JAVA EE: Spring Dependency Injection via Constructor [IOutputGenerator]

Friday, 13 July 2018

Spring Dependency Injection via Constructor [IOutputGenerator] | Spring Tutorial

Enjoying This Tutorial?

Subscribe to Ram N Java for high-quality, practical Java and Spring tutorials!

Introduction

Dependency Injection (DI) is the core design pattern powering the Spring Framework. By injecting dependencies rather than letting objects instantiate them directly, applications achieve loose coupling and easy testability. In this tutorial, we demonstrate constructor-based dependency injection using a practical interface-driven architecture featuring IOutputGenerator.

Why Use Constructor-Based Dependency Injection?

Ensures Immutability: Injected dependencies can be declared as final, preventing modification after bean creation.
Guarantees Initialization: Beans cannot be created in an incomplete or broken state; all required dependencies must be supplied during instantiation.
Coding to Interfaces: Promotes loose coupling by depending on interface contracts (e.g., IOutputGenerator) rather than concrete implementations.

Step-by-Step Code Example

Here is the architecture showing how an implementation of IOutputGenerator is injected into a service class through its constructor:

1. The Interface and Concrete Implementation

package com.ram.output;

public interface IOutputGenerator {
    void generateOutput();
}

// Concrete Implementation
public class JsonOutputGenerator implements IOutputGenerator {
    @Override
    public void generateOutput() {
        System.out.println("Generating output in JSON format...");
    }
}

2. The Consumer Class with Constructor Injection

package com.ram.output;

public class OutputHelper {
    private final IOutputGenerator outputGenerator;

    // Injected via constructor
    public OutputHelper(IOutputGenerator outputGenerator) {
        this.outputGenerator = outputGenerator;
    }

    public void generate() {
        outputGenerator.generateOutput();
    }
}

3. Wiring Beans in applicationContext.xml

<!-- Implementation Bean -->
<bean id="jsonOutputGenerator" class="com.ram.output.JsonOutputGenerator" />

<!-- Helper Bean with Constructor Injection -->
<bean id="outputHelper" class="com.ram.output.OutputHelper">
    <constructor-arg ref="jsonOutputGenerator" />
</bean>

4. Executing the Application

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

OutputHelper helper = (OutputHelper) context.getBean("outputHelper");
helper.generate();

Key Takeaway

Using <constructor-arg ref="..."/> allows you to swap out implementations of an interface (such as switching from JSON to CSV or XML generator) entirely within XML configuration without modifying a single line of client Java code.

Recommended Dependency Injection Tutorials from Ram N Java

Click here to watch on Youtube:
https://www.youtube.com/watch?v=MnfeSVN5UPo&list=UUhwKlOVR041tngjerWxVccw

Click the below Image to Enlarge:

Spring Dependency Injection via Constructor [IOutputGenerator] | Spring Tutorial

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>

IOutputGenerator.java

package com.ram.core;

public interface IOutputGenerator

{

    public abstract void generate();

}

JsonOutputGenerator.java

package com.ram.core.impl;

import com.ram.core.IOutputGenerator;

public class JsonOutputGenerator implements IOutputGenerator

{

    public void generate()

    {

        System.out.println("Generate Json file.");

    }

}

CsvOutputGenerator.java

package com.ram.core.impl;

import com.ram.core.IOutputGenerator;

public class CsvOutputGenerator implements IOutputGenerator

{

    public void generate()

    {

        System.out.println("Generate csv file.");

    }

}

OutputHelper.java

package com.ram.core;

public class OutputHelper

{

    private IOutputGenerator outputGenerator;

    public OutputHelper(IOutputGenerator outputGenerator)

    {

        this.outputGenerator = outputGenerator;

    }

    public IOutputGenerator getOutputGenerator()

    {

        return outputGenerator;

    }

}

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="csvOutputGenerator" class="com.ram.core.impl.CsvOutputGenerator" />

    <bean id="jsonOutputGenerator" class="com.ram.core.impl.JsonOutputGenerator" />

    <bean id="outputHelper" class="com.ram.core.OutputHelper">

        <constructor-arg  ref="jsonOutputGenerator"></constructor-arg>

    </bean>

</beans>

App.java

package com.ram.core;

import org.springframework.context.ApplicationContext;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class App

{

    public static void main(String[] args)

    {

        ApplicationContext context = new ClassPathXmlApplicationContext(

                "applicationContext.xml");

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

        OutputHelper outputHelper = (OutputHelper) context.getBean("outputHelper");

        System.out.println(

                "Got outputHelper object from the ApplicationContext(Spring container)");

        IOutputGenerator outputGenerator = outputHelper.getOutputGenerator();

        System.out.println(outputGenerator.getClass().getName());

        outputGenerator.generate();

    }

}

Output

Jun 24, 2018 9:35:03 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@5b37e0d2: startup date [Sun Jun 24 09:35:03 IST 2018]; root of context hierarchy

Jun 24, 2018 9:35:03 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions

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

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

Got outputHelper object from the ApplicationContext(Spring container)

com.ram.core.impl.JsonOutputGenerator

Generate Json file.