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Setter vs Constructor Injection in Spring Example
June 18, 2012
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Setter vs Constructor Injection in Spring Example

In this article we will discuss Constructor Injection and Setter Injection. These are two ways to define the dependency injection in the spring application. Spring Dependency Injection (DI) design pattern is used to define the object dependencies between each other. There are following two types in dependency-injection:

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

 

Setter and Constructor Injection in Spring Example

 

 

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;
   ...
   ...
}

Lets see with the example.  
Triangle.java

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()+")");
 }
}
Triangle class has three points of nodes to draw a triangle and define setter of all three points pointA, pointB and pointC (only setter is required in the spring framework). Here point is also a class with two points one is X-axis and another is Y-axis.

Point.java

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;
 }
}

Here we define the Point class with two properties int x, int y. Actually Point class is also a bean which is as a property of the Triangle bean. Now we will configure both beans to the configuration file(spring.xml).
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.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>
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 ref attribute specifies the reference of another bean or id of the bean element (Point)  corresponding property.
Here we set “pointA“, “pointB” and “pointC” properties name for the Triangle bean with ref id of another bean (Point) “pointA“, “pointB” and “pointC” (these id name may be different such like “point1“, “point2” and “point3“) respectively. Now “pointA” is the id of bean element of one Point class and so on. 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.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();
 }
}

 

Output:
Jun 23, 2012 12:30:25 PM org.springframework.context.support.AbstractApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@ab50cd: startup date [Sat Jun 23 12:30:25 IST 2012]; root of context hierarchy
Jun 23, 2012 12:30:26 PM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [spring.xml]
Jun 23, 2012 12:30:26 PM org.springframework.beans.factory.support.DefaultListableBeanFactory preInstantiateSingletons
INFO: Pre-instantiating singletons in org.springframework.beans.factory.support.DefaultListableBeanFactory@14c1103: defining beans [triangle,zeroPoint,Point2,Point3]; root of factory hierarchy

PointA is (0, 0)
PointB is (-20, 0)
PointC is (20, 0)

 

Spring Related Topics you may like

 

In Next Chapter we will discuss about Injecting Inner Beans in Spring with Example in Application.

                                          
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About The Author

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!

2 Comments

  1. Anonymous June 21, 2012

    good one..

  2. Anonymous June 23, 2012

    very nice.,