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Spring - Setter Injection using XML

We have seen Spring creating and managing beans/objects for you. Now, we will see how, by using setter methods spring is going to inject beans for you.

Say there is a class called 'Car' :

class Car {

String brand;

public String getBrand() {
return brand;
}

public void setBrand(String brand) {
this.brand = brand;
}
}

Also let us define the application-context.xml

application-context.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
http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">

<bean name = "car" class = "Car">
<property name = "brand" value = "Hyundai"/>
</bean>

</beans>

Just compare the contents of <bean ..> tag with the 'Car' class.

We have linked the 'Car' class in XML using the below code :

Also the 'Car' class has a property named 'brand'.

On seeing the above line Spring goes to the Setter method of the 'brand' property of the 'Car' class and injects the value 'Hyundai' to 'brand'.

public void setBrand(String brand) {
this.brand = brand;
}

Finally, in the main() method we define our application code to interact with spring.

import org.springframework.context.ApplicationContext;

public class FirstSpringApp {
public static void main(String[] args) {

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

Car car = (Car)applicationContext.getBean("car");

System.out.println("The Car brand is : "+car.getBrand());

}
}

A Complex Setter Injection

All the Cars rolls on Wheels. Now, let us say our 'Car' class holds a reference of a 'Wheel' class.

Car Class

class Car {

Wheel wheel;

public Wheel getWheel() {
return wheel;
}

public void setWheel(Wheel wheel) {
this.wheel = wheel;
}
}

Wheel Class

class Wheel {

String tyreType;

public String getTyreType() {
return tyreType;
}

public void setTyreType(String tyreType) {
this.tyreType = tyreType;
}

}

Now, since the 'Car' class is holding a reference of 'Wheel' . We will define them in 'application-context.xml'.

application-context.xml

Since, the 'Car' class is holding a 'Wheel' reference

class Car {

Wheel wheel;
...
...
}

We have instantiated 'Wheel' first (Also, initialized the 'tireType' property with value 'Thick Tire'):

Just note we have put the '<bean name =..>' as 'wheelReference'.

Then, we have taken this 'wheelReference' and put it in the 'ref' of 'wheel' property. And instantiated the 'Car'.

So, spring is using the setter method of 'Wheel', which belongs to 'Car' class to get the Setter injection done.

public void setWheel(Wheel wheel) {
this.wheel = wheel;
}

Finally, in the main() method we define our application code to interact with spring.

Just note, we have just asked spring to give us a 'Car' object.

Car car = (Car)applicationContext.getBean("car");

And in the background, spring has created the 'Wheel' object and injected into the 'Wheel' reference of 'Car' class.

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