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How to provide constructor initialization to a bean using index attribute in configuration file of Spring Framework ?.

A simple example to demonstrate, how to provide constructor initialization to a bean using constructor-arg tag's index attribute in configuration file of Spring Framework

-------- Scenario 1 ----------

1. Create a normal class having constructor say 'Dog' -


package com.hubberspot.spring;


public class Dog {

 private String breed;
 private int teeth;

 // Scenario 1 : Overloaded Constructor
 public Dog(String breed, int teeth){
  this.breed = breed;
  this.teeth = teeth;
 }
 
 public int getTeeth() {
  return teeth;
 }


 public void setTeeth(int teeth) {
  this.teeth = teeth;
 }


 public String getBreed() {
  return breed;
 }

 public void setBreed(String breed) {
  this.breed = breed;
 }

 public void move() {
  System.out.println("A Dog of breed "+ getBreed() +", " +
    "having "+ getTeeth() +" teeth moves slowly ...");
 }

}




2. Create a spring.xml file placed in classpath of your application -


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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN 2.0//EN" 
"http://www.springframework.org/dtd/spring-beans-2.0.dtd">

<beans>

 <!-- In order to create an object of bean we define its properties in bean 
  tag. The 'id' attribute value can be thought of as a reference to 'class' 
  attribute value -->
 <bean id="dog" class="com.hubberspot.spring.Dog">

  <!-- Scenario 1 : It misplaces the order of constructor args but because 
   of index Spring can resolve which is first arg and which is second arg. It 
   than checks and call the appropriate constructor if it exists. -->
   
  <constructor-arg index="1" value="20"></constructor-arg>
  <constructor-arg index="0" value="shepherd"></constructor-arg>

      <!-- Scenario 2 : It takes unordered constructor args with no index -->
    <!--   <constructor-arg value="20"></constructor-arg> 
             <constructor-arg value="shepherd"></constructor-arg> -->

 </bean>
</beans>



3. Create a Test class for the application (imp) -


package com.hubberspot.spring;

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


public class WildLifeApplication {

 public static void main(String[] args) {

  // ApplicationContext is a Spring interface which 
  // provides with the configuration for an application. 
  // It provides us with all the methods that BeanFactory 
  // provides. It loads the file resources in a older 
  // and generic manner. It helps us to publish events to the
  // listener registered to it. It also provides quick support
  // for internationalization. It provides us with the object 
  // requested, it reads the configuration file and provides
  // us with the necessary object required.
  // We are using concrete implementation of ApplicationContext
  // here called as ClassPathXmlApplicationContext because this 
  // bean factory reads the xml file placed in the classpath of 
  // our application. We provide ClassPathXmlApplicationContext
  // with a configuration file called as spring.xml placed
  // at classpath of our application. 
  ApplicationContext context = 
    new ClassPathXmlApplicationContext(("spring.xml"));

  // In order to get a object instantiated for a particular bean 
  // we call getBean() method of ClassPathXmlApplicationContext
  // passing it the id for which the object is to be needed. 
  // Here getBean() returns an Object. We need to cast it back 
  // to the Dog object. Without implementing new keyword we 
  // have injected object of Dog just by reading an xml 
  // configuration file.
  Dog dog = (Dog)context.getBean("dog");

  // Calling our functionality
  dog.move();

 }

}



Output of the Scenario 1 :







-------- Scenario 2 ----------

If Scenario 2 in spring.xml is un-commented and Scenario 1 is commented, than the Spring cannot convert "twenty" to int and throws java.lang.NumberFormatException. Think of it as Spring cannot perform something like    Integer.parseInt("String") .    

Output of the Scenario 2 :





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