Micronaut: Qualifying a bean By Annotation

  In my previous post, I explained how to qualify a bean by name. In this post, I am going to explain how to qualify a bean by building your own qualifiers using the Qualifier annotation. 

@Qualifier
@Retention(RUNTIME)
public @interface Bull {

}

@Qualifier
@Retention(RUNTIME)
public @interface Poodle {

}

@Singleton
public class DogService {

	private Dog dog1;
	private Dog dog2;

	public DogService(@Bull Dog dog1, @Poodle Dog dog2) {
		this.dog1 = dog1;
		this.dog2 = dog2;
	}

}

 

public DogService(@Bull Dog dog1, @Poodle Dog dog2)

dog1 is injected with an instance of BullDog and dog2 is injected with an instance of PoodleDog.

 

How is this injection possible?

@Bull Dog dog1;

Annotation value (Bull) + type being injected simple name (Dog) == (case-insensitive) == The simple name of a bean of type Engine (BullDog).

 

Find the below working application.

 

Step 1: Create new maven project ‘micronaut-qualify-by-annotation’.

 

Step 2: Update pom.xml with maven dependencies.

 

pom.xml

<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.sample.app</groupId>
	<artifactId>micronaut-qualify-by-annotation</artifactId>
	<version>1</version>

	<properties>
		<maven-compiler-plugin.version>3.8.1</maven-compiler-plugin.version>
		<micronaut.version>3.7.1</micronaut.version>

		<maven.compiler.target>15</maven.compiler.target>
		<maven.compiler.source>15</maven.compiler.source>
	</properties>

	<dependencies>

		<dependency>
			<groupId>io.micronaut</groupId>
			<artifactId>micronaut-inject-java</artifactId>
			<version>${micronaut.version}</version>
		</dependency>
	</dependencies>

	<build>
		<plugins>

			<plugin>
				<artifactId>maven-assembly-plugin</artifactId>
				<configuration>
					<archive>
						<manifest>
							<mainClass>com.sample.app.App</mainClass>
						</manifest>
					</archive>
					<descriptorRefs>
						<descriptorRef>jar-with-dependencies</descriptorRef>
					</descriptorRefs>
				</configuration>

				<executions>
					<execution>
						<id>make-assembly</id>
						<phase>package</phase>
						<goals>
							<goal>single</goal>
						</goals>
					</execution>
				</executions>
			</plugin>

		</plugins>
	</build>
</project>

 

Step 3: Define Dog interface.

 

Dog.java

package com.sample.app.interfaces;

public interface Dog {

	String aboutMe();
}

Step 4: Define BullDog and PoodleDog classes.

 

BullDog.java

package com.sample.app.models;

import com.sample.app.interfaces.Dog;

import jakarta.inject.Singleton;

@Singleton
public class BullDog implements Dog {

	@Override
	public String aboutMe() {
		return "I am Bull Dog";
	}

}

PoodleDog.java

package com.sample.app.models;

import com.sample.app.interfaces.Dog;

import jakarta.inject.Singleton;

@Singleton
public class PoodleDog implements Dog {

	@Override
	public String aboutMe() {
		return "I am Poodle Dog";
	}

}

Step 5: Define annotations.

 

Bull.java

package com.sample.app.annotations;

import jakarta.inject.Qualifier;
import java.lang.annotation.Retention;

import static java.lang.annotation.RetentionPolicy.RUNTIME;

@Qualifier
@Retention(RUNTIME)
public @interface Bull {

}

Poodle.java

package com.sample.app.annotations;

import static java.lang.annotation.RetentionPolicy.RUNTIME;

import java.lang.annotation.Retention;

import jakarta.inject.Qualifier;

@Qualifier
@Retention(RUNTIME)
public @interface Poodle {

}

Step 6: Define DogService class.

 

DogService.java

package com.sample.app.util;

import com.sample.app.annotations.Bull;
import com.sample.app.annotations.Poodle;
import com.sample.app.interfaces.Dog;

import jakarta.inject.Singleton;

@Singleton
public class DogService {

	private Dog dog1;
	private Dog dog2;

	public DogService(@Bull Dog dog1, @Poodle Dog dog2) {
		this.dog1 = dog1;
		this.dog2 = dog2;
	}

	public void aboutMe() {

		System.out.println(dog1.aboutMe());
		System.out.println(dog2.aboutMe());

	}

}

Step 7: Define main application class.

 

App.java

package com.sample.app;

import com.sample.app.util.DogService;

import io.micronaut.context.ApplicationContext;

public class App {

	public static void main(String[] args) {
		try (ApplicationContext context = ApplicationContext.run()) {
			DogService animalUtil = context.getBean(DogService.class);
			animalUtil.aboutMe();
		}
	}
}

Total project structure looks like below.

Build the project using mvn package command.

Navigate to the folder where pom.xml is located and execute the command ‘mvn package’.

 

Upon command successful execution, you can see the jar file ‘micronaut-qualify-by-annotation-1-jar-with-dependencies.jar’ in project target folder.

$ls ./target/
archive-tmp
classes
generated-sources
maven-archiver
maven-status
micronaut-qualify-by-annotation-1-jar-with-dependencies.jar
micronaut-qualify-by-annotation-1.jar
test-classes

Execute below command to run the application.

java -jar ./target/micronaut-qualify-by-annotation-1-jar-with-dependencies.jar

$java -jar ./target/micronaut-qualify-by-annotation-1-jar-with-dependencies.jar
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
I am Bull Dog
I am Poodle Dog

You can download this application from this link.

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