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Understanding Spring Qualifier Annotation

@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>

Output

Write Fiction Novels... 

You got booker prize...