Spring Part 03 - Constructor Injection

 Spring - Construction Injection

Maven Dependency:

<dependency>

  <groupId>org.springframework</groupId>

  <artifactId>spring-context</artifactId>

  <version>5.3.16</version>

</dependency>

Sample Program for Construction Injection using 'Spring Framework' 

beans_di.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

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

    <bean id="commonEngine" class="com.ah.di.Engine"/>


    <bean id="car1" class="com.ah.di.Car">

        <constructor-arg name="engine" ref="commonEngine"/>

    </bean>


    <bean id="car2" class="com.ah.di.Car">

        <constructor-arg name="engine" ref="commonEngine"/>

    </bean>

</beans>

package com.ah.di;

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

public class Test {
    public static void main(String[] args) {

   /*     Engine engine = new Engine();
        Car car = new Car(engine);
        car.callEngine();*/

/*        Car car = new Car(new Engine());
        car.callEngine();*/

        ApplicationContext context = new ClassPathXmlApplicationContext("beans_di.xml");
        Car carObj1 = context.getBean("car1",Car.class);
        carObj1.callEngine();

        Car carObj2 = context.getBean("car2",Car.class);
        carObj2.callEngine();
    }
}

package com.ah.di;

public class Engine {

    public Engine() {

        System.out.println("Engine constructor.");

    }

    public void engineInfo(){

        System.out.println("engineInfo method of Engine class called.");

    }

}

package com.ah.di;

public class Car {

    private Engine engine;

    public Car(Engine engine) {

        this.engine = engine;

    }

    public void callEngine(){

        if(engine==null){

            System.out.println("Engine is dead");

        }else{

            System.out.println("Engine is working");

            engine.engineInfo();

        }

    }

}

                                                                  Watch Demo