How to Inject Map having dependent Object in key and value using dependency injection by constructor

Enjoying this Java & Spring tutorial?

Subscribe to Ram N Java on YouTube

Introduction

In Spring Framework, constructor injection is the preferred way to ensure all mandatory dependencies are initialized properly. When working with collections such as a java.util.Map, scenarios often arise where both the key and the value are custom dependent objects rather than simple primitives or strings.

In this tutorial, you will learn how to configure Spring XML to inject a Map containing custom objects as both its keys and values using constructor dependency injection.

1. Create the Key and Value Model Classes

Let's create two separate model classes: Customer (to act as the Map key) and Car (to act as the Map value).

public class Customer {
    private String name;

    public Customer(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Customer [name=" + name + "]";
    }
}
  

public class Car {
    private String model;

    public Car(String model) {
        this.model = model;
    }

    @Override
    public String toString() {
        return "Car [model=" + model + "]";
    }
}
  

2. Create the Service or Container Class

Next, create a class named CarRentalService that accepts a Map<Customer, Car> through its constructor:

import java.util.Map;

public class CarRentalService {
    private Map<Customer, Car> rentalMap;

    public CarRentalService(Map<Customer, Car> rentalMap) {
        this.rentalMap = rentalMap;
    }

    public void displayRentalInfo() {
        for (Map.Entry<Customer, Car> entry : rentalMap.entrySet()) {
            System.out.println(entry.getKey() + " rented " + entry.getValue());
        }
    }
}
  

3. Configure Spring XML with <key-ref> and <value-ref>

In your applicationContext.xml, declare the dependent beans and then wire them inside the <map> element using key-ref and value-ref attributes inside <entry>:

<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.xsd">

    <!-- Key Beans -->
    <bean id="customer1" class="Customer">
        <constructor-arg value="John" />
    </bean>

    <!-- Value Beans -->
    <bean id="car1" class="Car">
        <constructor-arg value="Tesla Model 3" />
    </bean>

    <!-- Target Bean -->
    <bean id="rentalService" class="CarRentalService">
        <constructor-arg>
            <map>
                <entry key-ref="customer1" value-ref="car1" />
            </map>
        </constructor-arg>
    </bean>

</beans>
  

4. Run and Test the Code

Initialize the Spring context and call the display method to see your injected dependent objects:

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class MainApp {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
        CarRentalService service = (CarRentalService) context.getBean("rentalService");
        service.displayRentalInfo();
    }
}
  

Watch More Spring Tutorials

• How to Inject Map using Spring dependency injection by constructor? | Spring Tutorial
• How to Inject Set having Dependent Object using Spring dependency injection by constructor?
• How to Inject Primitive Value Using Spring dependency injection by constructor? | Spring Tutorial

---

Click here to watch on Youtube:
https://www.youtube.com/watch?v=3SY5lU9aoyE&list=UUhwKlOVR041tngjerWxVccw

Click the below Image to Enlarge:

How to Inject Map having dependent Object in key and value using dependency injection by constructor

How to Inject Map having dependent Object in key and value using dependency injection by constructor

How to Inject Map having dependent Object in key and value using dependency injection by constructor

How to Inject Map having dependent Object in key and value using dependency injection by constructor

How to Inject Map having dependent Object in key and value using dependency injection by constructor

How to Inject Map having dependent Object in key and value using dependency injection by constructor