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)



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