Spring. Dependency Injection with Annotaions and Autowiring

 Autowiring Injection Types

1. Constructor Injection

2. Setter Injection

3. Field Injections

Constructor Injection

<?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:context="http://www.springframework.org/schema/context"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans.xsd
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context.xsd">

   <!-- add entry to enable component scanning -->
   <context:component-scan base-package="com.example.demo"/>

</beans>

package com.example.demo;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component
public class TennisCoach implements Coach{

    private FortuneService fortuneService;

    @Autowired
    public TennisCoach(FortuneService fortuneService) {
        this.fortuneService = fortuneService;
    }

    @Override
    public String getDailyWorkout() {
        return "Practice your backhand volley";
    }

    @Override
    public String getDailyFortune() {
        return fortuneService.getFortune();
    }
}
Setter Injection
package com.example.demo;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component
public class TennisCoach implements Coach {

    private FortuneService fortuneService;

    //    @Autowired
//    public TennisCoach(FortuneService fortuneService) {
//        this.fortuneService = fortuneService;
//    }


    public TennisCoach() {
        System.out.println("TennisCoach: inside default constructor");
    }

    @Autowired
    public void setFortuneService(FortuneService fortuneService) {
        this.fortuneService = fortuneService;
    }

    @Override
    public String getDailyWorkout() {
        return "Practice your backhand volley";
    }

    @Override
    public String getDailyFortune() {
        return fortuneService.getFortune();
    }
}
Field Injection
    Inject dependencies by setting field values on your class directly (even private fields). 
Accomplished by using Java Reflection.

package com.example.demo;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component
public class TennisCoach implements Coach {

    @Autowired
    private FortuneService fortuneService;

    public TennisCoach() {
        System.out.println("TennisCoach: inside default constructor");
    }

    @Override
    public String getDailyWorkout() {
        return "Practice your backhand volley";
    }

    @Override
    public String getDailyFortune() {
        return fortuneService.getFortune();
    }
}
@Qualifier - defines which bean to use.
package com.example.demo;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.stereotype.Component;

@Component
public class TennisCoach implements Coach {

    @Autowired
    @Qualifier("happyFortuneService")
    private FortuneService fortuneService;

    public TennisCoach() {
        System.out.println("TennisCoach: inside default constructor");
    }

    @Override
    public String getDailyWorkout() {
        return "Practice your backhand volley";
    }

    @Override
    public String getDailyFortune() {
        return fortuneService.getFortune();
    }
}