Spring – Injecting Objects By Constructor Injection

Spring IoC (Inversion of Control) Container is the core of Spring Framework. It creates the objects, configures and assembles their dependencies, manages their entire life cycle. The Container uses Dependency Injection(DI) to manage the components that make up the application. It gets the information about the objects from a configuration file(XML) or Java Code or Java Annotations and Java POJO class. These objects are called Beans. Since the Controlling of Java objects and their lifecycle is not done by the developers, hence the name Inversion Of Control. The followings are some of the main features of Spring IoC.

• Creating Object for us,
• Managing our objects,
• Helping our application to be configurable,
• Managing dependencies

Spring Dependency Injection

Dependency Injection is the main functionality provided by Spring IOC(Inversion of Control). The Spring-Core module is responsible for injecting dependencies through either Constructor or Setter methods. The design principle of Inversion of Control emphasizes keeping the Java classes independent of each other and the container frees them from object creation and maintenance. These classes, managed by Spring, must adhere to the standard definition of Java-Bean. Dependency Injection in Spring also ensures loose coupling between the classes. There are two types of Spring Dependency Injection.

• Setter Dependency Injection (SDI)
• Constructor Dependency Injection (CDI)

To read more on Spring Dependency Injection please refer to this article: Spring Dependency Injection with Example

Constructor Injection

In Constructor Injection, the Dependency Injection will be injected with the help of constructors. Now to set the Dependency Injection as Constructor Dependency Injection in bean, it is done through the bean-configuration file. For this, the property to be set with the CDI is declared under the <constructor-arg> tag in the bean-config file.

So in this article, let’s learn how we are going to use Spring to inject our dependencies into our object values by Setter Injection. Object is a basic unit of Object-Oriented Programming and represents real-life entities. So generally in Java, we create objects of a class using the new keyword.

Test t = new Test();
// creating object of class Test
// t is the object

Implementation: Suppose we have a class named “Student” and inside the class, we want to inject the object of another class named “MathCheat”. And there is one method also present inside the class named “cheating()” something like that.

File: Student.java

public class Student 
{
    // Class data members
    private int id;
    private MathCheat mathCheat;

    // Method
    public void cheating() 
    {

        System.out.println("My ID is: " + id);

        mathCheat.mathCheating();
    }

}

File: MathCheat.java

public class MathCheat 
{
    public void mathCheating() 
    {
        System.out.println("And I Have Stated Math Cheating");
    }
}

So now we want to inject the object of MathCheat into the Student class by using the concept of Constructor Dependency Injection. So at first, we have to create the constructor inside the Student.java file. Now the Student.java file is something like this.

Java

// Java Program to Illustrate Student class

public class Student {

    // Class data members
    private int id;
    private MathCheat mathCheat;

    // Constructor of Student class
    public Student(int id, MathCheat mathCheat)
    {
        this.id = id;
        this.mathCheat = mathCheat;
    }

    // Method
    public void cheating()
    {
        System.out.println("My ID is: " + id);
        mathCheat.mathCheating();
    }
}

The only thing we need to change is in the beans.xml file. Now let’s create a Student Bean in the beans.xml file and inside the bean, you have to add your property’s name and its corresponding values inside the <constructor-arg> tag instead of the <property> tag, like this

Syntax: Standard

<bean id="AnyUniqueId" class="YourClassName">
  <constructor-arg name="attributes that you have defined in your class" value="And its corresponding values"/>
</bean>

For example: For this project, we can write something like this

<bean id="student" class="Student">
    <constructor-arg name="id" value="101"/>
    <constructor-arg name="mathCheat">
       <bean class="MathCheat"></bean>
    </constructor-arg>
</bean>

You can see how we created the bean of MathCheat class inside the Student bean. Below is the complete code for the beans.xml file

XML

<?xml version="1.0" encoding="UTF-8"?>
<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
       https://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="student" class="Student">
        <constructor-arg name="id" value="101"/>
        <constructor-arg name="mathCheat">
            <bean class="MathCheat"></bean>
        </constructor-arg>
    </bean>

</beans>

So for testing this stuff let’s create a main method and call the cheating() method inside the Student class. Below is the code for the Main.java file.

Application File 

Java

// Java Program to Illustrate Application Class

// Importing required classes
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

// Application class
// GFG class
public class Main {

    // Main driver method
    public static void main(String[] args)
    {

        // Implementing Spring IoC
        // Using ApplicationContext
        ApplicationContext context
            = new ClassPathXmlApplicationContext(
                "beans.xml");

        // Getting the bean student
        Student student
            = context.getBean("student", Student.class);

        // Calling the method inside main() method
        student.cheating();
    }
}

Output:

My ID is: 101
And I Have Stated Math Cheating

Another Approach (Right Approach)

So there is another approach to create the bean of  MathCheat class inside the beans.xml file. So you can write something like this using the “ref“. 

<bean id="mathCheatObjectValue" class="MathCheat"></bean>


<bean id="student" class="Student">
    <constructor-arg name="id" value="101"/>
    <constructor-arg name="mathCheat" ref="mathCheatObjectValue"/>
</bean>

Below is the complete code for the beans.xml file.

Example:

XML

<?xml version="1.0" encoding="UTF-8"?>
<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
       https://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="mathCheatObjectValue" class="MathCheat"></bean>

    <bean id="student" class="Student">
         <constructor-arg name="id" value="101"/>
         <constructor-arg name="mathCheat" ref="mathCheatObjectValue"/>
    </bean>

</beans>

So whenever you are going to run your application you are going to get the same output. Example