Spring @Qualifier annotation tutorial

last modified October 18, 2023

Spring @Qualifier annotation tutorial shows how to use @Qualifier to differentiate beans in a Spring application.

Spring is a popular Java application framework for creating enterprise applications.

Spring @Qualifier annotation

The @Qualifier annotation helps disambiguate bean references when Spring would otherwise not be able to do so.

Spring @Qualifier example

The application has different types of message beans. We differentiate between them with @Qualifier.

src ├───main │ ├───java │ │ └───com │ │ └───zetcode │ │ │ Application.java │ │ ├───bean │ │ │ IMessage.java │ │ │ Info.java │ │ │ Warning.java │ │ └───service │ │ MessageProducer.java │ └───resources │ logback.xml └───test └───java

This is the project structure.

pom.xml

<?xml version=“1.0” encoding=“UTF-8”?> <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 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion>

<groupId>com.zetcode</groupId>
<artifactId>qualifierannotation</artifactId>
<version>1.0-SNAPSHOT</version>

<properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.source>17</maven.compiler.source>
    <maven.compiler.target>17</maven.compiler.target>
    <spring-version>5.3.23</spring-version>

</properties>

<dependencies>

    <dependency>
        <groupId>ch.qos.logback</groupId>
        <artifactId>logback-classic</artifactId>
        <version>1.4.0</version>
    </dependency>

    <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-context</artifactId>
        <version>${spring-version}</version>
    </dependency>

    <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-core</artifactId>
        <version>${spring-version}</version>
    </dependency>
    
</dependencies>

<build>
    <plugins>
        <plugin>
            <groupId>org.codehaus.mojo</groupId>
            <artifactId>exec-maven-plugin</artifactId>
            <version>3.1.0</version>
            <configuration>
                <mainClass>com.zetcode.Application</mainClass>
            </configuration>
        </plugin>
    </plugins>
</build>

</project>

In the pom.xml file, we have basic Spring dependencies spring-core and spring-context and logging logback-classic dependency.

The exec-maven-plugin is used for executing Spring application from the Maven on the command line.

resources/logback.xml

<?xml version=“1.0” encoding=“UTF-8”?> <configuration> <logger name=“org.springframework” level=“ERROR”/> <logger name=“com.zetcode” level=“INFO”/>

<appender name="consoleAppender" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
        <Pattern>%d{HH:mm:ss.SSS} [%thread] %blue(%-5level) %magenta(%logger{36}) - %msg %n
        </Pattern>
    </encoder>
</appender>

<root>
    <level value="INFO" />
    <appender-ref ref="consoleAppender" />
</root>

</configuration>

The logback.xml is a configuration file for the Logback logging library.

com/zetcode/bean/IMessage.java

package com.zetcode.bean;

public interface IMessage {

String getMessage();

}

The IMessage interface has one method declaration.

com/zetcode/bean/Info.java

package com.zetcode.bean;

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

@Component @Qualifier(“info”) public class Info implements IMessage {

@Override
public String getMessage() {

    return "This is an information message";
}

}

The first implementation of the interface gives an information message. The @Qualifier is used to identify the bean.

com/zetcode/bean/Warning.java

package com.zetcode.bean;

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

@Component @Qualifier(“warning”) public class Warning implements IMessage {

public String getMessage() {

    return "This is a warning message";
}

}

The second implementation gives a warning message. It is also named with the @Qualifier.

com/zetcode/service/MessageProducer.java

package com.zetcode.service;

import com.zetcode.bean.IMessage; import org.slf4j.Logger; import org.slf4j.LoggerFactory; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.beans.factory.annotation.Qualifier; import org.springframework.stereotype.Service;

@Service public class MessageProducer {

private static final Logger logger = LoggerFactory.getLogger(MessageProducer.class);

@Autowired
@Qualifier("info")
private IMessage infoMessage;

@Autowired
@Qualifier("warning")
private IMessage warningMessage;

public void produce() {

    logger.info("{}", infoMessage.getMessage());
    logger.warn("{}", warningMessage.getMessage());
}

}

The MessageProducer injects two IMessage beans. To differentiate between them, we use @Qualifier annotations.

com/zetcode/Application.java

package com.zetcode;

import com.zetcode.service.MessageProducer; import org.slf4j.Logger; import org.slf4j.LoggerFactory; import org.springframework.context.annotation.AnnotationConfigApplicationContext; import org.springframework.context.annotation.ComponentScan;

@ComponentScan(basePackages = “com.zetcode.bean;com.zetcode.service”) public class Application {

private static final Logger logger = LoggerFactory.getLogger(Application.class);

public static void main(String[] args) {

    logger.info("Application starting");

    try (var ctx = new AnnotationConfigApplicationContext(Application.class)) {

        var messageProducer = (MessageProducer) ctx.getBean("messageProducer");
        messageProducer.produce();
    }
}

}

This is the main application class. It retrieves the messageProducer bean and call its produce method.

$ mvn -q exec:java 10:50:03.309 [com.zetcode.Application.main()] INFO com.zetcode.Application - Application starting 10:50:03.574 [com.zetcode.Application.main()] INFO com.zetcode.service.MessageProducer - This is an information message 10:50:03.574 [com.zetcode.Application.main()] WARN com.zetcode.service.MessageProducer - This is a warning message

We run the application.

In this article we have worked with Spring’s @Qualifier annotation.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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