@Qualifier Annotation

The @Qualifier annotation in Spring is used to differentiate a bean among the same type of bean objects.

If we have more than one bean of the same type and want to wire only one of them then use the @Qualifier annotation along with @Autowired to specify which exact bean will be wired.

If we don't use this annotation in the given project then we get an error like:

Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.studytonight.community.Award' available: expected single matching bean but found 3: bookerAward,nationalAward,pulitzerAward

In this topic, we are using the @Qualifier annotation to specify the dependency instance. The project is a Maven-Based Spring Project and contains the following files.

• App.java

• AppConfig.java

• Award.java

• BookerAward.java

• NationalAward.java

• PulitzerAward.java

• FictionWriter.java

• TechnicalWriter.java

• Writer.java

• pom.xml

Project Structure

Files Source Code:

//App.java

This file contains the code to create an IOC container for our application. The AnnotationConfigApplicationContext class is used to create an object for application context.

package com.studytonight.community;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class App {

	public static void main(String[] args) {
		
		AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
		Writer writer = context.getBean("fictionWriter", Writer.class);
		writer.write();
		writer.getAward();
		// Close the context
		context.close();
	}
}

// AppConfig.java

This is a configuration file in Java which is an alternate of the applicationContext.xml file that we created for the XML-based configuration example. The @Configuration annotation indicates that this is not a simple class but a configuration class and the @ComponentScan annotation is used to indicate the component location in our spring project.

package com.studytonight.community;

import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

@Configuration
@ComponentScan("com.studytonight.community")
public class AppConfig {

}

// Award.java

This is an interface Award that contains a award() abstract method. Each class that implements this interface will have to override the award() method.

package com.studytonight.community;

public interface Award {
	
	void award();

}

// BookerAward.java

This is a component class that is marked using the @Component annotation and implements the Award interface.

package com.studytonight.community;

import org.springframework.stereotype.Component;

@Component
public class BookerAward implements Award {

	@Override
	public void award() {
		
		System.out.println("You got booker prize...");
	}
}

// FictionWriter.java

This is another component class that is marked using the @Component annotation and implements the Writer interface.

package com.studytonight.community;

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

@Component
public class FictionWriter implements Writer {

	@Autowired
	@Qualifier("bookerAward")
	private Award award;

	@Override
	public void write() {
		
		System.out.println("Write Fiction Novels...");
	}

	@Override
	public void getAward() {
		
		award.award();
	}
}

// NationalAward.java

This is another component class that is marked using the @Component annotation and implements the Award interface.

package com.studytonight.community;

import org.springframework.stereotype.Component;

@Component
public class NationalAward implements Award{
	
	public void award() {
		
		System.out.println("Your Writting got National Award!");
	}
}

// PulitzerAward.java

This is another component class that is marked using the @Component annotation and implements the Award interface.

package com.studytonight.community;

import org.springframework.stereotype.Component;

@Component
public class PulitzerAward implements Award {

	@Override
	public void award() {
		
		System.out.println("You have won Pulitzer Award.");
	}
}

// TechnicalWriter.java

This is another component class that is marked using the @Component annotation and implements the Writer interface.

package com.studytonight.community;

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

@Component
public class TechnicalWriter implements Writer{

	@Autowired
	@Qualifier("pulitzerAward")
	private Award award;

	@Override
	public void write() {
		
		System.out.println("Write technical stuffs...");
	}

	@Override
	public void getAward() {
		award.award();
		
	}
}

// Writer.java

package com.studytonight.community;

public interface Writer {
	
	void write();
	void getAward();
}

// pom.xml

This file contains all the dependencies of this project such as spring jars, servlet jars, etc. Put these dependencies into your project to run the application.

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.studytonight</groupId>
  <artifactId>springproject</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <dependencies>
  <!-- https://mvnrepository.com/artifact/org.springframework/spring-web -->
  <dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-core</artifactId>
			<version>${spring.version}</version>
		</dependency>
		<dependency>
			<groupId>org.springframework</groupId>
			<artifactId>spring-context</artifactId>
			<version>${spring.version}</version>
		</dependency>
	</dependencies>
	<properties>
		<spring.version>5.2.8.RELEASE</spring.version>
	</properties>
  <build>
    <sourceDirectory>src</sourceDirectory>
    <plugins>
      <plugin>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.8.1</version>
        <configuration>
          <source>1.8</source>
          <target>1.8</target>
        </configuration>
      </plugin>
    </plugins>
    
  </build>
</project>

Write Fiction Novels...
You got booker prize...