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How to create beans by invoking a Constructor (Constructor - Injection) in Spring ?.

Updated on September 17, 2014

In Spring, you can create a bean by invoking its constructor. Creating beans in Spring container through constructor is equivalent to new operator for creating objects in Java. In order to create a Spring bean by invoking its constructor, you have to follow below 5 steps -

Steps to perform -

  1. Create simple Spring bean class – “Item”.
  2. Create Spring bean configuration file – “beans.xml”.
  3. Configure Item bean through constructor-arg tag.
  4. Create a Test class – “ItemTest”.
  5. Run the “ItemTest” class.

In below video tutorial and this hub, lets discuss above steps in great detail and see how to create beans by invoking constructor in Spring framework.

Video tutorial - How to create beans by invoking a constructor in Spring.

Below video tutorial, demonstrate constructor - injection in Spring.

Step 1 - Create simple Spring bean class – “Item”.

How it works -

  1. Create a package into which item bean is placed. In this case, its com dot hubberspot dot spring.
  2. Create a public class by name Item. This class will be Spring bean class for the example.
  3. Create instance variables name and price, of type String and integer.
  4. Create a parametrized constructor taking in parameters as name and price. This constructor is been called by Spring container in order to perform creation of bean.
  5. Create a method named as "itemInfo". This method will print information about name and price of current Item instance.

Item.java

package com.hubberspot.spring;

public class Item {
	
	private String name;
	private int price;
	
	public Item(String name , int price) {
		this.name = name;
		this.price = price;
	}
	
	public void itemInfo() {
		System.out.println("Item name - " + name);
		System.out.println("Item price - " + price);
	}
}

Step 2 and 3 - Create Spring bean configuration file – “beans.xml” and configure Item bean through constructor-arg tag.

How it works -

  1. In Spring configuration file, bean tag provides information to spring container about each individual bean. Lets provide information for Item bean. It has two attributes -
    • id - It takes in a value which uniquely identifies a particular bean in Spring container.
  2. class - It takes in a value which takes in a fully qualified class name.
  3. constructor-arg defines name and value for constructor defined in Item class. Here, one constructor-arg tag is for name and another for price. When Spring bean is requested, spring container instantiate Item instance and call parametrized constructor by reading constructor-arg tag. In this case, Item instance has name value as "i-phone" and price value as 40000.

Spring configuration file - beans.xml

<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-3.0.xsd">
		
	<bean id="item" class="com.hubberspot.spring.Item">
		<constructor-arg name="name" value="i-phone"></constructor-arg>
		<constructor-arg name="price" value="40000"></constructor-arg>
	</bean>

</beans>

Step 4 - Create a Test class – “ItemTest”.

How it works -

  1. Create Application context by providing name of spring configuration file "beans.xml".
  2. Get Item bean by id, from application context. Application instantiates Item bean by invoking constructor.
  3. Call itemInfo method by itemBean instantiated above.

ItemTest.java

package com.hubberspot.spring;

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

public class ItemTest {

	public static void main(String[] args) {

		ApplicationContext context = new ClassPathXmlApplicationContext("beans.xml");	
		
		Item itemBean = (Item) context.getBean("item");
		
		itemBean.itemInfo();

	}

}

Output of the example -

Upon running ItemTest class, itemInfo method gets executed. This method prints information for Item instance as -

Item name - i-phone

Item price - 40000

Image below shows Eclipse IDE console, which prints the output of the example.

Step 5 - Run the “ItemTest” class.

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