Constructor Injection with overloaded constructors in Spring

In the previous tutorial, I introduced you all about Constructor Injection in Spring. But because we can have overloaded constructors in Java, so in this tutorial, I will show you all how we can declare those overloaded constructors in the Spring container.

Let’s consider the following example:

package com.huongdanjava.springoverloadedconstructorinjection;

public class Student {

  private String name;
  private int age;

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

  public Student(int age) {
    this.age = age;
  }

  public Student(String name, int age) {
    this.name = name;
    this.age = age;
  }

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

}

package com.huongdanjava.springoverloadedconstructorinjection;

public class Student {

private String name;

private int age;

public Student(String name) {

this.name = name;

}

public Student(int age) {

this.age = age;

}

public Student(String name, int age) {

this.name = name;

this.age = age;

}

@Override

public String toString() {

return "Student [name=" + name + ", age=" + age + "]";

}

As you see, the Student object has 3 constructors, 2 of them are overloaded constructors. Now, we will consider each case!

But first of all, I will create a new Maven project:

I will use Java 17 for this example application:

<properties>
  <maven.compiler.source>17</maven.compiler.source>
  <maven.compiler.target>17</maven.compiler.target>
</properties>

<maven.compiler.source>17</maven.compiler.source>

<maven.compiler.target>17</maven.compiler.target>

</properties>

Spring dependency is as follows:

<dependency>
  <groupId>org.springframework</groupId>
  <artifactId>spring-context</artifactId>
  <version>6.0.2</version>
</dependency>

<groupId>org.springframework</groupId>

<artifactId>spring-context</artifactId>

</dependency>

The Application class has the content below:

package com.huongdanjava.springoverloadedconstructorinjection;

import org.springframework.beans.factory.BeanFactory;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Application {

  public static void main(String[] args) {
    BeanFactory context = new ClassPathXmlApplicationContext("spring.xml");

    Student student = (Student) context.getBean("student");
    System.out.println(student.toString());
  }
}

package com.huongdanjava.springoverloadedconstructorinjection;

import org.springframework.beans.factory.BeanFactory;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Application {

public static void main(String[] args) {

BeanFactory context = new ClassPathXmlApplicationContext("spring.xml");

Student student = (Student) context.getBean("student");

System.out.println(student.toString());

}

OK, let’s get started.

With overloaded constructors, we need to specify the data type for each argument in the constructor.

For example, I want to initialize a Student object with the name “Khanh”. I will declare in Spring container as below:

<bean id="student" class="com.huongdanjava.springoverloadedconstructorinjection.Student">
  <constructor-arg type="java.lang.String" value="Khanh" />
</bean>

<constructor-arg type="java.lang.String" value="Khanh" />

</bean>

With this declaration, Spring will call the constructor:

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

public Student(String name) {

this.name = name;

}

to initialize the Student object because the data type of the name variable is String.

Result:

If now I want to initialize the Student object with an age is 30, then I need to change the data type as below:

<bean id="student" class="com.huongdanjava.springoverloadedconstructorinjection.Student">
  <constructor-arg type="int" value="30" />
</bean>

<constructor-arg type="int" value="30" />

</bean>

Now, Spring will call the constructor:

public Student(int age) {
  this.age = age;
}

public Student(int age) {

this.age = age;

}

to initialize the Student object and the result will be as below:

With the constructor has many arguments, we declare it as normal:

<bean id="student" class="com.huongdanjava.springoverloadedconstructorinjection.Student">
  <constructor-arg value="Khanh" />
  <constructor-arg value="30" />
</bean>