Dependency injection by Constructor in Spring Container:

Dependency injection by Constructor in Spring Container:

We can inject the dependency by constructor in spring framework, use the <constructor-arg> sub tag of <beans> tag in the bean configuration file and supply the values.
If the constructor parameter type is simple type, to supply value we can use “value” attribute.
If the constructor parameter type is Object type, to supply value we can use “ref” attribute.
Let’s see the sample example to inject sample (primitive and standard based values) and reference (object) type values. Follow the below steps for Constructor dependency injection (DI).

Step-1: Project Structure.

Step-2: Create Spring Bean class with the required dependencies.

Step-3: Define the parameterized constructor to support for constructor DI.

Step-4: Use the <constructor-arg> tag in bean configuration file.

Step-5: Get the bean object from beans configuration file in Client app.

Step-6: Required jar/dependency configuration in pom.xml file.

Step-7: Output of Application

Step-1: Project Structure.

Step-2: Create Spring Bean class with the required dependencies:

Creating a Department.java class with required fields, in this example I have created with 3 fields, ie., deptNo, deptName and deptDesc, all are String data type.

	package com.gurugubellitech.bean;

	public class Department {
	private String deptNo;
	private String deptName;
	private String deptDesc;

	/**
	* @return the deptNo
	*/
	public String getDeptNo() {
	return deptNo;
	}

	/**
	* @param deptNo
	* the deptNo to set
	*/
	public void setDeptNo(String deptNo) {
	this.deptNo = deptNo;
	}

	/**
	* @return the deptName
	*/
	public String getDeptName() {
	return deptName;
	}

	/**
	* @param deptName
	* the deptName to set
	*/
	public void setDeptName(String deptName) {
	this.deptName = deptName;
	}

	/**
	* @return the deptDesc
	*/
	public String getDeptDesc() {
	return deptDesc;
	}

	/**
	* @param deptDesc
	* the deptDesc to set
	*/
	public void setDeptDesc(String deptDesc) {
	this.deptDesc = deptDesc;
	}

	}

view raw Department.java hosted with ❤ by GitHub

Step-3: Define the parameterized constructor to support for constructor DI:

Creating a Employee.java class with required fields and with 2 constructors like below

	package com.gurugubellitech.bean;

	public class Employee {
	private String empNo;
	private String firstName;
	private String lastName;
	private Department department;

	/**
	* @return the empNo
	*/
	public String getEmpNo() {
	return empNo;
	}

	/**
	* @param empNo
	* the empNo to set
	*/
	public void setEmpNo(String empNo) {
	this.empNo = empNo;
	}

	/**
	* @return the firstName
	*/
	public String getFirstName() {
	return firstName;
	}

	/**
	* @param firstName
	* the firstName to set
	*/
	public void setFirstName(String firstName) {
	this.firstName = firstName;
	}

	/**
	* @return the lastName
	*/
	public String getLastName() {
	return lastName;
	}

	/**
	* @param lastName
	* the lastName to set
	*/
	public void setLastName(String lastName) {
	this.lastName = lastName;
	}

	public Department getDepartment() {
	return department;
	}

	public void setDepartment(Department department) {
	this.department = department;
	}

	/**
	* @param empNo
	* @param firstName
	* @param lastName
	*/
	public Employee(String empNo, String firstName, String lastName) {
	this.empNo = empNo;
	this.firstName = firstName;
	this.lastName = lastName;
	}

	/**
	* @param empNo
	* @param firstName
	* @param lastName
	* @param department
	*/
	public Employee(String empNo, String firstName, String lastName,
	Department department) {
	this.empNo = empNo;
	this.firstName = firstName;
	this.lastName = lastName;
	this.department = department;
	}

	}

view raw Employee.java hosted with ❤ by GitHub

Step-4: Use the <constructor-arg> tag in bean configuration file:

Below is the beans configuration file, provided by spring framework, we need to configure all the bean object information in this file as per the spring API. In the below file we configured Employee and Department object information, and getting this object in TestApp.java class by using bean class name.

	<?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">

	<!-- Data about Employee1 -->
	<bean id="employee1" class="com.gurugubellitech.bean.Employee">
	<constructor-arg name="empNo" value="EMP1001"/>
	<constructor-arg name="firstName" value="Gurugubelli"/>
	<constructor-arg name="lastName" value="Technologies"/>
	</bean>
	<bean id="employee2" class="com.gurugubellitech.bean.Employee">
	<constructor-arg name="empNo" value="EMP1002"/>
	<constructor-arg name="firstName" value="Gurugubelli"/>
	<constructor-arg name="lastName" value="Technologies"/>
	<constructor-arg name="department" ref="dept2"/>
	</bean>
	<bean id="dept2" class="com.gurugubellitech.bean.Department">
	<property name="deptNo" value="10"></property>
	<property name="deptName" value="CS and IS"></property>
	<property name="deptDesc" value="Department of Computer science And information systems"></property>
	</bean>

	<!-- Data about Employee2 -->
	<bean id="employee3" class="com.gurugubellitech.bean.Employee">
	<constructor-arg name="empNo" value="EMP1003"/>
	<constructor-arg name="firstName" value="Gurugubelli"/>
	<constructor-arg name="lastName" value="Tech"/>
	<constructor-arg name="department" ref="dept3"/>
	</bean>
	<bean id="dept3" class="com.gurugubellitech.bean.Department">
	<property name="deptNo" value="20"></property>
	<property name="deptName" value="M and S"></property>
	<property name="deptDesc" value="Department of Mathematics And Statistics"></property>
	</bean>

	</beans>

view raw my-beans.xml hosted with ❤ by GitHub

Step-5: Get the bean object from beans configuration file in Client app:

TestApp.java class is a main method class and getting bean objects using bean name in my-beans.xml file configuration with the help of ApplicationContext object provided by spring framework.

In this example we are getting two types of objects from beans configuration file. One object is Employee object and another object also Employee object with Department object. (Department object is available in Employee object as per the Employee.java class declaration.)

	package com.gurugubellitech.client;

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

	import com.gurugubellitech.bean.Department;
	import com.gurugubellitech.bean.Employee;

	public class TestApp {
	private static ApplicationContext context;

	public static void main(String[] args) {
	String configFile = "my-beans.xml";
	context = new ClassPathXmlApplicationContext(configFile);
	Employee emp1 = context.getBean("employee1", Employee.class);

	System.out.println("\n======= Employee Details with out Address =======");
	System.out.println("Employee ID : " + emp1.getEmpNo());
	System.out.println("First Name : " + emp1.getFirstName());
	System.out.println("Last Name : " + emp1.getLastName());

	Employee emp2 = context.getBean("employee2", Employee.class);

	System.out.println("\n======= Employee2 Details with Dept info =======");
	System.out.println("Employee ID : " + emp2.getEmpNo());
	System.out.println("First Name : " + emp2.getFirstName());
	System.out.println("Last Name : " + emp2.getLastName());
	Department dept2 = emp2.getDepartment();
	System.out.println("Dept NO : " + dept2.getDeptNo());
	System.out.println("Dept Name : " + dept2.getDeptName());
	System.out.println("Dept Desc : " + dept2.getDeptDesc());

	Employee emp3 = context.getBean("employee3", Employee.class);
	System.out.println("\n======= Employee3 Details with Dept info =======");
	System.out.println("Employee ID : " + emp3.getEmpNo());
	System.out.println("First Name : " + emp3.getFirstName());
	System.out.println("Last Name : " + emp3.getLastName());
	Department dept3 = emp3.getDepartment();
	System.out.println("Dept NO : " + dept3.getDeptNo());
	System.out.println("Dept Name : " + dept3.getDeptName());
	System.out.println("Dept Desc : " + dept3.getDeptDesc());
	}
	}

view raw TestApp.java hosted with ❤ by GitHub

Step-6: Required jar/dependency configuration in pom.xml file:

Configured required spring dependencies/jar information in pom.xml file

	<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.gurugubellitech</groupId>
	<artifactId>maven-spring-constructor-injection</artifactId>
	<version>0.0.1-SNAPSHOT</version>

	<dependencies>
	<!-- https://mvnrepository.com/artifact/org.springframework/spring-beans -->
	<dependency>
	<groupId>org.springframework</groupId>
	<artifactId>spring-beans</artifactId>
	<version>5.0.2.RELEASE</version>
	</dependency>
	<!-- https://mvnrepository.com/artifact/org.springframework/spring-core -->
	<dependency>
	<groupId>org.springframework</groupId>
	<artifactId>spring-core</artifactId>
	<version>5.0.2.RELEASE</version>
	</dependency>
	<!-- https://mvnrepository.com/artifact/org.springframework/spring-context -->
	<dependency>
	<groupId>org.springframework</groupId>
	<artifactId>spring-context</artifactId>
	<version>5.0.2.RELEASE</version>
	</dependency>

	</dependencies>
	</project>