Dependency Injection with Setter Methods

Today we are going to understand about one of the main features of Spring Framework - Dependency Injection and how to use this feature within a Spring Project using the Eclipse Integrated Development Environment(IDE) application.

Dependency Injection is a feature of Spring Framework which allows us inject primitive values as well as an object(bean) value into another object(bean). In dependency injection, a dependency could be a primitive value or an object(bean). An injection is the passing of a such dependency(primitive value or an object) to a dependent object that would use it.

By setter methods
By constructor

To understand this concept of dependency Injection by the setter method, let's proceed with an example. In the upcoming section, we are going to create a few loosely coupled Java classes also known as beans, configure these beans in a configuration xml file, load and instantiate the beans and perform dependency injection using the Spring Framework.

Creating the Java class - OutdoorSport

We are going to create a java class named OutdoorSport within the decodejava package and this class contains a message info().

package decodejava;

public class OutdoorSport
{
	
	public void info() 
	{
		System.out.println("-An Outdoor Sport-");	
	}

}

Dependent class with an OutdoorSport object - Tennis

Next, we are going to add another Java class named Tennis and it is going to contain a String object and an OutdoorSport reference, hence Tennis class is dependent on OutdoorSport class.

These objects of String class and the OutdoorSport class will be injected into a Tennis instance/bean by an automatic call to the setter methods in the Tennis class by the Spring Container, when the Tennis bean is created by it using the configuration xml file(to be created in the upcoming section).

package decodejava;

public class Tennis 
{

	OutdoorSport sport;
	String message;


	public void getMessage() 
	{
		System.out.println("Message is : " + message);
		sport.info();
	}

	
	//Setter method to set the message
	public void setMessage(String message) 
	{
		this.message = message;
	}
	
	
	//Setter method to set the OutdoorSport object
	public void setSport(OutdoorSport sport)
	{
		this.sport = sport;
	}
	
}

Adding the Utility class that calls the Spring API

Next, we are going to create another class named - Utility, which is a simple java class. The Utility class uses the ApplicationContext container(an interface) of Spring Framework by creating its instance using its implemented class FileSystemXmlApplicationContext, which -
- Loads the configuration xml file - config.beans.xml
- Creates the Tennis bean, calls setter methods i.e. setMessage() & setSport(), creates the OutdoorSport bean specified in it and injects it into Tennis bean.
- Accesses the bean specified in the configuration file by calling the getBean() method of ApplicationContext container.
- Calls the method of Tennis class - getMessage().
Utility.java
```
package decodejava;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.FileSystemXmlApplicationContext;

public class Utility 
{
	public static void main(String[] args) 
	{
		ApplicationContext context = new FileSystemXmlApplicationContext("classpath:config.beans.xml");
		Tennis tennisBean = context.getBean("TennisBean", Tennis.class);
		tennisBean.getMessage();
	}
}
```
Adding a configuration file

Next, we are going to add a configuration file to our project. This configuration document is an Extensible Markup Language(XML) file, ending with .xml extension and we are going to name file as config.beans.xml.

In this file, we have configured a Tennis instance with a unique id and a String object and an OutdoorSport instance is injected into the Tennis bean using the setter methods in Tennis class..

config.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.xsd">


<bean id ="TennisBean" class="decodejava.Tennis">
	<property name="message" value="Let's play Tennis!"></property>
	<property name="sport" ref="OutdoorSportBean"></property>
</bean>


<bean id ="OutdoorSportBean" class="decodejava.OutdoorSport">
</bean>


</beans>
```
This mapping document has a parent <beans> tag as the root element and its individual child elements, each with a <bean> tag, containing all the attributes such as -
- The id attribute within the <bean> element defines a unique id associated with the bean.
- The class attribute within the <bean> element defines the name of the class of this bean.
- The name child attribute refers to the name of a property in the Tennis class i.e. sport and message.
- The value child attribute refers to the value of a property in the Tennis class i.e. property message, is automatically assigned a value Let's play Tennis! by calling the setter method - setMessage(), when the bean is created by Spring Container.
- The ref child attribute refers to the reference name of the property which is matched with the id of another bean(another class - OutdoorSport) declared within a different <bean> tag. The leads to calling of setter method - setSport() by the Spring container.
Adding JARs
- We are going to add some JARs files to our Java project. These JARs are required in order to successfully execute a Spring project.
  
  All these JARs are included in the folder named libs(within our Spring installation folder). So, we need to add all the JARs in the libs folder to our build path of our Java project.
  
  Note : Those who don't know how to add JARs to the build path of your Java project in Eclipse IDE, please refer to our section Adding JARs to your Spring project in Eclipse.
Directory Structure of Project

The picture above depicts how and where to arrange classes and interfaces comprising this Spring Project, in a specific directory structure.

Project Folder - SpringDependencyInjection is the name of our Project and it is a top-level directory.
- Besides this, the project folder also contains a configuration file i.e. config.beans.xml, used to configure the beans of classes in the project.
- The project folder also contains all the needed Spring JARs present in the libs folder of Spring Framework installation.
Execution

Finally, after executing Utility class, you will get the following output within the Console window. This output shown below, shows how the Utility class has used the ApplicationContext container of Spring Framework to load the configuration xml file - config.beans.xml, access the beans specified in it, instantiate the Tennis class and OutdoorSport class. It also injects OutdoorSport bean into Tennis bean and calls their methods.
```
Jul 06, 2018 1:02:02 PM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.support.FileSystemXmlApplicationContext@1ee0005: startup date [Fri Jul 06 13:02:02 IST 2018]; root of context hierarchy
Jul 06, 2018 1:02:03 PM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [config.beans.xml]
Playing Tennis
-An Outdoor Sport-
```
This concludes performing the dependency injection by injecting a bean into another bean using the setter methods. In the next article, we are going to show you how to perform the dependency injection using the constructor.

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