JAVA EE: Spring Dependency Injection via setter method [IOutputGenerator]

Friday, 13 July 2018

Spring Dependency Injection via setter method [IOutputGenerator] | Spring Tutorial

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Introduction

In Spring Framework, Dependency Injection (DI) allows you to build loosely coupled applications by letting the IoC container handle object creation and wiring. Along with constructor injection, setter-based dependency injection is a primary way to inject dependencies into Spring beans. In this tutorial, we demonstrate how to inject an implementation of the IOutputGenerator interface into a helper bean using setter methods in XML configuration.

Why Use Setter-Based Dependency Injection?

Flexibility & Optional Dependencies: Setter injection is ideal for optional dependencies or configurations that may need reconfiguration or reassignment later.
Prevents Circular Dependency Issues: Unlike constructor injection, setter injection allows Spring to resolve circular references between beans more smoothly.
Interface-Driven Design: Injecting via an interface like IOutputGenerator keeps the consumer class completely decoupled from concrete output generator classes (e.g., CSV, JSON, or XML).

Step-by-Step Code Example

Here is a full breakdown showing the interface contract, implementation class, setter wiring in the consumer, and the Spring XML configuration:

1. The IOutputGenerator Interface and Implementation

package com.ram.output;

public interface IOutputGenerator {
    void generateOutput();
}

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

2. The Consumer Class with Setter Method

package com.ram.output;

public class OutputHelper {
    private IOutputGenerator outputGenerator;

    // Setter method for dependency injection
    public void setOutputGenerator(IOutputGenerator outputGenerator) {
        this.outputGenerator = outputGenerator;
    }

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

3. Configuring Beans in applicationContext.xml

In XML, the <property> element invokes the matching setter method using the ref attribute:

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

<!-- Helper Bean with Setter Injection -->
<bean id="outputHelper" class="com.ram.output.OutputHelper">
    <property name="outputGenerator" ref="csvOutputGenerator" />
</bean>

4. Executing with ApplicationContext

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

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

Key Takeaway

Setter dependency injection with <property ref="..."/> provides clear and decoupled architecture. By coding against the IOutputGenerator interface, switching between different output implementations requires only updating the XML bean definition without altering your core application logic.

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Spring Dependency Injection via setter method [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 IOutputGenerator getOutputGenerator()

    {

        return outputGenerator;

    }

    public void setOutputGenerator(IOutputGenerator outputGenerator)

    {

        this.outputGenerator = 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">

        <property name="outputGenerator">

            <ref bean="jsonOutputGenerator" />

        </property>

    </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 23, 2018 10:25:13 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@5b37e0d2: startup date [Sat Jun 23 10:25:13 IST 2018]; root of context hierarchy

Jun 23, 2018 10:25:13 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.