Now that we have our POJO and a basic configuration for the message bean, we can introduce our first dependency injection example. Through the Spring beans XML file you can configure your bean to initialize with an argument for the constructor, and then assign the arguments. Spring essentially “injects” the argument into your bean. This is referred to as constructor injection.

The following example passes in the String message using a constructor. The class is the same as the one in Basic Bean Creation except the default message on the message variable has been cleared and is now set to null. A single parameter constructor has been added to set a message.

                
public class ConstructorMessage {

    private String message = null;

    /**
     * Constructor
     */
    public ConstructorMessage(String message) {
        this.message = message;
    }

    /**
     * Gets message.
     */
    public String getMessage() {
        return message;
    }

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

}
                
            

The configuration for this bean is exactly the same as in the previous example, but now we have a new element, the constructor-arg. The constructor-arg element injects a message into the bean using the constructor-arg element's value attribute.

                
<?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="message"
          class="org.springbyexample.di.xml.ConstructorMessage">
        <constructor-arg value="Spring is fun." />
    </bean>

</beans>