Spring Boot @ConditionalOnClass Annotation Example

In this section we will learn about @ConditionalOnClass Annotation.


The @ConditionalOnClass can be used to register a bean only when the given class(es) can be found on the class-path. We can define the fully qualified class name either via a String or via a Class<?> reference.

For example, when providing library/framework classes to other developers, this annotation can be used to active certain beans only based on the existing of specific classes, e.g. driver-classes etc.

The @ConditionalOnClass annotation may be used on any class annotated with @Configuration, @Component, @Service & @Repository or on methods annotated with @Bean.


1. Using @ConditionalOnClass on @Bean method

To illustrate the use of @ConditionalOnClass, we will create two simple service classes, SMSNotificationService class and  EmailNotificationService class.

public class EmailNotificationService {

    public EmailNotificationService() {
        System.out.println("Inside EmailNotificationService Constructor");
    }

    public void sendEmailNotification()
    {
        System.out.println("Sending Email Notification");
    }
}


package com.knf.dev.demo.service;

public class SMSNotificationService {

    public SMSNotificationService() {
        System.out.println("Inside SMSNotificationService Constructor");
    }

    public void sendSmsNotification()
    {
        System.out.println("Sending SMS Notification");
    }
}


Next, we will create a simple class which will be required to call on @ConditionalOnClass annotation because this class will be executed only when the required class is found.

public class NotificationProvider {
}


Next, we will create a Spring Config class and use annotation @ConditionalOnClass in various ways, such as, by name, value.

@Configuration
public class AppConfig {

    @Bean
    @ConditionalOnClass(name = 
            "com.knf.dev.demo.provider.NotificationProvider")
    public SMSNotificationService smsNotificationService()
    {
        return new SMSNotificationService();
    }

    @Bean
    @ConditionalOnClass(NotificationProvider.class)
    public EmailNotificationService emailNotificationService()
    {
        return new EmailNotificationService();
    }
}

Here, SMSNotificationService bean and EmailNotificationService bean will register to the application context if NotificationProvider class is found.


2. Using @ConditionalOnClass on @Service class

@Service
@ConditionalOnClass(name=
        "com.knf.dev.demo.provider.NotificationProvider")
public class TwitterNotificationService {

    public TwitterNotificationService() {
        System.out.println("Inside TwitterNotificationService Constructor");
    }

    public void sendTwitterNotification()
    {
        System.out.println("Sending Twitter Notification");
    }
}

Here TwitterNotificationService bean will register to the application context if NotificationProvider class is found.

The following example creates a Spring Boot web application which uses @ConditionalOnClass annotation.

Project Directory



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
   https://maven.apache.org/xsd/maven-4.0.0.xsd">
   <modelVersion>4.0.0</modelVersion>
   <parent>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-parent</artifactId>
      <version>3.0.1</version>
      <relativePath/> <!-- lookup parent from repository -->
   </parent>
   <groupId>com.knf.dev.demo</groupId>
   <artifactId>spring-boot-conditionalonclass-annotation-example</artifactId>
   <version>0.0.1-SNAPSHOT</version>
   <name>spring-boot-conditionalonclass-annotation-example</name>
   <description>Demo project for Spring Boot</description>
   <properties>
      <java.version>17</java.version>
   </properties>
   <dependencies>
      <dependency>
         <groupId>org.springframework.boot</groupId>
         <artifactId>spring-boot-starter-web</artifactId>
      </dependency>
      <dependency>
         <groupId>org.springframework.boot</groupId>
         <artifactId>spring-boot-starter-test</artifactId>
         <scope>test</scope>
      </dependency>
   </dependencies>

   <build>
      <plugins>
         <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
         </plugin>
      </plugins>
   </build>

</project>


EmailNotificationService.java

package com.knf.dev.demo.service;

public class EmailNotificationService {

    public EmailNotificationService() {
        System.out.println("Inside EmailNotificationService Constructor");
    }

    public void sendEmailNotification()
    {
        System.out.println("Sending Email Notification");
    }
}


SMSNotificationService.java

package com.knf.dev.demo.service;

public class SMSNotificationService {

    public SMSNotificationService() {
        System.out.println("Inside SMSNotificationService Constructor");
    }

    public void sendSmsNotification()
    {
        System.out.println("Sending SMS Notification");
    }
}


NotificationProvider.java

package com.knf.dev.demo.provider;

public class NotificationProvider {
}


AppConfig.java

package com.knf.dev.demo.config;

import com.knf.dev.demo.provider.NotificationProvider;
import com.knf.dev.demo.service.EmailNotificationService;
import com.knf.dev.demo.service.SMSNotificationService;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class AppConfig {

    @Bean
    @ConditionalOnClass(name =
            "com.knf.dev.demo.provider.NotificationProvider")
    public SMSNotificationService smsNotificationService()
    {
        return new SMSNotificationService();
    }

    @Bean
    @ConditionalOnClass(NotificationProvider.class)
    public EmailNotificationService emailNotificationService()
    {
        return new EmailNotificationService();
    }
}


TwitterNotificationService.java

package com.knf.dev.demo.service;

import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.stereotype.Service;

@Service
@ConditionalOnClass(name=
        "com.knf.dev.demo.provider.NotificationProvider")
public class TwitterNotificationService {

    public TwitterNotificationService() {
        System.out.println("Inside TwitterNotificationService Constructor");
    }

    public void sendTwitterNotification()
    {
        System.out.println("Sending Twitter Notification");
    }
}


Run the application - Application.java

package com.knf.dev.demo;

import com.knf.dev.demo.service.EmailNotificationService;
import com.knf.dev.demo.service.SMSNotificationService;
import com.knf.dev.demo.service.TwitterNotificationService;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;

@SpringBootApplication
public class Application  {


   public static void main(String[] args) {
           ConfigurableApplicationContext context =
                   SpringApplication.run(Application.class, args);

       SMSNotificationService smsNotificationService =
               context.getBean(SMSNotificationService.class);
       smsNotificationService.sendSmsNotification();

       EmailNotificationService emailNotificationService =
               context.getBean(EmailNotificationService.class);
       emailNotificationService.sendEmailNotification();

       TwitterNotificationService twitterNotificationService =
               context.getBean(TwitterNotificationService.class);
       twitterNotificationService.sendTwitterNotification();

       context.close();
   }
}

Application is the entry point that sets up the Spring Boot application. The @SpringBootApplication annotation enables auto-configuration and component scanning. 

Let's run this Spring boot application from either IntelliJ IDEA IDE by right click - Run 'Application.main()'
Or you can use the below maven command to run:

mvn spring-boot:run

Console Output:
Inside TwitterNotificationService Constructor
Inside SMSNotificationService Constructor
Inside EmailNotificationService Constructor
Sending SMS Notification
Sending Email Notification
Sending Twitter Notification


Download Source Code

$ git clone https://github.com/knowledgefactory4u/spring-boot-annotations

More related topics,

Spring Core Annotations

Spring Web Annotations

Spring Boot Annotations

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