Spring @Autowired Annotation With Constructor Injection Example

In this post, We will learn about the Spring @Autowired Annotation With Constructor Injection Example using a Demo Project.

Spring Dependency Injection

The dependency Injection is a fundamental aspect of the Spring framework through which the Spring framework container injects objects or instances into other objects or “dependencies”.
In another word, We can say that dependency Injection promotes loose coupling of software components and moves the responsibility of managing components onto the Spring container.

Types of Spring Dependency Injection:

Setter Based Dependency Injection:

In setter-based DI, the container will call setter methods of the class after invoking a no-argument constructor or no-argument static factory method to instantiate the bean.

Constructor Based Dependency Injection:

The constructor-based dependency injection is accomplished when the Spring container invokes a class constructor with a number of arguments and each representing a dependency on the other class.

When to use setter injection and constructor injection?

The Spring documentation recommends using constructor-based injection for mandatory dependencies, and setter-based injection for optional Dependency

@Autowired annotation

Spring @Autowired annotation is mainly used for automatic dependency injection. This annotation may be applied to before class variables and methods for auto wiring byType. We can also use @Autowired annotation on the constructor for constructor-based spring auto wiring.

Now Let’s try to understand Constructor Based dependency injection(DI) using @Autowired Annotation With the help of the below demo Project.

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.kkhindigyan.org</groupId>
  <artifactId>AutowiredAnnotationWithConstructorDI</artifactId>
  <version>0.0.1-SNAPSHOT</version>
    <properties>
		<org.springframework.version>5.2.13.RELEASE</org.springframework.version>
	</properties>
	<dependencies>
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-context</artifactId>
			<version>${org.springframework.version}</version>
		</dependency>
	</dependencies>
</project>

<groupId>com.kkhindigyan.org</groupId>

<artifactId>AutowiredAnnotationWithConstructorDI</artifactId>

<version>0.0.1-SNAPSHOT</version>

<org.springframework.version>5.2.13.RELEASE</org.springframework.version>

</properties>

<groupId>org.springframework</groupId>

<artifactId>spring-context</artifactId>

<version>${org.springframework.version}</version>

</dependency>

</dependencies>

</project>

Employee.java

package com.kkhindigyan.model;

import org.springframework.beans.factory.annotation.Autowired;

public class Employee {

	private Integer employeeId;
	private String employeeName;
	private Double salary;
	
	private Address address;
	
	@Autowired
	public Employee(Integer employeeId, String employeeName, Double salary, Address address) {
		super();
		this.employeeId = employeeId;
		this.employeeName = employeeName;
		this.salary = salary;
		this.address = address;
	}

	public Integer getEmployeeId() {
		return employeeId;
	}

	public String getEmployeeName() {
		return employeeName;
	}

	public Double getSalary() {
		return salary;
	}

	public Address getAddress() {
		return address;
	}


	@Override
	public String toString() {
		return "Employee [employeeId=" + employeeId + ", employeeName=" + employeeName + ", salary=" + salary + "]";
	}
}

package com.kkhindigyan.model;

import org.springframework.beans.factory.annotation.Autowired;

public class Employee {

private Integer employeeId;

private String employeeName;

private Double salary;

private Address address;

@Autowired

public Employee(Integer employeeId, String employeeName, Double salary, Address address) {

super();

this.employeeId = employeeId;

this.employeeName = employeeName;

this.salary = salary;

this.address = address;

}

public Integer getEmployeeId() {

return employeeId;

}

public String getEmployeeName() {

return employeeName;

}

public Double getSalary() {

return salary;

}

public Address getAddress() {

return address;

}

@Override

public String toString() {

return "Employee [employeeId=" + employeeId + ", employeeName=" + employeeName + ", salary=" + salary + "]";

}

Address.java

package com.kkhindigyan.model;

public class Address {

	private String addressLine1;
	private String addressLine2;
	private String city;
	private Long zipCode;
	
	public String getAddressLine1() {
		return addressLine1;
	}
	public void setAddressLine1(String addressLine1) {
		this.addressLine1 = addressLine1;
	}
	public String getAddressLine2() {
		return addressLine2;
	}
	public void setAddressLine2(String addressLine2) {
		this.addressLine2 = addressLine2;
	}
	public String getCity() {
		return city;
	}
	public void setCity(String city) {
		this.city = city;
	}
	public Long getZipCode() {
		return zipCode;
	}
	public void setZipCode(Long zipCode) {
		this.zipCode = zipCode;
	}
	@Override
	public String toString() {
		return "Address [addressLine1=" + addressLine1 + ", addressLine2=" + addressLine2 + ", city=" + city
				+ ", zipCode=" + zipCode + "]";
	}
}

package com.kkhindigyan.model;

public class Address {

private String addressLine1;

private String addressLine2;

private String city;

private Long zipCode;

public String getAddressLine1() {

return addressLine1;

}

public void setAddressLine1(String addressLine1) {

this.addressLine1 = addressLine1;

}

public String getAddressLine2() {

return addressLine2;

}

public void setAddressLine2(String addressLine2) {

this.addressLine2 = addressLine2;

}

public String getCity() {

return city;

}

public void setCity(String city) {

this.city = city;

}

public Long getZipCode() {

return zipCode;

}

public void setZipCode(Long zipCode) {

this.zipCode = zipCode;

}

@Override

public String toString() {

return "Address [addressLine1=" + addressLine1 + ", addressLine2=" + addressLine2 + ", city=" + city

+ ", zipCode=" + zipCode + "]";

}

applicationContext.xml

To enable @Autowired annotation in Spring Framework we have to use <context:annotation-config/> tag in the config file as below.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:context="http://www.springframework.org/schema/context"
	xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        https://www.springframework.org/schema/context/spring-context.xsd">
	
	<context:annotation-config/>
	<bean id="employee" class="com.kkhindigyan.model.Employee" scope="prototype">
		<constructor-arg name="employeeId" value="102883" type="java.lang.Integer" index="0"></constructor-arg>
		<constructor-arg name="employeeName" value="Sean" type="java.lang.String" index="1"></constructor-arg>
		<constructor-arg name="salary" value="80000" type="java.lang.Double" index="2"></constructor-arg>
		<!-- <constructor-arg name="address" ref="address" type="com.kkhindigyan.model.Address" index="3"></constructor-arg> -->
	</bean>

	<bean id="address" class="com.kkhindigyan.model.Address">
		<property name="addressLine1" value=""></property>
		<property name="addressLine2" value=""></property>
		<property name="city" value="Delhi"></property>
		<property name="zipCode" value="959049"></property>
	</bean>

</beans>        
	@Override
	public String toString() {
		return "Address [addressLine1=" + addressLine1 + ", addressLine2=" + addressLine2 + ", city=" + city
				+ ", zipCode=" + zipCode + "]";
	}
}

<?xml version="1.0" encoding="UTF-8"?>

<beans xmlns="http://www.springframework.org/schema/beans"

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xmlns:context="http://www.springframework.org/schema/context"

xsi:schemaLocation="http://www.springframework.org/schema/beans

https://www.springframework.org/schema/beans/spring-beans.xsd

http://www.springframework.org/schema/context

https://www.springframework.org/schema/context/spring-context.xsd">

<context:annotation-config/>

<constructor-arg name="employeeId" value="102883" type="java.lang.Integer" index="0"></constructor-arg>

<constructor-arg name="employeeName" value="Sean" type="java.lang.String" index="1"></constructor-arg>

<constructor-arg name="salary" value="80000" type="java.lang.Double" index="2"></constructor-arg>

</bean>

</bean>

</beans>

@Override

public String toString() {

return "Address [addressLine1=" + addressLine1 + ", addressLine2=" + addressLine2 + ", city=" + city

+ ", zipCode=" + zipCode + "]";

}

ClientTest.java

package com.kkhindigyan.org;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.AbstractApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.kkhindigyan.model.Address;
import com.kkhindigyan.model.Employee;

public class ClientTest {

	public static void main(String[] args) {
		
		ApplicationContext applicationContext = null;
		
		try {
			
			//Creating Instance of ApplicationContext Spring Container
			applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
			
			//Asking Spring Container to return Spring bean with Specific Id or name
			Employee employee = (Employee) applicationContext.getBean("employee");
			System.out.println(employee);
			Address address = employee.getAddress();
			//Address address = (Address) applicationContext.getBean("address");
			System.out.println(address);
			
		} catch (Exception e) {
			e.printStackTrace();
		}finally {
			if (applicationContext != null)
				((AbstractApplicationContext) applicationContext).close();
		}
	}
}

package com.kkhindigyan.org;

import org.springframework.context.ApplicationContext;

import org.springframework.context.support.AbstractApplicationContext;

import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.kkhindigyan.model.Address;

import com.kkhindigyan.model.Employee;

public class ClientTest {

public static void main(String[] args) {

ApplicationContext applicationContext = null;

try {

//Creating Instance of ApplicationContext Spring Container

applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");

//Asking Spring Container to return Spring bean with Specific Id or name

Employee employee = (Employee) applicationContext.getBean("employee");

System.out.println(employee);

Address address = employee.getAddress();

//Address address = (Address) applicationContext.getBean("address");

System.out.println(address);

} catch (Exception e) {

e.printStackTrace();

}finally {

if (applicationContext != null)

((AbstractApplicationContext) applicationContext).close();

}

If you run ClientTest.java as Java Application then it will give the below output:

Employee [employeeId=102883, employeeName=Sean, salary=80000.0]
Address [addressLine1=, addressLine2=, city=Delhi, zipCode=959049]

1 2	Employee [employeeId=102883, employeeName=Sean, salary=80000.0] Address [addressLine1=, addressLine2=, city=Delhi, zipCode=959049]

That’s all about Spring @Autowired Annotation With Constructor Injection Example

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