Wednesday, March 26, 2014

Spring Tutorial 02 - My First 'Hello World' Program in Spring

Before writing any program in spring, let’s first setup our Spring environment in eclipse.


Step 1 - Create Java Project:
The first step is to create a simple Java Project using Eclipse. Go to the option File -> New -> Project and select Java Project from the wizard list.







Now name your project as ‘SpringTutorial_02’ using the wizard window as follows:



The structure of the project would be like as follows.




Step 2 - Add Required Libraries:

Now we need to add the libraries of spring to give support of the spring in this project. Select the project ---> Right Click --->Property --->Java Build Path as follows.





Now add the spring libraries.



List of Libraries.


  • antlr-runtime-3.0.1
  • org.springframework.aop-3.1.0.M2
  • org.springframework.asm-3.1.0.M2
  • org.springframework.aspects-3.1.0.M2
  • org.springframework.beans-3.1.0.M2
  • org.springframework.context.support-3.1.0.M2
  • org.springframework.context-3.1.0.M2
  • org.springframework.core-3.1.0.M2
  • org.springframework.expression-3.1.0.M2
  • commons-logging-1.1.1

Step 3 - Create Source Files:


Now let’s create source files in this project. First we need to create a package called org.javaIsEasy.springTutorial. To do this, right click on src in package explorer section and follow the option : New -> Package. Next we will create HelloWorld.java and MainClass.java files under the org.javaIsEasy.springTutorial package.

HelloWorld.java


package org.javaIsEasy.springTutorial;

public class HelloWorld {
    private String message;

    public void setMessage(String message){
       this.message  = message;
    }

    public void getMessage(){
       System.out.println("Your Message : " + message);
    }
 }


MainClass.Java


package org.javaIsEasy.springTutorial;

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

public class MainClass {

 public static void main(String[] args) {
  ApplicationContext context = new ClassPathXmlApplicationContext("org/javaIsEasy/springTutorial /Beans.xml");

  HelloWorld obj = (HelloWorld) context.getBean("helloWorld");

  obj.getMessage();
 }
}

***********************************************************************************

Java files have been created now we need to create 

There are following two important points to note about the main program:


  • First step is to create application context where we used framework API ClassPathXmlApplicationContext(). This API loads beans configuration file and eventually based on the provided API, it takes care of creating and initializing all the objects ie. beans mentioned in the configuration file.
  • Second step is used to get required bean using getBean() method of the created context. This method uses bean ID to return a generic object which finally can be casted to actual object. Once you have object, you can use this object to call any class method.



Step 4 - Create Bean Configuration File:
You need to create a Bean Configuration xml file which would be created in org.javaIsEasy.springTutorial package.



We have given 'Beans.xml' to this configuration file. We can give any name to this file as we want to. Only the classpath and the name of this configuration file needs to be maintained which is being used in the java files. 

The Beans.xml is used to assign unique IDs to different beans and to control the creation of objects with different values without impacting any of the Spring source files. For example, using below file you can pass any value for "message" variable and so you can print different values of message without impacting HelloWorld.java and MainClass

After creating the Beans.xml file, our project structure would be as follows.




Beans.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="helloWorld" class="org.javaIsEasy.springTutorial.HelloWorld">
       <property name="message" value="Java Is Easy !!"/>
   </bean>

</beans>


When Spring application gets loaded into the memory, Framework makes use of the above configuration file to create all the beans defined and assign them a unique ID as defined in <bean> tag. You can use <property> tag to pass the values of different variables used at the time of object creation.


Step 5 - Running the Program:
Let's run this program and see what result would be coming.




************************************************************************************************
OUTPUT : Your Message : Java Is Easy !!
************************************************************************************************


Congratulations, you have created your first Spring Application successfully. You can see the flexibility of above Spring application by changing the value of "message" property and keeping both the source files unchanged. Further, let us start doing something more interesting in next few chapters.