XML Context based bean cannot perform constructor injection : No default constructor found

Question

I am learning Spring framework (without Spring Boot, in order to understand how it works under the hoods).

I got this beans defined in applicationContext.xml file:

    <bean id="connectionXML" class="com.springLearning.ConnectionXML"/>
    <bean id="xmlBasedDAO" class="com.springLearning.XmlBasedDAO">
        <property name="XMLJdbcConnection" ref="connectionXML" />
    </bean>

This is my DAO class:

    public class XmlBasedDAO {

        private XMLJdbcConnection XMLJdbcConnection;


        public XMLJdbcConnection getXMLJdbcConnection() {
            return XMLJdbcConnection;
        }

        public void setXMLJdbcConnection(XMLJdbcConnection XMLJdbcConnection) {
            this.XMLJdbcConnection = XMLJdbcConnection;
        }
}

This is my ConnectionXML class:

    public class ConnectionXML implements XMLJdbcConnection {
       @Override
       public void connect() {
           System.out.println("Connected");
       }
   }

Which implements this interface :

    public interface XMLJdbcConnection {
        void connect();
    }

Now with a setter injection (as shown above), everything works fine.

But here is the problem: if I switch to a defined constructor injection like so:

    public class XmlBasedDAO {

        private XMLJdbcConnection XMLJdbcConnection;

        public XmlBasedDAO(XMLJdbcConnection XMLJdbcConnection) {
            this.XMLJdbcConnection = XMLJdbcConnection;
        }

        public XMLJdbcConnection getXMLJdbcConnection() {
            return XMLJdbcConnection;
        }
    
}

I got this exception:

Error creating bean with name 'xmlBasedDAO' defined in class path resource [applicationContext.xml]: Instantiation of bean failed;

nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.springLearning.XmlBasedDAO]: No default constructor found; nested exception is java.lang.NoSuchMethodException: com.springLearning.XmlBasedDAO.

Why can't I define a constructor in my class with this configuration? Why just a setter injection will work?

I read a lot of questions and the docs too but i did not found an answer for this specific case.

Answer 1

In Spring you can perform DI either by using Constructor Injection or by Setter Injection.

<constructor-arg> tag is used in constructor injection

<property> tag is used in setter injection

You are not changing your xml for constructor injection, you could do this:

<bean id="connectionXML" class="com.springLearning.ConnectionXML"/>
<bean id="xmlBasedDAO" class="com.springLearning.XmlBasedDAO">
    <constructor-arg>
     <ref bean="connectionXML"/> 
    </constructor-arg>  
</bean>

you can check out this tutorial