How does this works

[Arun Giridharan]

I'm learning a bit on Spring , so i didn't understand how this Execution happens because my A class object is depend on B class object , b instance is again depend on a ;

public class A {
	
	private B b;
	
	A(B b){
		System.out.println("Inside A constructor");
		this.b = b;
	}
	
	A(){
		System.out.println("Inside A constructor");
	}

	public B getB() {
		return b;
	}

	public void setB(B b) {
		this.b = b;
	}
	@Override
	public String toString() {
	
		return "A";
	}
}

public class B {
	
	private A a;
	
	B(A a){
		System.out.println("Inside B constructor");
		this.a = a;
	}

	
	B(){
		System.out.println("Inside B constructor");
	}

	public A getA() {
		return a;
	}


	public void setA(A a) {
		this.a = a;
	}

	@Override
	public String toString() {
	
		return "B";
	}
}

My beans.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN" "http://www.springframework.org/dtd/spring-beans.dtd">

<beans>
	<bean id = "a" class="com.cluster.spring.ioc.A">
	 	<!--<constructor-arg index="0" type="com.home.spring.ioc.B" ref = "b"/>   
	  --><property name="b" ref="b"/> 	
	</bean>
	
	<bean id = "b" class="com.cluster.spring.ioc.B">
	 	<!--<constructor-arg index="0" type="com.home.spring.ioc.A" ref = "a"/>  
	  	--><property name="a" ref="a"/> 
	</bean>
</beans>

My Manger Class

package com.cluster.spring.ioc;



import org.apache.log4j.BasicConfigurator;

import org.springframework.beans.factory.BeanFactory;

import org.springframework.beans.factory.xml.XmlBeanFactory;

import org.springframework.core.io.FileSystemResource;



public class Manager {

	

	static {

		BasicConfigurator.configure();

	}



	public static void main(String[] args) {



		BeanFactory beanFactory = new XmlBeanFactory(new FileSystemResource("./src/beans.xml"));

	//	B b = (B)beanFactory.getBean("b");

		

		A a = (A)beanFactory.getBean("a");

		System.out.println(a.toString());

	}

}

Can some one explain me , how DI happens here (Dependency Injection).

[Mark Spritzler]

First, I recommend learning the latest version of Spring. The sample code you use BeanFactory instead of ApplicationContext. This means that you are using at least 5 year old material, as today to start up Spring you should create an ApplicationContext instead.

Now, in your case you changed your dependency to be property dependency. When you had constructor dependency, those dependencies are mandatory, but how can you create one before the other if they depend on each other both via constructor args. Just think in Java can you do

A a = new A(b);
B b = new B(a);

No because the first line doesn't work, there is no instance of B created yet, and if you tried to reverse the two lines, same problem but the other way.

But I can do

A a = new A();
B b = new B();
a.setB(b);
b.setA(a);

That works.

Now, I would take that circular dependency and see it as a code smell and refactor the code so that there is no circular dependency.

Hope that helps

Mark

[Arun Giridharan]

Yep! That Helps

Thank You