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Spring ─ IoC Containers

The Spring container is at the core of the Spring Framework. The container will create the objects, wire them together, configure them, and manage their complete life cycle from creation till destruction. The Spring container uses DI to manage the components that make up an application. These objects are called Spring Beans, which we will discuss in the next chapter.

The container gets its instructions on what objects to instantiate, configure, and assemble by reading the configuration metadata provided. The configuration metadata can be represented either by XML, Java annotations, or Java code. The following diagram represents a high-level view of how Spring works. The Spring IoC container makes use of Java POJO classes and configuration metadata to produce a fully configured and executable system or application.

Spring provides the following two distinct types of containers.

Sr. No.	Container & Description
1	Spring BeanFactory Container This is the simplest container providing the basic support for DI and is defined by the org.springframework.beans.factory.BeanFactoryinterface. The BeanFactory and related interfaces, such as BeanFactoryAware, InitializingBean, DisposableBean, are still present in Spring for the purpose of backward compatibility with a large number of third-party frameworks that integrate with Spring.
2	Spring ApplicationContext Container This container adds more enterprise-specific functionality such as the ability to resolve textual messages from a properties file and the ability to publish application events to interested event listeners. This container is defined by theorg.springframework.context.ApplicationContext interface.

Spring BeanFactory Container

This is the simplest container providing the basic support for DI and defined by the org.springframework.beans.factory.BeanFactory interface. The BeanFactory and related interfaces, such as BeanFactoryAware, InitializingBean, DisposableBean, are still present in Spring for the purpose of backward compatibility with a large number of third-party frameworks that integrate with Spring.

There are a number of implementations of the BeanFactory interface that are come straight out-of-the-box with Spring. The most commonly used BeanFactory implementation is the XmlBeanFactory class. This container reads the configuration metadata from an XML file and uses it to create a fully configured system or application.

The BeanFactory is usually preferred where the resources are limited like mobile devices or applet-based applications. Thus, use an ApplicationContext unless you have a good reason for not doing so.

Example

Let us take a look at a working Eclipse IDE in place and take the following steps to create a Spring application:

Steps	Description
1	Create a project with a name SpringExample and create a packagecom.jtc under the src folder in the created project.
2	Add the required Spring libraries using Add External JARs option as explained in the Spring Hello World Example chapter.
3	Create Java classes HelloWorld and MainApp under the com.jtcpackage.
4	Create Beans configuration file Beans.xml under the src folder.
5	The final step is to create the content of all the Java files and Bean Configuration file. Finally, run the application as explained below.

Here is the content of HelloWorld.java file:

package com.jtc;
public class HelloWorld {
private String message;
public void setMessage(String message){
this.message = message;
}
public void getMessage(){
System.out.println("Your Message : " + message);
}
}

Following is the content of the second file MainApp.java

package com.jtc;
import org.springframework.beans.factory.InitializingBean;
import org.springframework.beans.factory.xml.XmlBeanFactory;
import org.springframework.core.io.ClassPathResource;
public class MainApp {
public static void main(String[] args) {
XmlBeanFactory factory = new XmlBeanFactory
(new ClassPathResource("Beans.xml"));
HelloWorld obj = (HelloWorld) factory.getBean("helloWorld");
obj.getMessage();
}
}

Following two important points should be noted about the main program:

The first step is to create a factory object where we used the framework APIXmlBeanFactory() to create the factory bean andClassPathResource() API to load the bean configuration file available in CLASSPATH. The XmlBeanFactory() API takes care of creating and initializing all the objects, i.e. beans mentioned in the configuration file.
The second step is used to get the required bean using getBean() method of the created bean factory object. This method uses bean ID to return a generic object, which finally can be casted to the actual object. Once you have the object, you can use this object to call any class method.

Following is the content of the bean configuration file Beans.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-3.0.xsd">
<bean id="helloWorld" class="com.jtc.HelloWorld">
<property name="message" value="Hello World!"/>
</bean>
</beans>

Once you are done with creating the source and the bean configuration files, let us run the application. If everything is fine with your application, it will print the following message:

Your Message : Hello World!

Spring ApplicationContext Container

The Application Context is Spring's advanced container. Similar to BeanFactory, it can load bean definitions, wire beans together, and dispense beans upon request. Additionally, it adds more enterprise-specific functionality such as the ability to resolve textual messages from a properties file and the ability to publish application events to interested event listeners. This container is defined by the org.springframework.context.ApplicationContext interface.

The ApplicationContext includes all functionality of the BeanFactory. It is generally recommended over BeanFactory. BeanFactory can still be used for lightweight applications like mobile devices or applet-based applications.

The most commonly used ApplicationContext implementations are:

FileSystemXmlApplicationContext: This container loads the definitions of the beans from an XML file. Here you need to provide the full path of the XML bean configuration file to the constructor.
ClassPathXmlApplicationContext: This container loads the definitions of the beans from an XML file. Here you do not need to provide the full path of the XML file but you need to set CLASSPATH properly because this container will look like bean configuration XML file in CLASSPATH.
WebXmlApplicationContext: This container loads the XML file with definitions of all beans from within a web application.

We already have seen an example on ClassPathXmlApplicationContext container in Spring Hello World Example, and we will talk more about XmlWebApplicationContext in a separate chapter when we will discuss web-based Spring applications. So let us see one example on FileSystemXmlApplicationContext.

Example

Let us have a working Eclipse IDE in place and take the following steps to create a Spring application:

Steps	Description
1	Create a project with a name SpringExample and create a packagecom.jtc under the src folder in the created project.
2	Add required Spring libraries using Add External JARs option as explained in the Spring Hello World Example chapter.
3	Create Java classes HelloWorld and MainApp under the com.jtcpackage.
4	Create Beans configuration file Beans.xml under the src folder.
5	The final step is to create the content of all the Java files and Bean Configuration file and run the application as explained below.

Here is the content of HelloWorld.java file:

Following is the content of the second file MainApp.java:

package com.jtc;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.FileSystemXmlApplicationContext;
public class MainApp {
public static void main(String[] args) {
ApplicationContext context = new FileSystemXmlApplicationContext
("C:/Users/ZARA/workspace/HelloSpring/src/Beans.xml");
HelloWorld obj = (HelloWorld) context.getBean("helloWorld");
obj.getMessage();
}
}

Following two important points should be noted about the main program:

The first step is to create factory object where we used framework APIFileSystemXmlApplicationContext to create the factory bean after loading the bean configuration file from the given path. TheFileSystemXmlApplicationContext() API takes care of creating and initializing all the objects ie. beans mentioned in the XML bean configuration file.
The second step is used to get the required bean using getBean() method of the created context. This method uses bean ID to return a generic object, which finally can be casted to the actual object. Once you have an object, you can use this object to call any class method.

Following is the content of the bean configuration file Beans.xml

Once you are done with creating the source and bean configuration files, let us run the application. If everything is fine with your application, it will print the following message:

Your Message : Hello World!

The ApplicationContext container includes all functionality of the BeanFactorycontainer, so it is generally recommended over BeanFactory. BeanFactory can still be used for lightweight applications like mobile devices or applet-based applications where data volume and speed is significant.

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Spring ─ IoC Containers