Dependency Injection by Constructor Example

We can inject the dependency by constructor. The <constructor-arg> subelement of <bean> is used for constructor injection. Here we are going to inject

primitive and String-based values
Dependent object (contained object)
Collection values etc.

Injecting primitive and string-based values

Let's see the simple example to inject primitive and string-based values. We have created three files here:

Employee.java
applicationContext.xml
Test.java

Employee.java

It is a simple class containing two fields id and name. There are four constructors and one method in this class.

package com.javatpoint;

public class Employee {
private int id;
private String name;

public Employee() {System.out.println("def cons");}

public Employee(int id) {this.id = id;}

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

public Employee(int id, String name) {
	this.id = id;
	this.name = name;
}

void show(){
	System.out.println(id+" "+name);
}

}

applicationContext.xml

We are providing the information into the bean by this file. The constructor-arg element invokes the constructor. In such case, parameterized constructor of int type will be invoked. The value attribute of constructor-arg element will assign the specified value. The type attribute specifies that int parameter constructor will be invoked.

<?xml version="1.0" encoding="UTF-8"?>
<beans
	xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:p="http://www.springframework.org/schema/p"
	xsi:schemaLocation="http://www.springframework.org/schema/beans
                http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">

<bean id="e" class="com.javatpoint.Employee">
<constructor-arg value="10" type="int"></constructor-arg>
</bean>

</beans>

Test.java

This class gets the bean from the applicationContext.xml file and calls the show method.

package com.javatpoint;

import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.xml.XmlBeanFactory;
import org.springframework.core.io.*;

public class Test {
	public static void main(String[] args) {
		
		Resource r=new ClassPathResource("applicationContext.xml");
		BeanFactory factory=new XmlBeanFactory(r);
		
		Employee s=(Employee)factory.getBean("e");
		s.show();
		
	}
}

Output:10 null

download this example

Injecting string-based values

If you don't specify the type attribute in the constructor-arg element, by default string type constructor will be invoked.

....
<bean id="e" class="com.javatpoint.Employee">
<constructor-arg value="10"></constructor-arg>
</bean>
....

If you change the bean element as given above, string parameter constructor will be invoked and the output will be 0 10.

Output:0 10

You may also pass the string literal as following:

....
<bean id="e" class="com.javatpoint.Employee">
<constructor-arg value="Sonoo"></constructor-arg>
</bean>
....

Output:0 Sonoo

You may pass integer literal and string both as following

....
<bean id="e" class="com.javatpoint.Employee">
<constructor-arg value="10" type="int" ></constructor-arg>
<constructor-arg value="Sonoo"></constructor-arg>
</bean>
....

Output:10 Sonoo

download this example (developed using Myeclipse IDE)
download this example (developed using Eclipse IDE)

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