Autowire Spring Beans using XML Configuration

Let’s look at the examples to Autowire the Spring beans by constructor, by type and by their names using Spring XML configuration.

Class Structure

• Product – a POJO class to store product info
• ProductDAO – an interface that we will use in a dependent service class
• ProductDAOImpl – an implementation of ProductDAO returns list of mocked Product instances
• ProductService – an interface that we will use in the application to hold a service instance
• ProductServiceImpl – an implementation of ProductService and the dependent of ProductDAO instance
• Application – It will get the ProductService instance to fetch products

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>xml-config-autowire</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 a Product model class which will hold the instances returned by the DAO layer.

package com.techstackjournal.model;

public class Product {

	private String productId;
	private String productName;
	private double price;

	public Product() {

	}

	public Product(String productId, String productName, double price) {
		super();
		this.productId = productId;
		this.productName = productName;
		this.price = price;
	}

	public String getProductId() {
		return productId;
	}

	public void setProductId(String productId) {
		this.productId = productId;
	}

	public String getProductName() {
		return productName;
	}

	public void setProductName(String productName) {
		this.productName = productName;
	}

	public double getPrice() {
		return price;
	}

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

	@Override
	public String toString() {
		return "Product [productId=" + productId + ", productName=" + productName + ", price=" + price + "]";
	}

}

Data Access Layer

In this layer, we’ll create ProductDAO interface, which will be implemented by ProductDAOImpl. Also, ProductServiceImpl will use ProductDAO 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.Product;

public interface ProductDAO {

	List<Product> findAll();

}

ProductDAOImpl implements ProductDAO 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.Product;

public class ProductDAOImpl implements ProductDAO {
	public List<Product> findAll() {
		List<Product> products = new ArrayList<Product>();
		products.add(new Product("1", "Mobile", 400));
		products.add(new Product("2", "Book", 10));
		return products;
	}
}

Service Layer

In this Service Layer, we’ll be creating ProductService interface and ProductServiceImpl class.

package com.techstackjournal.service;

import java.util.List;

import com.techstackjournal.model.Product;

public interface ProductService {

	List<Product> findAll();

}

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

package com.techstackjournal.service;

import java.util.List;

import com.techstackjournal.dao.ProductDAO;
import com.techstackjournal.model.Product;

public class ProductServiceImpl implements ProductService {

	private ProductDAO productDao;
	
	public ProductServiceImpl() {

	}

	public ProductServiceImpl(ProductDAO productDao) {
		super();
		this.productDao = productDao;
	}

	public List<Product> findAll() {
		return productDao.findAll();
	}

	public void setProductDao(ProductDAO productDao) {
		this.productDao = productDao;
	}

}

Main Application

In this class, we’ll be fetching the ProductService instance from Spring Container using ClassPathXmlApplicationContext.

package com.techstackjournal.app;

import java.util.List;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.techstackjournal.model.Product;
import com.techstackjournal.service.ProductService;

public class Application {

	public static void main(String[] args) {

		ApplicationContext appContext = new ClassPathXmlApplicationContext("appContext.xml");

		ProductService service = appContext.getBean("productService", ProductService.class);

		List<Product> products = service.findAll();

		for (Product product : products) {
			System.out.println(product);
		}

	}

}

Spring Application Context XML

Finally, in this appContext.xml file we’ll define 2 beans to get the instances of type ProductDAOImpl and ProductServiceImpl, respectively.

The location of the file should be under src/main/resources directory.

Autowire by Constructor

By defining the autowire attribute with value constructor, we can instruct Spring Container to inject ProductDAOImpl instance into ProductServiceImpl via its constructor.

<?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 name="productDao" 
	      class="com.techstackjournal.dao.ProductDAOImpl" />

	<bean name="productService" 
		  class="com.techstackjournal.service.ProductServiceImpl" autowire="constructor" />
		  
</beans>

Autowire by Type

To instruct Spring Container to wire the beans by type, we initialize the autowire attribute with byType. This will match the available bean types in Spring context with the dependency requirements in ProductServiceImpl and wires them together automatically.

<bean name="productService" 
		  class="com.techstackjournal.service.ProductServiceImpl" autowire="byType" />

Autowire by Name

To instruct Spring container to wire the beans by names of the reference variables, we initialize the attribute autowire with byName value.

<bean name="productService" 
		  class="com.techstackjournal.service.ProductServiceImpl" autowire="byName" />