to inject those dependencies when it creates the bean. This process is basically the inverse (so it is known as IoC) of the bean itself controlling the instantiation or location of its dependencies, by using the direct construction of classes, or a mechanism.

There are two main base packages of the Spring Framework's IoC container: org.springframework.beans, and org.springframework.context. The BeanFactory interface provides some of the advanced-level configuration mechanisms to manage any type of object. ApplicationContext includes all the functionalities of BeanFactory, and acts as a subinterface of it. In fact, ApplicationContext is also recommended over BeanFactory, and provides more supporting infrastructure that enables: easier integration with Spring's AOP features and transaction; message resource handling in terms of internationalization and event publication; and application layer-specific contexts such as WebApplicationContext for use in web applications.

The interface org.springframework.context.ApplicationContext is represented as the Spring IoC container, and it is in complete control of a bean's life cycle and responsible for instantiating, configuring, and assembling the beans.

The container gets all the instructions to instantiate, configure, and assemble, by scanning bean configuration metadata. The configuration metadata can be represented using the following methods:

• XML-based configuration
• Annotation-based configuration
• Java-based configuration

We will learn these methods in more detail in Chapter 2, Spring Best Practices and Bean Wiring Configurations.

The following diagram represents a simple representation of the Spring Container process towards creating a fully configured application:

The following example shows the basic structure of XML-based configuration metadata:

<?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">

 <!-- All the bean configuration goes here -->
<bean id="..." class="...">

</bean>

<!-- more bean definitions go here -->

</beans>

The id attribute is a string that you use to identify the individual bean definition. The class attribute defines the type of bean, and uses the fully qualified class name. The value of the id attribute refers to collaborating objects.