Aware Injection | Interface Injection

 ApplicationContextAware Injection Example:-

If underlying container injects objects/values to target class only when target class implements certain interface either directly or indirectly then it is called interface injection.

It is also known as Aware Injection because by implementing certain interface in target class, we made the container aware that it has to inject objects/values to target class.

Here target class(Vehicle) is implementing ApplicationContextAware interface so it is also known as ApplicationContextAware Injection.

Here Aware injection is not used to inject simple and regular values like name, age, driver class name, datasource etc. It is used only to inject container managed special object and values like bean ids, BeanFactory object, ApplicationContext object and etc...

Project Structure :- (Three packages) 

com.nt.beans --> Engine.java --> Vehicle.java com.nt.cfgs --> applicationContext.xml com.nt.client --> ClientTest.java

        Engine.java

package com.nt.beans; public class Engine { public Engine() { System.out.println("Engine ::0-param constructor"); } public void start() { System.out.println("Engine started...."); } public void stop() { System.out.println("Engine stopped...."); } }

        Vehicle.java

package com.nt.beans; import org.springframework.beans.BeansException; import org.springframework.context.ApplicationContext; import org.springframework.context.ApplicationContextAware; public class Vehicle implements ApplicationContextAware { private ApplicationContext ctx=null; public Vehicle()  { System.out.println("Vehicle ::0-param constructor"); } public void setApplicationContext(ApplicationContext ctx) throws BeansException  { System.out.println("Vehicle.setApplicationContext()"); this.ctx=ctx; } public void journey(String source, String destination)  { Engine engine=null; engine=ctx.getBean("engg",Engine.class); engine.start(); System.out.println("Journey is going on from "+source+" to "+destination); engine.stop(); } }

        applicationContext.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-4.3.xsd"> <bean id="engg" class="com.nt.beans.Engine"/> <bean id="vehicle" class="com.nt.beans.Vehicle"/> </beans>

        ClientTest.java

package com.nt.client; import org.springframework.context.ApplicationContext; import org.springframework.context.support.AbstractApplicationContext; import org.springframework.context.support.ClassPathXmlApplicationContext; import com.nt.beans.Vehicle; public class ClientTest { public static void main(String[] args) { ApplicationContext ctx=null; ctx=new ClassPathXmlApplicationContext("com/nt/cfgs/applicationContext.xml"); Vehicle v= ctx.getBean("vehicle",Vehicle.class); v.journey("Hyderabad", "Banglore"); ((AbstractApplicationContext) ctx).close(); } }

Output :- 

Engine ::0-param constructor Vehicle ::0-param constructor Vehicle.setApplicationContext() Engine started.... Journey is going on from Hyderabad to Banglore Engine stopped....