Learn Spring basics through Helloworld Program

Spring jump start

Spring-enabled applications are like any Java application. They are made up of several classes, each performing a specific purpose within the application. What makes Spring-enabled applications different, however, is how these classes are configured and introduced to each other. Typically, a Spring application has an XML file that describes how to configure the classes, known as the Spring configuration file.

The first class that our Springified Hello World example needs is a service class whose purpose is to print the infamous greeting. Listing 1.1 shows GreetingService.java, an interface that defines the contract for our service class.

Listing 1.1 The GreetingService interface separates the service's implementation from its interface.

package com.springinaction.chapter01.hello;

public interface GreetingService {

   public void sayGreeting();

}

GreetingServiceImpl.java (listing 1.2) implements the GreetingService interface. Although it's not necessary to hide the implementation behind an interface, it's highly recommended as a way to separate the implementation from its contract.

Listing 1.2 GreetingServiceImpl.java: Responsible for printing the greeting

package com.springinaction.chapter01.hello;

public class GreetingServiceImpl implements GreetingService {

   private String greeting;

   public GreetingServiceImpl() {}

   public GreetingServiceImpl(String greeting) {

      this.greeting = greeting;

   }

   public void sayGreeting() {

      System.out.println(greeting);

   }

   public void setGreeting(String greeting) {

      this.greeting = greeting;

   }

}

The GreetingServiceImpl class has a single property: the greeting property. This property is simply a String that holds the text that is the message that will be printed when the sayGreeting() method is called. You may have noticed that the greeting can be set in two different ways: by the constructor or by the property's setter method.

What's not apparent just yet is who will make the call to either the constructor or the setGreeting() method to set the property. As it turns out, we're going to let the Spring container set the greeting property. The Spring configuration file (hello.xml) in listing 1.3 tells the container how to configure the greeting service.

Listing 1.3 Configuring Hello World in Spring

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

<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN//EN"

   "http://www.springframework.org/dtd/spring-beans.dtd">

<beans>

   <bean id="greetingService"

         class="com.springinaction.chapter01.hello.GreetingServiceImpl">

      <property name="greeting">

         <value>Buenos Dias!</value>

      </property>

   </bean>

</beans>

The XML file in listing 1.3 declares an instance of a GreetingServiceImpl in the Spring container and configures its greeting property with a value of "Buenos Dias!" Let's dig into the details of this XML file a bit to understand how it works.

At the root of this simple XML file is the <beans> element, which is the root element of any Spring configuration file. The <bean> element is used to tell the Spring container about a class and how it should be configured. Here, the id attribute is used to name the bean greetingService and the class attribute specifies the bean's fully qualified class name.

Within the <bean> element, the <property> element is used to set a property, in this case the greeting property. By using <property>, we're telling the Spring container to call setGreeting() when setting the property.

The value of the greeting is defined within the <value> element. Here we've given the example a Spanish flair by choosing "Buenos Dias" instead of the traditional "Hello World."

The following snippet of code illustrates roughly what the container does when instantiating the greeting service based on the XML definition in listing 1.3:²

GreetingServiceImpl greetingService = new GreetingServiceImpl();

greetingService.setGreeting("Buenos Dias!");

Similarly, we may choose to have Spring set the greeting property through GreetingServiceImpl's single argument constructor. For example:

<bean id="greetingService"

      class="com.springinaction.chapter01.hello.GreetingServiceImpl">

   <constructor-arg>

      <value>Buenos Dias!</value>

   </constructor-arg>

</bean>

The following code illustrates how the container will instantiate the greeting service when using the <constructor-arg> element:

GreetingServiceImpl greetingService =

   new GreetingServiceImpl("Buenos Dias");

The last piece of the puzzle is the class that loads the Spring container and uses it to retrieve the greeting service. Listing 1.4 shows this class.

Listing 1.4 The Hello World main class

package com.springinaction.chapter01.hello;

import java.io.FileInputStream;

import org.springframework.beans.factory.BeanFactory;

import org.springframework.beans.factory.xml.XmlBeanFactory;

public class HelloApp {

   public static void main(String[] args) throws Exception {

      BeanFactory factory =

         new XmlBeanFactory(new FileInputStream("hello.xml"));

      GreetingService greetingService =

         (GreetingService) factory.getBean("greetingService");

      greetingService.sayGreeting();

   }

}