Spring Core

Spring Autowiring by Constructor

In Spring “Autowiring by Constructor means” autowiring is actually autowiring by type it means if data type of a bean is same as the data type of other bean constructor argument, auto wire it.Spring container looks at the properties of the beans on which autowire attribute is set to constructorin the XML configuration file. It then tries to match and wire its properties with the beans defined by the same data type of constructor argument in the configuration file.
For example, if a “circle” bean exposes an “center” constructor argument of Point data type, Spring will find these “center” in current container and wire it automatically. And if no matching found, just do nothing.

 

You can enable this feature via autowire=”constructor” like below :

 

Normally, you wire the bean explicitly, via ref attribute like this :

<bean class="com.dineshonjava.sdnext.autoWiring.tutorial.Circle" id="circle">
 <constructor-arg><ref bean="center"></ref>
  </constructor-arg></bean>  <bean class="com.dineshonjava.sdnext.autoWiring.tutorial.Point" id="center">  <property name="x" value="0"></property>  <property name="y" value="0"></property>  </bean>

With autowire by constructor enabled, you do not need to declares the property tag anymore. As long as the “center” beans are same data type as the constructor argument of “circle” bean, which is “center“, Spring will wire it automatically.

Lets see Exaple

Circle.java

package com.dineshonjava.sdnext.autoWiring.tutorial;

public class Circle
{
 private Point center;
  
 /**
  * @param center the center to set
  */
 public Center(Point center) {
  this.center = center;
 }

 public void draw()
 {
  System.out.println("Center of circle is ("+center.getX()+", "+center.getY()+")");
 }
}


Point.java

package com.dineshonjava.sdnext.autoWiring.tutorial;

public class Point
{
 private int x;
 private int y;
 /**
  * @return the x
  */
 public int getX() {
  return x;
 }
 /**
  * @param x the x to set
  */
 public void setX(int x) {
  this.x = x;
 }
 /**
  * @return the y
  */
 public int getY() {
  return y;
 }
 /**
  * @param y the y to set
  */
 public void setY(int y) {
  this.y = y;
 }
}


spring.xml

<beans xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://www.springframework.org/schema/beans" xsi:schemalocation="http://www.springframework.org/schema/beans  
    http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">

<bean autowire="constructor" class="com.dineshonjava.sdnext.autoWiring.tutorial.Circle" id="circle">
  </bean>
 <bean class="com.dineshonjava.sdnext.autoWiring.tutorial.Point" id="center">
  <property name="x" value="0"></property>
  <property name="y" value="0"></property>
 </bean>
</beans>


DrawingApp.java

package com.dineshonjava.sdnext.autoWiring.tutorial;

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


/**
 * @author Dinesh Rajput
 *
 */
public class DrawingApp 
{
 /**
  * @param args
  */
 public static void main(String[] args) 
 {
ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");
  Circle circle = (Circle) context.getBean("circle");
  circle.draw();
 }
}

 

Now run this application and get following output on the console.

Output:
Jun 25, 2012 7:55:23 PM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [spring.xml]
Jun 25, 2012 7:55:24 PM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons
INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@1592174: defining beans [circle ,center]; root of factory hierarchy

Center of Circle is (0, 0)

 

Spring Related Topics you may like

 

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Dinesh Rajput

Dinesh Rajput is the chief editor of a website Dineshonjava, a technical blog dedicated to the Spring and Java technologies. It has a series of articles related to Java technologies. Dinesh has been a Spring enthusiast since 2008 and is a Pivotal Certified Spring Professional, an author of a book Spring 5 Design Pattern, and a blogger. He has more than 10 years of experience with different aspects of Spring and Java design and development. His core expertise lies in the latest version of Spring Framework, Spring Boot, Spring Security, creating REST APIs, Microservice Architecture, Reactive Pattern, Spring AOP, Design Patterns, Struts, Hibernate, Web Services, Spring Batch, Cassandra, MongoDB, and Web Application Design and Architecture. He is currently working as a technology manager at a leading product and web development company. He worked as a developer and tech lead at the Bennett, Coleman & Co. Ltd and was the first developer in his previous company, Paytm. Dinesh is passionate about the latest Java technologies and loves to write technical blogs related to it. He is a very active member of the Java and Spring community on different forums. When it comes to the Spring Framework and Java, Dinesh tops the list!

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