JAVA EE: Factory Design Pattern

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Tuesday, 31 July 2018

What is Spring DI with a static factory method that returns the instance of another class?

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Introduction

In Spring XML configuration, the IoC container can create beans not only by invoking constructors or static methods of the same class, but also by invoking a static factory method located on a completely different class. This capability is widely used when working with legacy factory utility classes, third-party libraries, or complex factory design patterns.

Why Use a Static Factory Method of Another Class?

Factory Separation: Keeps product classes clean and stateless while delegating construction logic to a separate static factory class.
Third-Party Integration: Allows Spring to manage objects instantiated by external factories without needing modifications to their source code.
Simple XML Configuration: Uses the class attribute to specify the factory class and factory-method to specify the static method returning the target object.

Step-by-Step Configuration Example

Consider a scenario where a separate class, BeanFactory, contains a static method to produce instances of Printer:

1. The Factory and Product Classes

package com.ram.factory;

import com.ram.service.Printer;

public class BeanFactory {
    // Static factory method returning an instance of another class
    public static Printer getPrinterInstance() {
        return new Printer();
    }
}

2. Configuring in applicationContext.xml

The class attribute points to the factory class, while factory-method invokes the static method:

<!-- Bean created by calling a static method on BeanFactory -->
<bean id="printer" 
      class="com.ram.factory.BeanFactory" 
      factory-method="getPrinterInstance" />

3. Retrieving the Bean in Java

ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");

Printer printer = (Printer) context.getBean("printer");
printer.print();

Key Takeaway

When you specify class and factory-method, Spring calls that static method to obtain the resulting bean instance. The bean registered in the container is the object returned by the factory method, not the factory class itself.

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What is Spring DI with a static factory method that returns the instance of another class?

pom.xml

<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%20http://maven.apache.org/maven-v4_0_0.xsd">

    <modelVersion>4.0.0</modelVersion>

    <groupId>com.ram.core</groupId>

    <artifactId>SpringDemo</artifactId>

    <packaging>jar</packaging>

    <version>1.0-SNAPSHOT</version>

    <name>SpringDemo</name>

    <url>http://maven.apache.org</url>

    <properties>

        <spring.version>5.0.2.RELEASE</spring.version>

    </properties>

    <dependencies>

        <dependency>

            <groupId>junit</groupId>

            <artifactId>junit</artifactId>

            <version>3.8.1</version>

            <scope>test</scope>

        </dependency>

        <!-- Spring 5 dependencies -->

        <dependency>

            <groupId>org.springframework</groupId>

            <artifactId>spring-core</artifactId>

            <version>${spring.version}</version>

        </dependency>

        <dependency>

            <groupId>org.springframework</groupId>

            <artifactId>spring-context</artifactId>

            <version>${spring.version}</version>

        </dependency>

    </dependencies>

</project>

Animal.java

package com.ram.model;

public interface Animal

{

    public abstract void eat();

}

Cow.java

package com.ram.model;

public class Cow implements Animal

{

    public void eat()

    {

        System.out.println("Cow eats grass..");

    }

}

Dog.java

package com.ram.model;

public class Dog implements Animal

{

    public void eat()

    {

        System.out.println("Dog eats chicken bone...");

    }

}

AnimalFactory.java

package com.ram.factory;

import com.ram.model.Animal;

import com.ram.model.Dog;

public class AnimalFactory

{

    public static Animal getAnimal()

    {

        return new Dog();// return any one instance, either Dog or Cow

        // return new Cow();

    }

}

applicationContext.xml

<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-4.3.xsd">

    <bean id="animal" class="com.ram.factory.AnimalFactory"

        factory-method="getAnimal">

    </bean>

</beans>

App.java

package com.ram.core;

import org.springframework.context.ApplicationContext;

import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.ram.model.Animal;

public class App

{

    public static void main(String[] args)

    {

        ApplicationContext context = new ClassPathXmlApplicationContext(

                "applicationContext.xml");

        System.out.println("---------------------------------------");

        Animal animal = (Animal)context.getBean("animal");

        System.out.println(

                "Got " + animal +" object from the ApplicationContext[Spring Container]");

        animal.eat();

    }

}

Output

Jul 05, 2018 9:24:36 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@5b37e0d2: startup date [Thu Jul 05 09:24:36 IST 2018]; root of context hierarchy

Jul 05, 2018 9:24:36 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions

INFO: Loading XML bean definitions from class path resource [applicationContext.xml]

---------------------------------------

Got com.ram.model.Dog@3ffc5af1 object from the ApplicationContext[Spring Container]

Dog eats chicken bone...