7.0 Dependency injection with Annotations and Autowiring

Spring Auto Wiring - For Dependency injection spring  can use auto wiring. Spring will look for a class that matches the property , match can be class or interface. After finding the match spring will automatically inject it and it is known as spring auto wiring.


Example of Constructor Injection using Annotations
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BaseBallCoach.java
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package com.demo.spring;

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

@Component
public class BaseBallCoach implements Coach {

@Autowired
public BaseBallCoach(FortuneService thefortuneservce){
fortuneservce=thefortuneservce;
}


@Override
public String getDailyWorkout() {
return "practice 30 mins daily";
}


@Override
public String getFortuneService() {
// TODO Auto-generated method stub
return fortuneservce.getFortuneService();
}


}

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applicationContext.xml
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<?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">

 <!-- initializing component scanning -->

 <context:component-scan base-package="com.demo.spring"></context:component-scan>

</beans>

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MyApp.java
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package com.demo.spring;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class MyApp {

public static void main(String[] args) {

ClassPathXmlApplicationContext context=new ClassPathXmlApplicationContext("applicationContext.xml");

Coach obj=context.getBean(Coach.class);

System.out.println(obj.getDailyWorkout());

System.out.println(obj.getFortuneService());

context.close();

}

}

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FortuneService.java
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package com.demo.spring;

public interface FortuneService {

public String getFortuneService();

}

Example of Setter Injection using Annotations
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BaseBallCoach.java
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package com.demo.spring;

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

@Component
public class BaseBallCoach implements Coach {


public BaseBallCoach(){
System.out.println("BaseBallCoach : inside no-arg constructor");
}

@Autowired
public void setFortuneservce(FortuneService fortuneservce) {
System.out.println("BaseBallCoach : inside fortuneservice setter");
this.fortuneservce = fortuneservce;
}

@Override
public String getDailyWorkout() {
return "practice 30 mins daily";
}



@Override
public String getFortuneService() {
return fortuneservce.getFortuneService();
}

}

Example of Any Method Injection using Annotations

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BaseBallCoach.java
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package com.demo.spring;

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

import org.springframework.beans.factory.annotation.Qualifier;

import org.springframework.stereotype.Component;

@Component

public class BaseBallCoach implements Coach {

private FortuneService fortuneservce;

public BaseBallCoach(){

System.out.println("BaseBallCoach : inside no-arg constructor");

}

@Autowired

public void doRandomStuff(FortuneService fortuneservce) {

System.out.println("BaseBallCoach : inside doRandomStuff method");

this.fortuneservce = fortuneservce;

}

@Override

public String getDailyWorkout() {

return "practice 30 mins daily";

}

@Override

public String getFortuneService() {

return fortuneservce.getFortuneService();

}

}

Example of Field Injection using Annotations

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BaseBallCoach.java
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package com.demo.spring;

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

import org.springframework.beans.factory.annotation.Qualifier;

import org.springframework.stereotype.Component;

@Component

public class BaseBallCoach implements Coach {

@Autowired

private FortuneService fortuneservce;

public BaseBallCoach(){

System.out.println("BaseBallCoach : inside no-arg constructor");

}

@Override

public String getDailyWorkout() {

return "practice 30 mins daily";

}

@Override

public String getFortuneService() {

// TODO Auto-generated method stub

return fortuneservce.getFortuneService();

}

}

When there is multiple dependency injection is available

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RandomFortuneService .java
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package com.demo.spring;

import org.springframework.stereotype.Component;

@Component

public class RandomFortuneService implements FortuneService {

@Override

public String getFortuneServie() {

return "Hello random service";

}

}

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ERROR
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Feb 26, 2018 3:45:16 AM org.springframework.context.support.AbstractApplicationContext prepareRefresh

INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@41975e01: startup date [Mon Feb 26 03:45:16 IST 2018]; root of context hierarchy

Feb 26, 2018 3:45:16 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions

INFO: Loading XML bean definitions from class path resource [applicationContext.xml]

BaseBallCoach : inside no-arg constructor

Feb 26, 2018 3:45:16 AM org.springframework.context.support.AbstractApplicationContext refresh

WARNING: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'baseBallCoach': Unsatisfied dependency expressed through field 'fortuneservce'; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.demo.spring.FortuneService' available: expected single matching bean but found 2: happyFortuneService,randomFortuneService

Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'baseBallCoach': Unsatisfied dependency expressed through field 'fortuneservce'; nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.demo.spring.FortuneService' available: expected single matching bean but found 2: happyFortuneService,randomFortuneService

at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:581)

at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:91)

at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:367)

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1340)

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:582)

at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:502)

at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:312)

at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:228)

at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:310)

at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:200)

at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:756)

at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:868)

at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:549)

at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:144)

at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:85)

at com.demo.spring.MyApp.main(MyApp.java:8)

Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.demo.spring.FortuneService' available: expected single matching bean but found 2: happyFortuneService,randomFortuneService

at org.springframework.beans.factory.config.DependencyDescriptor.resolveNotUnique(DependencyDescriptor.java:215)

at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1111)

at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1060)

at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:578)

... 15 more

Resolution using Qualifier Annotation

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BaseBallCoach.java
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package com.demo.spring;

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

import org.springframework.beans.factory.annotation.Qualifier;

import org.springframework.stereotype.Component;

@Component

public class BaseBallCoach implements Coach {

@Autowired

@Qualifier("happyFortuneService")

private FortuneService fortuneservce;

public BaseBallCoach(){

System.out.println("BaseBallCoach : inside no-arg constructor");

}

@Override

public String getDailyWorkout() {

return "practice 30 mins daily";

}

@Override

public String getFortuneServie() {

return fortuneservce.getFortuneService();

}

}