Spring - Dependency Injection

Java components / classes should be as independent as possible of other Java classes.

This increases the possibility to reuse these classes and to test them independently of other classes(Unit Testing).

To decouple Java components from other Java components the dependency to a certain other class should get injected into them rather that the class itself creates / finds this object.

A class A has a dependency to class B if class uses class B as a variable.

If dependency injection is used then the class B is given to class A via:

1.The constructor of the class A - this is then called construction injection

1.A setter - this is then called setter injection

The general concept between dependency injection is called Inversion of Control.

A class should not configure itself but should be configured from outside.

A design based on independent classes / components increases the re-usability and possibility to test the software.

For example, if a class A expects a Dao (Data Access object) for receiving the data from a database you can easily create another

test object which mocks the database connection and inject this object into A to test A without having an actual database connection.

A software design based on dependency injection is possible with standard Java.

Spring just simplifies the use of dependency injection by providing a standard way of providing the configuration and by managing the reference to the created objects.

Spring - Setter Injection Example

InjectSetter.java

package com.jtc;
public class InjectSetter {
private String message = null;
public String getMessage() {
return message;
}
public void setMessage(String message) {
this.message = message;
}
}

TestInjectSetter.java

package com.jtc;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class TestInjectSetter {
public static void main(String[] args) {
BeanFactory beanfactory = new ClassPathXmlApplicationContext("context.xml");
InjectSetter bean = (InjectSetter) beanfactory.getBean("welcome");
System.out.println(bean.getMessage());
}
}

context.xml

<?xml version="1.0" encoding="UTF-8"?>

<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="welcome" class="com.jtc.InjectSetter">
      <property name="message" value="Setter Injection!"/>
    
   </bean>
   


</beans>

Below is the code for constructor Injection
InjectConstructor.java

package com.jtc;
public class InjectConstructor {
private String message = null;
public InjectConstructor(String message) {
this.message = message;
}
public String getMessage() {
return message;
}
public void setMessage(String message) {
this.message = message;
}

TestInjectConstructor.java

package com.jtc;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class TestInjectConstructor {
public static void main(String[] args) {
BeanFactory beanfactory = new ClassPathXmlApplicationContext("context.xml");
InjectConstructor bean = (InjectConstructor) beanfactory.getBean("welcome");
System.out.println(bean.getMessage());
}
}

context.xml

<?xml version="1.0" encoding="UTF-8"?>

<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="welcome" class="com.jtc.InjectConstructor">
      <constructor-arg
	  
	  value="Constructor Injection!"/>
	  
    
   </bean>
   


</beans>

Add Below Libraries

commons-logging-1.1.1
spring-aop-4.1.6.RELEASE
spring-aspects-4.1.6.RELEASE
spring-beans-4.1.6.RELEASE
spring-context-4.1.6.RELEASE
spring-context-support-4.1.6.RELEASE
spring-core-4.1.6.RELEASE
spring-expression-4.1.6.RELEASE
spring-instrument-4.1.6.RELEASE
spring-instrument-tomcat-4.1.6.RELEASE
spring-jdbc-4.1.6.RELEASE
spring-jms-4.1.6.RELEASE
spring-messaging-4.1.6.RELEASE
spring-orm-4.1.6.RELEASE
spring-oxm-4.1.6.RELEASE
spring-test-4.1.6.RELEASE
spring-tx-4.1.6.RELEASE
spring-web-4.1.6.RELEASE
spring-webmvc-4.1.6.RELEASE
spring-webmvc-portlet-4.1.6.RELEASE
spring-websocket-4.1.6.RELEASE

OutPut

Setter Injection!
Constructor Injection!

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Spring - Dependency Injection

If dependency injection is used then the class B is given to class A via:

Spring - Setter Injection Example