Spring IoC : Annotation on Constructor

@Autowired can also be used with Constructor by which Spring Container detects the bean instance using byType process but inject the bean property using Constructor Injection. Using @Autowired with the Constructor, there is not need to configuring XML with the <constructor-arg> tag. It is same as constructor autowiring prcoess with a difference of mapping style. 

Example showing the Annotation on Constructor :
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 {
  
 A a;                             // Bean Property
public Hello(){                   //Default Constructor 
     System.out.println("Hello instance ");
  }

  @Autowired                        //Annotation on properties
  public Hello(A a){    
     System.out.println("Arg Cons");//Constructor Injection
     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
Arg Cons

In the above example ,

1. Instance of bean A will be created by calling default constructor.

2. Instance of bean Hello will be created by calling argumenetd constructor.

3. Bean property will be injected using Constructor Injection.

Note : In the above example has output of argumented constructor hence uses Constructor Injection for injecting bean property.