Basic Hello World Implementation

Create a simple POJO class with a string property:

public class Greeting {
    private String message;
    
    public String getMessage() {
        return message;
    }
    
    public void setMessage(String message) {
        this.message = message;
    }
    
    @Override
    public String toString() {
        return "Greeting{message='" + message + "'}";
    }
}

Configure the Spring bean in XML configuration:

<?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
       https://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="greeting" class="com.example.pojo.Greeting">
        <property name="message" value="Spring Framework"/>
    </bean>

</beans>

Test the configuartion with a unit test:

public class SpringTest {
    @Test
    public void testGreetingBean() {
        ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
        Greeting greeting = (Greeting) context.getBean("greeting");
        System.out.println(greeting.toString());
    }
}

Dependency Injection Example

Define a data access interface:

public interface CustomerRepository {
    void fetchCustomers();
}

Implement the interface with different data sources:

public class DefaultCustomerRepository implements CustomerRepository {
    @Override
    public void fetchCustomers() {
        System.out.println("Fetching customers from default repository");
    }
}

public class DatabaseCustomerRepository implements CustomerRepository {
    @Override
    public void fetchCustomers() {
        System.out.println("Fetching customers from database");
    }
}

Create a service that depends on the repository:

public interface CustomerService {
    void retrieveCustomers();
}

public class CustomerServiceImpl implements CustomerService {
    private CustomerRepository repository;
    
    public void setRepository(CustomerRepository repository) {
        this.repository = repository;
    }
    
    @Override
    public void retrieveCustomers() {
        repository.fetchCustomers();
    }
}

Configure dependency injection in Spring XML:

<?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
       https://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="databaseRepo" class="com.example.repository.DatabaseCustomerRepository"/>
    <bean id="defaultRepo" class="com.example.repository.DefaultCustomerRepository"/>
    
    <bean id="customerService" class="com.example.service.CustomerServiceImpl">
        <property name="repository" ref="defaultRepo"/>
    </bean>

</beans>

Test the service with dependency injection:

public class CustomerServiceTest {
    @Test
    public void testCustomerRetrieval() {
        ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
        CustomerService service = (CustomerService) context.getBean("customerService");
        service.retrieveCustomers();
    }
}