Spring IoC: Annotation on Properties

@Autowired is used on Properties, which means Spring Container detects the bean instance using the byType process but injection is not with Setter method.This is how byType auto-wiring process is different from Annotation based auto-wiring. Hence annotation tells no need to write any setter method or constructor for injecting the bean property.

Example showing the Annotation on Properties :
There are two bean class A and Hello and Hello is having dependency of A.

A.java

public class A {  } 

Hello.java

public class Hello {
  
 @Autowired           //Annotation on properties
 A a;
public Hello(){      //Default Constructor 
     System.out.println("Hello instance ");
  }
  
  public Hello(A a){    //Constructor Injection
     System.out.println("Arg Cons");
     this.a = a;
  }

  public void setA(A a) { //Setter Injection
     this.a = a;
  }

   public static void main(String[] args) {
  
       ApplicationContext ctx= new ClassPathXmlApplicationContext("SpringCore.xml");
       Hello hello =(Hello)ctx.getBean("hello");
 }

}

SpringCore.xml

<bean id="a" class="com.def.A"> 
</bean>

<bean id="hello" class="com.def.Hello" >
</bean>

Output

Def A cons
Hello instance

In the above example
1. Instance of bean A will be created using default constructor.
2. Instance of bean Hello will be created using default constructor.
3. Bean property will be injected but without setter and constructor injection.

Note : In the above example output doesn't involve any constructor or setter injection hence proved that it follows different approach for injecting the bean property rather using Constructor or Setter Injection.

Note : Annotation on properties rather follows different approach to inject the bean property.