Java based Spring Configuration

In the previous articles, we saw how to configure the beans based on XML and annotations. In this example, we’ll learn how we can configure beans based Java configuration, a third approach to configure Spring beans. We’ll follow the similar class design by coding to interfaces.

Class Structure

• Item – a POJO class to store item info
• ItemDAO – an interface that we will use in a dependent service class
• ItemDAOImpl – an implementation of ItemDAO returns list of mocked Item instances
• ItemService – an interface that we will use in the application to hold a service instance
• ItemServiceImpl – an implementation of ItemService and the dependent of ItemDAO instance
• Application – It will get the ItemService instance to fetch items
• AppConfig – a configuration class replacing the XML based configuration

Creation of Maven Project

First create a simple Maven project by skipping archtype selection in STS.

Add below pom.xml file which can be found at the root of the project.

<project xmlns="http://maven.apache.org/POM/4.0.0"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<groupId>com.techstackjournal</groupId>
	<artifactId>java-config</artifactId>
	<version>0.0.1-SNAPSHOT</version>

	<dependencies>
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-context</artifactId>
			<version>4.3.2.RELEASE</version>
		</dependency>
	</dependencies>

	<build>
		<plugins>
			<plugin>
				<groupId>org.apache.maven.plugins</groupId>
				<artifactId>maven-compiler-plugin</artifactId>
				<version>3.2</version>
				<configuration>
					<source>1.8</source>
					<target>1.8</target>
				</configuration>
			</plugin>
		</plugins>
	</build>
</project>

Right click on the project, and select Maven=>Update Project. This will download all the required jar files for the application.

Creating the Model Class

Let’s create an Item model class which will hold the instances returned by the DAO layer.

package com.techstackjournal.model;

public class Item {

	private String itemId;
	private String itemName;
	private double price;

	public Item() {

	}

	public Item(String itemId, String itemName, double price) {
		super();
		this.itemId = itemId;
		this.itemName = itemName;
		this.price = price;
	}

	public String getItemId() {
		return itemId;
	}

	public void setItemId(String itemId) {
		this.itemId = itemId;
	}

	public String getItemName() {
		return itemName;
	}

	public void setItemName(String itemName) {
		this.itemName = itemName;
	}

	public double getPrice() {
		return price;
	}

	public void setPrice(double price) {
		this.price = price;
	}

	@Override
	public String toString() {
		return "Item [itemId=" + itemId + ", itemName=" + itemName + ", price=" + price + "]";
	}

}

Data Access Layer

In this layer, we’ll create ItemDAO interface, which will be implemented by ItemDAOImpl. Also, ItemServiceImpl will use ItemDAO interface to call the findAll method. We call this approach as coding to interface. With this approach, we can decrease the dependence on the implementation class.

package com.techstackjournal.dao;

import java.util.List;

import com.techstackjournal.model.Item;

public interface ItemDAO {

	List<Item> findAll();

}

ItemDAOImpl implements ItemDAO and provides a findAll method. In this findAll method, we are not writing the actual data access logic to simplify the subject in focus. We’ll just mock the findAll method by returning a collection object.

package com.techstackjournal.dao;

import java.util.ArrayList;
import java.util.List;

import com.techstackjournal.model.Item;

public class ItemDAOImpl implements ItemDAO {
	public List<Item> findAll() {
		List<Item> items = new ArrayList<Item>();
		items.add(new Item("1", "Mobile", 400));
		items.add(new Item("2", "Book", 10));
		return items;
	}
}

Service Layer

In this Service Layer, we’ll be creating ItemService interface and ItemServiceImpl class.

package com.techstackjournal.service;

import java.util.List;

import com.techstackjournal.model.Item;

public interface ItemService {

	List<Item> findAll();

}

ItemServiceImpl implements ItemService interface. This class is in HAS-A relation with an object of type ItemDAO. But you can see that we are not creating any instance of type ItemDAO. Spring Container will wire the instance of ItemServiceImpl with the instance of type ItemDAO either by constructor or by type ItemDAO or by the name of the reference variable itemDao.

package com.techstackjournal.service;

import java.util.List;

import com.techstackjournal.dao.ItemDAO;
import com.techstackjournal.model.Item;

public class ItemServiceImpl implements ItemService {

	private ItemDAO itemDao;
	
	public ItemServiceImpl() {

	}

	public ItemServiceImpl(ItemDAO itemDao) {
		super();
		this.itemDao = itemDao;
	}

	public List<Item> findAll() {
		return itemDao.findAll();
	}

	public void setItemDao(ItemDAO itemDao) {
		this.itemDao = itemDao;
	}

}

Main Application

In this class, we’ll be fetching the ItemService instance from Spring Container using AnnotationConfigApplicationContext by passing AppConfig.class as a constructor argument.

package com.techstackjournal.app;

import java.util.List;

import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;

import com.techstackjournal.model.Item;
import com.techstackjournal.service.ItemService;

public class Application {

	public static void main(String[] args) {

		ApplicationContext appContext = new AnnotationConfigApplicationContext(AppConfig.class);

		ItemService service = appContext.getBean("itemService", ItemService.class);

		List<Item> items = service.findAll();

		for (Item item : items) {
			System.out.println(item);
		}

	}

}

Java based Configuration

Finally, in this class we will define the beans using Java based configuration replacing the Spring XML based configuration (applicationContext.xml) file. In this class we’ll define 2 beans to get the instances of type ItemDAOImpl and ItemServiceImpl, respectively.

Java based Configuration with Constructor Injection

While creating the ItemServiceImpl bean, we’ll make use of its constructor having ItemDAO to address the dependency by passing the instance of ItemDAOImpl.

package com.techstackjournal.app;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

import com.techstackjournal.dao.ItemDAO;
import com.techstackjournal.dao.ItemDAOImpl;
import com.techstackjournal.service.ItemService;
import com.techstackjournal.service.ItemServiceImpl;

@Configuration
public class AppConfig {

	@Bean(name = "itemDao")
	public ItemDAO getItemDAO() {
		return new ItemDAOImpl();
	}

	@Bean(name = "itemService")
	public ItemService getItemService() {
		return new ItemServiceImpl(getItemDAO());
	}

}

Java based Configuration with Setter Injection

To resolve the dependencies of ItemServiceImpl using setter injection we’ll change the above getItemService method as below.

	@Bean(name = "itemService")
	public ItemService getItemService() {
		ItemServiceImpl itemServiceImpl = new ItemServiceImpl();
		itemServiceImpl.setItemDao(getItemDAO());
		return itemServiceImpl;
	}