JAVA EE: How to Inject Set Collection using Spring dependency injection by constructor?

Friday, 25 May 2018

How to Inject Set Collection using Spring dependency injection by constructor? | Spring Tutorial

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Constructor Injection with Set Collections

In the Spring Framework, Constructor Injection is a best practice used to pass mandatory dependencies into a bean at the time of instantiation. When a bean property requires a collection of unique elements, Spring provides dedicated XML configuration elements to inject a java.util.Set directly through the constructor.

By injecting collections via constructor arguments, your beans remain immutable and fully initialized from the moment they are created by the IoC container.

Configuring <set> in Spring XML

To pass a Set to a constructor, use the <constructor-arg> tag combined with the inner <set> element. Individual values inside the set are defined using the <value> sub-element:

The Spring container automatically parses these values, constructs the appropriate Set implementation (typically a LinkedHashSet to preserve insertion order), and passes it to the corresponding constructor.

Step-by-Step Execution Workflow

XML Loading: The Spring container loads bean metadata using ClassPathXmlApplicationContext.
Constructor Matching: The container inspects the target class constructors to find a matching signature accepting a Set.
Collection Assembly: The container populates the configured items into a concrete Set collection.
Instantiation: The target bean is instantiated with the fully prepared collection injected via its constructor.

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How to Inject Set Collection using Spring dependency injection by constructor? | 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>

FruitShop.java

package com.ram.core;

import java.util.Set;

public class FruitShop

{

    private String shopName;

    private Set<String> setOfAvailableFruits;

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

    {

        System.out.println(

                "FruitShop(String shopName, Set<String> 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="fruitShop" class="com.ram.core.FruitShop">

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

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

            <set>

                <value>apple</value>

                <value>pineapple</value>

                <value>orange</value>

            </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 25, 2018 9:13:55 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@5b37e0d2: startup date [Wed Apr 25 09:13:55 IST 2018]; root of context hierarchy

Apr 25, 2018 9:13:55 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions

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

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

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

Got fruitShop object from the ApplicationContext(Spring Container)

FruitShop [shopName=Fruit Mart, setOfAvailableFruits=[apple, pineapple, orange]]