Spring Autowiring调用错误的构造函数

Question

I'm learning Springs with annotations and auto wiring. I tried three types of auto wiring

1. Constructor
2. Setter injection
3. Method

This is my configuration

<?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 for component scanning -->
    <context:component-scan base-package="com.aht.spring.entity"></context:component-scan>

</beans>

These are my entities

Coach.java

package com.aht.spring.entity.coach;

public interface Coach {

    String getDailyWorkOut();

    String getDailyFortune();
}

FortuneService

package com.aht.spring.entity.fortune;

public interface FortuneService {

    String getFortune();
}

HappyFortuneService

package com.aht.spring.entity.fortune;

import org.springframework.stereotype.Component;

@Component
public class HappyFortuneService implements FortuneService {

    public String getFortune() {
        return "Feel energetic for first half of trainning";
    }
}

FootBallCoach

package com.aht.spring.entity.coach;

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

import com.aht.spring.entity.fortune.FortuneService;

@Component
public class FootBallCoach implements Coach {

    private FortuneService fortuneService;

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

    public String getDailyWorkOut() {
        return "Practice one-on-one for 2 hours";
    }

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

CricketCoach

package com.aht.spring.entity.coach;

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

import com.aht.spring.entity.fortune.FortuneService;

@Component
public class CricketCoach implements Coach {

    private FortuneService fortuneService;

    public CricketCoach() {
        System.out.println("Default constructor");
    }

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

    public String getDailyWorkOut() {
        return "Practice out field tips";
    }

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

BaseBallCoach

package com.aht.spring.entity.coach;

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

import com.aht.spring.entity.fortune.FortuneService;

@Component
public class BaseBallCoach implements Coach {

    private FortuneService fortuneService;

    public String getDailyWorkOut() {
        return "Practice curve whole day";
    }

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

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

I've three classes for executing three ways of auto wiring, Constructor and Setter worked fine, but when method wise auto wiring was done a wrong constructor was called. One thing I missed in my in my BaseBallCoach class was a default constructor, but anyhow compiler will automatically generate one for me right?

This is my CoachMethodInjectionApp.java where I executed method auto wiring

package com.aht.spring.app;

import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.aht.spring.entity.coach.Coach;

public class CoachMethodInjectionApp {

    public static void main(String[] args) {

        //  read config-file
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");

        //  get beans
        Coach coach = context.getBean("baseBallCoach", Coach.class);

        //  get dependencies
        System.out.println(coach.getDailyFortune());

        //  get daily workout
        System.out.println(coach.getDailyWorkOut());

        //  close context
        context.close();
    }
}

This was the output

Default constructor
Feel energetic for first half of trainning
Practice curve whole day

First line of output is what I don't understand Default constructor , why the constructor of CricketCoach executing??

Answer 1

As CricketCoach class is annotated with @Component and the package is scanned when the Spring container starts it will create an instance of type CricketCoach by calling the no-arg constructor

All the beans defined in the xml file or annotated with @Component or any extension of @Component from scanned packages will be created at start time of the spring container no matter if they will be used or not

Answer 2

You're creating a new instance of Coach class here: Coach coach = context.getBean("baseBallCoach", Coach.class); Everytime new instance is created it is executing a constructor. In which you have call to System.out.println("Default constructor"); . You may try removing @Component from Cricket Coach imlpementation.