Spring - Autowiring by Name & Type

Let us define the 'Car' and 'Wheel' to understand Autowiring in detail:

Car Class

class Car {

Wheel wheel;

public Wheel getWheel() {
return wheel;
}

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

Wheel Class

class Wheel {

String tireType;

public String getTireType() {
return tireType;
}

public void setTireType(String tireType) {
this.tireType = tireType;
}

}

Autowiring by Name

Let us understand 'Autowiring by Name' by looking at 'application.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" autowire="byName">

</bean>

<bean name = "wheel" class = "Wheel">
<property name = "wheel" value="Thick Tire"/>
</bean>

</beans>

Now, if we compare this 'application-context.xml' with the 'application-context.xml' defined in the previous page, we can note the below changes:

1. We have added 'autowire="byName"' to the <bean ..> of 'Car'.

2. And removed

3. And most importantly changed the <bean name=...> from 'wheelReference'

to <bean name=...> as 'wheel'.

How does Autowiring by Name happens?

By using the above code we are asking Spring to check for the variable names in the 'Car' class:

class Car {
Wheel wheel;
}

When it finds there is a variable named 'wheel', it starts to search in 'application-context.xml' with '<bean name=...>' as 'wheel'.

And once it finds there is a bean with name 'wheel',

It injects the 'wheel' object to the 'Car' class using the setter method of 'wheel' in 'Car' class.

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

Autowiring by Type

Autowiring by Type is also similar to Autowiring by Name with a one liner change.

Instead of writing autowire="byName":

Just change it to autowire="byType"

and rest all remains the same.

How does Autowiring by Type happens?

By using the above code we are asking Spring to check for the data types in the 'Car' class:

class Car {
Wheel wheel;
}

When it finds there is a variable of type 'Wheel', it starts to search in 'application-context.xml' with '<bean ... class=...>' as 'Wheel'.

And once it finds there is a bean with class as 'Wheel',

It injects the 'wheel' object to the 'Car' class using the setter method of 'wheel' in 'Car' class.

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

Note : Autowiring by Type cannot happen if there are two or more variables of the same datatype. i.e. If 'Car' class contains two variables of type'Wheel', Autowiring by Type does not work.

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