Micronaut: Replace existing bean using @Replaces annotation

@Replaces annotation allows a a bean to specify that it replaces another bean. Note that the bean to be replaced cannot be an Infrastructure bean.

@Singleton
public class MySQLDataSource implements DataSource {

	@Override
	public String toString() {
		return "MySQLDataSource";
	}

}


@Singleton
@Replaces(MySQLDataSource.class)
public class FileDataSource implements DataSource{

	@Override
	public String toString() {
		return "FileDataSource";
	}

	
}

 

In the above snippet, FileDataSource replaces the MySQLDataSource bean.

 

Find the below working application.

 

Step 1: Create new maven project ‘micronaut-replace-bean’.

 

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-replace-bean</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 bean classes.

 

DataSource.java

package com.sample.app.datasources;

import jakarta.inject.Singleton;

@Singleton
public interface DataSource {

	
}

 

MySQLDataSource.java

package com.sample.app.datasources;

import jakarta.inject.Singleton;

@Singleton
public class MySQLDataSource implements DataSource {

	@Override
	public String toString() {
		return "MySQLDataSource";
	}

}

 

FileDataSource.java

package com.sample.app.datasources;

import io.micronaut.context.annotation.Replaces;
import jakarta.inject.Singleton;

@Singleton
@Replaces(MySQLDataSource.class)
public class FileDataSource implements DataSource{

	@Override
	public String toString() {
		return "FileDataSource";
	}

	
}

 

Step 4: Define main application class.

 

App.java

package com.sample.app;

import com.sample.app.datasources.DataSource;

import io.micronaut.context.ApplicationContext;;

public class App {

	public static void main(String[] args) {
		try (ApplicationContext applicationContext = ApplicationContext.run()) {
			DataSource dataSource = applicationContext.getBean(DataSource.class);

			System.out.println(dataSource);
			
		}
	}
}

 

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-replace-bean-1-jar-with-dependencies.jar’ in project target folder.

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

Execute below command to run the application.

java -jar ./target/micronaut-replace-bean-1-jar-with-dependencies.jar

$ java -jar ./target/micronaut-replace-bean-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.
FileDataSource

You can download this application from this link.

 

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