Setter and Constructor Injection in Spring Example

In Spring Dependency Injection (DI) design pattern is used to define the object dependencies between each other. There are two types in dependency-injection:

1. The constructor-injection type and

2. The setter-injection type . 

Historically the setter-injection type come from spring, whereas constructor-injection type are from PicoContainer and Google GUICE (Googles lightweight dependency injection framework).

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Constructor-Injection: The basic idea with constructor-injection is that the object has no defaults and instead you have a single constructor where all of the collaborators and values need to be supplied before you can instantiate the object.

Now we see an Example which has Constructor Injection type dependency.

Triangle.java

package com.dineshonjava.sdnext.constructorInjection.tutorial;

/**
 * @author Dinesh Rajput
 *
 */
public class Triangle
{
 private String type;
 private int    height;
 /**
  * @param type the type to set
  */
 /*public void setType(String type) {
  this.type = type;
 }*/
 
 public Triangle(String type)
 {
  this.type = type;
 }
 public Triangle(int height)
 {
  this.height = height;
 }
 public Triangle(String type, int height)
 {
  this.type   = type;
  this.height = height;
 }
 public void draw()
 {
  System.out.println(this.type+" triangle is drawn of height "+this.height);
 }
}

The Triangle bean class has two attributes viz. type and height. All the two attributes are set using the constructor injection.
spring.xml

<beans xmlns:aop="http://www.springframework.org/schema/aop" xmlns:p="http://www.springframework.org/schema/p" xmlns:security="http://www.springframework.org/schema/security" xmlns:tx="http://www.springframework.org/schema/tx" 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-2.5.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-2.5.xsd
     http://www.springframework.org/schema/security http://www.springframework.org/schema/security/spring-security-2.0.4.xsd 
        http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.5.xsd 
  http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.5.xsd">
  
 <bean class="com.dineshonjava.sdnext.constructorInjection.tutorial.Triangle" id="triangle">
 <constructor-arg index="0" type="java.lang.String" value="Equilateral"></constructor-arg><constructor-arg index="1" type="int" value="20"></constructor-arg></bean>  </beans> 

The id attribute of the bean element is used to specify the bean name and the class attribute is used to specify the fully qualified class name of the bean. The constructor-arg element with in the bean element is used to inject property value via constructor injection. The index attribute of the constructor-arg element represents the constructor argument sequences which passes to the constructor of the class
such as..  
      Triangle(String type, int height)
      at index = 0 argument is type;
      and at index = 1 argument is height;
      and so on.
 If any class has one argument constructor then there are no need to specify the index attribute of the bean in the bean configuration in spring. i.e. We can say index attribute used to distinguished to arguments of the constructor. Now second attribute is type of the constructor-arg it is used for type of the arguments of the constructor. 
Such as..
          Triangle(String type, int height)
          then type = "java.lang.String"
         and type = "int"
and the value attribute specifies the corresponding property value.

Here we set "Equilateral " and "20" for the Triangle bean properties type and height respectively. The following Main class is used to get the Triangle bean from the Spring IoC container and display its value it to the Triangle  . 

DrawingApp.java

package com.dineshonjava.sdnext.constructorInjection.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) 
 {
 //Triangle triangle = new Triangle();
 //BeanFactory factory =new XmlBeanFactory(new FileSystemResource("spring.xml"));
 ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");
 Triangle triangle = (Triangle) context.getBean("triangle");
 triangle.draw();
  }
}

Now we run this example the following output on the console.

Output:

Jun 20, 2012 12:12:25 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@145d068: startup date [Wed Jun 20 00:12:25 IST 2012]; root of context hierarchy

Jun 20, 2012 12:12:25 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions

INFO: Loading XML bean definitions from class path resource [spring.xml]

Jun 20, 2012 12:12:26 AM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons

INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@14c1103: defining beans [triangle]; root of factory hierarchy

Equilateral triangle is drawn of height 20

Setter Injection: The basic-ideas is that you have a no argument-constructor which creates the object with reasonable-defaults . The user of the object can then call setters on the object to override the collaborators of the object in order to wire the object graph together or to replace the key collaborators with test-doubles.

Now we see an Example which has Constructor Injection type dependency.

Triangle.java

package com.sdnext.constructorInjection.tutorial;

/**
 * @author Dinesh Rajput
 *
 */
public class Triangle
{
 private String type;
 private int    height;
 /**
  * @param type the type to set
  */
 public void setType(String type) {
  this.type = type;
 }
 
        /**
  * @param height the height to set
  */
 public void setHeight(String height) {
  this.height= height;
 }
 
 public void draw()
 {
  System.out.println(this.type+" triangle is drawn of height "+this.height);
 }
}

The Triangle bean class has two attributes viz. type and height. All the two attributes are set using the setter injection.


spring.xml

<beans xmlns:aop="http://www.springframework.org/schema/aop" xmlns:p="http://www.springframework.org/schema/p" xmlns:security="http://www.springframework.org/schema/security" xmlns:tx="http://www.springframework.org/schema/tx" 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-2.5.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-2.5.xsd
     http://www.springframework.org/schema/security http://www.springframework.org/schema/security/spring-security-2.0.4.xsd 
        http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.5.xsd 
  http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.5.xsd">
  
 <bean class="com.dineshonjava.sdnext.constructorInjection.tutorial.Triangle" id="triangle">
   <property name="type" value="Equilateral">
   <property name="height" value="20">
 </property></property></bean> 
 </beans>  

The id attribute of the bean element is used to specify the bean name and the class attribute is used to specify the fully qualified class name of the bean. The property element with in the bean element is used to inject property value via setter injection. The name attribute of the property element represents the bean attribute and the value attribute specifies the corresponding property value.

Here we set "Equilateral " and "20" for the Triangle bean properties type and height respectively. The following Main class is used to get the Triangle bean from the Spring IoC container and display its value it to the Triangle  . 

DrawingApp.java

package com.dineshonjava.sdnext.constructorInjection.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) 
 {
 //Triangle triangle = new Triangle();
 //BeanFactory factory =new XmlBeanFactory(new FileSystemResource("spring.xml"));
 ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");
 Triangle triangle = (Triangle) context.getBean("triangle");
 triangle.draw();
  }
}

Now we run this example the following output on the console.

Output:

Jun 20, 2012 12:12:25 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@145d068: startup date [Wed Jun 20 00:12:25 IST 2012]; root of context hierarchy

Jun 20, 2012 12:12:25 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions

INFO: Loading XML bean definitions from class path resource [spring.xml]

Jun 20, 2012 12:12:26 AM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons

INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@14c1103: defining beans [triangle]; root of factory hierarchy

Equilateral triangle is drawn of height 20

Injecting Object(User define) to the Bean

In the above examples we have been seen that how to define the bean(Triangle) definition in the configuration file (spring.xml) and initialized its associated properties(type and height). In previous examples We only used primitive data types (int, long, double etc.) and java defined class such as like String, Integer, etc(for example type is String and height is int).

pubic class Triangle{
   private int height;
   private String type;
   private double area;
   ...
   ...
}

Now we will define a bean(Triangle) with user define properties (Point class).

pubic class Triangle{
   private Point pointA;
   private Point pointB;
   private Point pointC;
   ...
   ...
}

pubic class Point{
   private int X;
   private int Y;
   ...
   ...
} 

package com.dineshonjava.sdnext.injectingObject.tutorial;

public class Triangle 
{
 private Point pointA;
 private Point pointB;
 private Point pointC;
 
 /**
  * @param pointA the pointA to set
  */
 public void setPointA(Point pointA) {
  this.pointA = pointA;
 }

 /**
  * @param pointB the pointB to set
  */
 public void setPointB(Point pointB) {
  this.pointB = pointB;
 }

 /**
  * @param pointC the pointC to set
  */
 public void setPointC(Point pointC) {
  this.pointC = pointC;
 }

 public void draw()
 {
  System.out.println("PointA is ("+pointA.getX()+", "+pointA.getY()+")");
  System.out.println("PointB is ("+pointB.getX()+", "+pointB.getY()+")");
  System.out.println("PointC is ("+pointC.getX()+", "+pointC.getY()+")");
 }
}

package com.dineshonjava.sdnext.injectingObject.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;
 }
}

<beans xmlns:aop="http://www.springframework.org/schema/aop" xmlns:p="http://www.springframework.org/schema/p" xmlns:security="http://www.springframework.org/schema/security" xmlns:tx="http://www.springframework.org/schema/tx" 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-2.5.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-2.5.xsd
     http://www.springframework.org/schema/security http://www.springframework.org/schema/security/spring-security-2.0.4.xsd 
        http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.5.xsd 
  http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.5.xsd">
  
                <bean class="com.dineshonjava.sdnext.injectingObject.tutorial.Triangle" id="triangle">
   <property name="pointA" ref="pointA"></property>
   <property name="pointB" ref="pointB"></property>
   <property name="pointC" ref="pointC"></property>
  </bean>
  
  <bean class="com.dineshonjava.sdnext.injectingObject.tutorial.Point" id="pointA">
   <property name="x" value="0"></property>
   <property name="y" value="0"></property>
  </bean>
  
  <bean class="com.dineshonjava.sdnext.injectingObject.tutorial.Point" id="pointB">
   <property name="x" value="-20"></property>
   <property name="y" value="0"></property>
  </bean>
  
  <bean class="com.dineshonjava.sdnext.injectingObject.tutorial.Point" id="pointC">
   <property name="x" value="20"></property>
   <property name="y" value="0"></property>
  </bean>
</beans>

package com.dineshonjava.sdnext.injectingObject.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) 
 {
  //BeanFactory factory =new XmlBeanFactory(new FileSystemResource("spring.xml"));
  ApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");
  Triangle triangle = (Triangle) context.getBean("triangle");
  triangle.draw();
 }
}

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