Spring XML Configuration Example

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.