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Autowiring with Constructor



In our last tutorial, we understood how to perform Dependency Injection using byType value of autowire attribute. In this tutorial we are going to understand about how to perform Dependency Injection using the constructor value of autowire attribute.

Recap - Autowiring is a feature provided by Spring Framework which automate the process of wiring the beans in a Spring Project. To perform autowiring of beans, Spring provides us an attribute -autowire.


Attribute autowire may have any of the three values -


What does autowire="constructor" do?


When a class A contains a property/reference of class B and also has a constructor with argument type class B, the use of attribute autowire="constructor" within the class A bean(configured in xml file) leads the Spring container to look for the bean(in xml) of type class B(as defined in the constructor argument) and inject it into the class A bean.

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 attribute - autowire with its value - constructor.



Creating the Java class - Company


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

package decodejava;

public class Company
{
	
	public void info() 
	{
		System.out.println("Company A");	
	}

}





Dependent Class containing a Company object - Employee


Next, we are going to add another Java class named Employee and it is going to contain a String object, a primitive int and an Company reference, hence Employee class is dependent on Company class as it contains its reference.

We are also going to create a constructor(a must when using autowire="constructor") of Employee class with argument type Company, String and a primitive int.

The objects of String class and Company class and primitive int value will be injected into a Employee instance/bean by an automatic call to the constructor of Employee class by the Spring Container, when the Employee bean is created by it using the configuration xml file(to be created in the upcoming section).


package decodejava;

public class Employee
{

	Company company ;
	String name;
	int yearsOfExp;
	
	//Constructor is a must when using autowire="constructor"
	public Employee(Company company, String name, int yearsOfExp)
	{
		this.company = company;
		this.name = name;
		this.yearsOfExp = yearsOfExp;
	}

	
	public Company getCompany()
	{
		return company;
	}
	
	public String getName()
	{
		return name;
	}
	
	public int getYearsOfExp()
	{
		return yearsOfExp;
	}
}





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 -

Eventually this class calls the method of Employee class - getName() and getYearsOfExp.

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");
		Employee employeeBean = context.getBean("EmployeeBean", Employee.class);
		System.out.println("Employee Name : " + employeeBean.getName());
		System.out.println("Years of experience : " + employeeBean.getYearsOfExp());
		employeeBean.getCompany().info();
	}
}





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 an Employee bean with a unique id, a String object, a primitive int value and a Company bean is injected into the Employee bean, using the autowire="constructor" attribute.


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 ="EmployeeBean" class="decodejava.Employee" autowire = "constructor">
	<constructor-arg index="1" value="Employee1"/>
	<constructor-arg index="2" value="7"/>
</bean>


<bean id ="CompanyBean" class="decodejava.Company">
</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 -




Adding JARs








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 - SpringDependencyInjectionAutowireByConstructor is the name of our Project and it is a top-level directory.






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 Company class and Employee class.

It also injects a String object, a primitive int value and a Company bean into Employee bean and calls the methods of the respective classes.

Jul 17, 2018 11:21:23 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.support.FileSystemXmlApplicationContext@1ee0005: startup date [Tue Jul 17 11:21:23 IST 2018]; root of context hierarchy
Jul 17, 2018 11:21:23 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [config.beans.xml]
Employee Name : Employee1
Years of experience : 7
Company A


This concludes performing the dependency injection by injecting a bean into another bean using the autowire attribute value - constructor. In the next article, we are going to show you how to perform the dependency injection using the autowiring with annotations.




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