Constructor-based Dependency Injection by R4R Team

Constructor-based Dependency Injection:-
We can inject the dependency by constructor. The subelement of is used for constructor injection. Constructor-based DI is accomplished by the container invoking a constructor with a number of arguments, each representing a dependency. Calling a static factory method with specific arguments to construct the bean is nearly equivalent, and this discussion treats arguments to a constructor and to a static factory method similarly.

Let us have working Eclipse IDE in place and follow the following steps to create a Spring application:

1 Create a project with a name SpringExample and create a package com.r4r.in under the src folder in the created project.
2 Add required Spring libraries using Add External JARs .
3 Create Java classes Test, Message and MainApp under the com.r4r.in.
4 Create Beans configuration file Beans.xml under the src folder.
5 The final step is to create the content of all the Java files and Bean Configuration file and run the application as explained below.
 

1 Create Test.java file.

package com.r4r.in;

public class Test {

private Message msg;

public Test(Message msg) {

System.out.println("Inside constructor Message." );

this.msg = msg;

}

public void show() {

msg.showmsg();

} }

2 Create another dependent class file Message.java.

package com.r4r.in;

public class Message {

public Message(){

System.out.println("Inside Message constructor." );

} public void showmsg() {

System.out.println("Inside showmsg." );

} }

3 Create MainApp.java file.

package com.r4r.in;

import org.springframework.context.ApplicationContext;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class MainApp {

public static void main(String[] args) {

ApplicationContext context = new ClassPathXmlApplicationContext("Beans.xml");

Test t = (Test) context.getBean("test");

t.show();

} }

4 Following is the configuration file Beans.xml which has configuration for the constructor-based injection:

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

"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">

"test" class="com.r4r.in.Test">

"test"/>

"msg" class="com.r4r.in.Message">

5 Run the Application .if everything is ok then show the message.

Inside Message constructor.

Inside constructor Message.

Inside showmsg.