In this article, I will be demonstrating how to configure a standalone Spring application using XML configuration. I will be demonstrating both setter and constructor injection. I will be using Eclipse and Maven. In order to get a basic introduction to Spring, you can check out this article.

Project Set-up

Step 1 - Create a new Maven Project in Eclipse. You can refer to this article on how to create a Maven project in Eclipse.

Step 2 - Add Spring dependencies to the maven pom file. You can add the following:

<dependencies>
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-core</artifactId>
      <version>5.2.2.RELEASE</version>
    </dependency>
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-context</artifactId>
      <version>5.2.2.RELEASE</version>
    </dependency>
  </dependencies>

Creating Beans

Create the following bean classes:

MessageDAO:

public class MessageDAO {
  
  public String getMessage() {
    return "Hello World";
  }

}

MessageDAO is a simple class that has only one method getMessage()This returns a String value.

MessageService:

public class MessageService {
  
  private MessageDAO messageDAO;

  public void printMessage() {
    String message = messageDAO.getMessage();
    System.out.println(message);
  }

  public MessageDAO getMessageDAO() {
    return messageDAO;
  }

  public void setMessageDAO(MessageDAO messageDAO) {
    this.messageDAO = messageDAO;
  }

}

MessageService has a method printMessage. It uses the MessageDAOto obtain the message and prints it. It has a private field corresponding to MessageDAOand getter/setter methods for it.

Configuration File (With Setter Injection)

Create a file ApplicationContext.xml in the src/main/resources folder.Add the following:

<?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.xsd">
  <bean id="messageService"
    class="learnjava.demo.MessageService">
    <property name="messageDAO" ref="messageDAO" />
  </bean>
  <bean id="messageDAO"
    class="learnjava.demo.MessageDAO">
  </bean>
</beans>

This ApplicationContext.xml is the spring configuration file. It specifies each bean in the application via the “bean” tag. Since the messageServicerequires the messageDAO, it specifies this via a propertytag.

Writing Main Code

Create a class Main.java with the following code:

public class Main {
  public static void main(String[] args) {
    ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("ApplicationContext.xml");
    MessageService messageService = applicationContext.getBean("messageService", MessageService.class);
    messageService.printMessage();
    
  }
}

The org.springframework.context..ApplicationContextinterface represents the Spring Container. This code first creates a ClassPathXmlApplicationContextinstance which is an implementation of the ApplicationContextinterface. There are several other implementations too. The ClassPathXmlApplicationContextis used in case of an XML configuration.

ApplicationContexthas a method getBean. The code invokes this method in order to obtain theMessageService. The code then invokes the printMessagemethod.

So on execution, this code prints the following output:

Hello World

Using Constructor Injection

In order to use constructor injection you need to make the following modifications:

Beans

The MessageService bean needs to have a constructor instead of getter/setter methods as follows:

public class MessageService {
  
  private MessageDAO messageDAO;

  public MessageService(MessageDAO messageDAO) {
    super();
    this.messageDAO = messageDAO;
  }

  public void printMessage() {
    String message = messageDAO.getMessage();
    System.out.println(message);
  }
}

Configuration file

The configuration file needs to specify that constructor injection is being used as follows:

<?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.xsd">
  <bean id="messageService"
    class="learnjava.demo.MessageService">
    <constructor-arg ref="messageDAO"/>
  </bean>
  <bean id="messageDAO"
    class="learnjava.demo.MessageDAO">
  </bean>
</beans>

Here, instead of specifying the propertytag for the messageServicebean, the constructor-argis specified.

Rest of the code remains the same. So when you execute the code with these changes, it will print the same output as before.

Further Learning

• Spring MasterClass
• Spring Tutorial For Beginners
• Step by Step Spring MVC Tutorial
• Spring Framework in Easy Steps

Conclusion

So in this article, we saw what is XML configuration in Spring. We saw how to configure a standalone Spring application using setter and constructor injection.