JAVA EE: How to Inject Set having Dependent Object using Spring dependency injection by constructor?

Friday, 25 May 2018

How to Inject Set having Dependent Object using Spring dependency injection by constructor?

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Constructor Dependency Injection with Collections

In the Spring Framework, Dependency Injection (DI) allows you to manage object dependencies cleanly. While injecting simple types is straightforward, real-world enterprise applications often require injecting collections of custom dependent objects—such as a Set<Address> or Set<Answer>—via constructor injection.

How Spring Handles a Set of Dependent Objects

Spring's IoC container uses the <constructor-arg> tag combined with the <set> and nested <ref bean="..."/> tags in the XML configuration. This ensures that unique instances of your custom bean are instantiated and passed directly into the target class constructor during bean creation.

Key Steps Covered in This Tutorial

Model Creation: Defining the dependent object class and the primary class with a constructor accepting a Set.
XML Bean Configuration: Configuring <constructor-arg> using <set> containing <ref> elements.
Application Context Loading: Initializing ClassPathXmlApplicationContext to fetch the configured bean.
Output Verification: Iterating through the injected Set to display the dependent object details.

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How to Inject Set having Dependent Object using Spring dependency injection by constructor?

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>

Fruit.java

package com.ram.model;

public class Fruit

{

    private String name;

    private String color;

    public Fruit(String name, String color)

    {

        System.out.println("Fruit(String name, String color) "

                + "constructor is called by the Spring container");

        this.name = name;

        this.color = color;

    }

    @Override

    public String toString()

    {

        return "Fruit [name=" + name + ", color=" + color + "]";

    }

}

FruitShop.java

package com.ram.core;

import java.util.Set;

import com.ram.model.Fruit;

public class FruitShop

{

    private String shopName;

    private Set<Fruit> setOfAvailableFruits;

    public FruitShop(String shopName, Set<Fruit> setOfAvailableFruits)

    {

        System.out.println(

                "FruitShop(String shopName, Set<Fruit> setOfAvailableFruits) "

                        + "constructor is called by the Spring container");

        this.shopName = shopName;

        this.setOfAvailableFruits = setOfAvailableFruits;

    }

    public void displayFruitShopInfo()

    {

        System.out.println("FruitShop [shopName=" + shopName

                + ", setOfAvailableFruits=" + setOfAvailableFruits + "]");

    }

}

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="fruit1" class="com.ram.model.Fruit">

        <constructor-arg type="String" value="Apple"></constructor-arg>

        <constructor-arg type="String" value="Red"></constructor-arg>

    </bean>

    <bean id="fruit2" class="com.ram.model.Fruit">

        <constructor-arg type="String" value="Grapes"></constructor-arg>

        <constructor-arg type="String" value="Violet"></constructor-arg>

    </bean>

    <bean id="fruitShop" class="com.ram.core.FruitShop">

        <constructor-arg type="String" value="Fruit Mart"></constructor-arg>

        <constructor-arg type="java.util.Set">

            <set>

                <ref bean="fruit1" />

                <ref bean="fruit2" />

            </set>

        </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("---------------------------------------");

        FruitShop fruitShop = (FruitShop) context.getBean("fruitShop");

        System.out.println(

                "Got fruitShop object from the ApplicationContext(Spring Container)");

        fruitShop.displayFruitShopInfo();

    }

}

Output

Apr 27, 2018 9:28:32 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@5b37e0d2: startup date [Fri Apr 27 09:28:32 IST 2018]; root of context hierarchy

Apr 27, 2018 9:28:32 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions

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

Fruit(String name, String color) constructor is called by the Spring container

Fruit(String name, String color) constructor is called by the Spring container

FruitShop(String shopName, Set<Fruit> setOfAvailableFruits) constructor is called by the Spring container

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

Got fruitShop object from the ApplicationContext(Spring Container)

FruitShop [shopName=Fruit Mart, setOfAvailableFruits=[Fruit [name=Apple, color=Red], Fruit [name=Grapes, color=Violet]]]