How to Inject Set having Dependent Object using Spring dependency injection by Setter method

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Introduction

In Spring Framework, configuring collection dependencies often involves collections that must maintain uniqueness, such as a java.util.Set. When your model requires a set containing custom dependent collaborator objects rather than basic literals, Spring provides the <set> element along with <ref bean="..."/> tags to seamlessly inject them via setter-based dependency injection.

Why Inject a Set of Dependent Objects?

• Ensuring Uniqueness: A Set guarantees that duplicate collaborator references are automatically eliminated based on the object's equals() and hashCode() contracts.
• Declarative Collaborator Wiring: By leveraging nested <ref bean="..."/> elements inside <set>, Spring connects independent bean instances without manual code instantiation.
• Loose Coupling: Dependent beans can be configured and managed independently in XML, making testing and maintenance straightforward.

Step-by-Step Code Example

Here is an example showing how to inject a Set<Address> of dependent objects into a Customer bean using setter injection:

1. The Dependent and Consumer Classes

package com.ram.model;

import java.util.Objects;
import java.util.Set;

// Dependent Object
public class Address {
    private String city;
    private String state;

    public void setCity(String city) {
        this.city = city;
    }

    public void setState(String state) {
        this.state = state;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Address address = (Address) o;
        return Objects.equals(city, address.city) && Objects.equals(state, address.state);
    }

    @Override
    public int hashCode() {
        return Objects.hash(city, state);
    }

    @Override
    public String toString() {
        return "Address [city=" + city + ", state=" + state + "]";
    }
}

// Consumer Class holding the Set
public class Customer {
    private String name;
    private Set<Address> addresses;

    public void setName(String name) {
        this.name = name;
    }

    // Setter method for Set injection
    public void setAddresses(Set<Address> addresses) {
        this.addresses = addresses;
    }

    public void display() {
        System.out.println("Customer Name: " + name);
        System.out.println("Addresses:");
        for (Address address : addresses) {
            System.out.println(address);
        }
    }
}

2. Configuring applicationContext.xml

Define the dependent beans first, then reference them inside the <set> element:

<!-- Dependent Address Beans -->
<bean id="address1" class="com.ram.model.Address">
    <property name="city" value="Bangalore" />
    <property name="state" value="Karnataka" />
</bean>

<bean id="address2" class="com.ram.model.Address">
    <property name="city" value="Chennai" />
    <property name="state" value="Tamil Nadu" />
</bean>

<!-- Customer Bean with Set Injection -->
<bean id="customer" class="com.ram.model.Customer">
    <property name="name" value="Ram" />
    <property name="addresses">
        <set>
            <ref bean="address1" />
            <ref bean="address2" />
        </set>
    </property>
</bean>

3. Loading and Executing with ApplicationContext

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

Customer customer = (Customer) context.getBean("customer");
customer.display();

Key Takeaway

Using <set> with nested <ref bean="..."/> elements enables Spring to populate distinct collaborator beans into a Java Set using setter injection, keeping component configurations flexible and strictly organized.

Recommended Collection & Dependency Injection Tutorials from Ram N Java

• How to Inject Array having Dependent Object using Spring dependency injection by Setter method?
• How to Inject Map having Dependent Object in key and value using DI by Setter method?
• How to Inject a Map having a Dependent Object in value using dependency injection by Setter method?

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How to Inject Set having Dependent Object using Spring dependency injection by Setter method

How to Inject Set having Dependent Object using Spring dependency injection by Setter method

How to Inject Set having Dependent Object using Spring dependency injection by Setter method

How to Inject Set having Dependent Object using Spring dependency injection by Setter method