Spring Boot @ConditionalOnExpression Annotation Example

In this section we will learn about @ConditionalOnExpression Annotation.


The @ConditionalOnExpression annotation allows configurations based on the result of a SpEL expression. 

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

1. Using @ConditionalOnExpression on @Bean method

To illustrate the use of @ConditionalOnExpression, we will develop a basic notification system. To keep things simple for now, let's assume we want to send email notifications and sms notifications.

Define our application custom configuration properties,

notification.enabled=true
notification.sms.enabled=true
notification.email.enabled=false
notification.twitter.enabled=true

Next, we'll need to create a simple service to send a notification. For example, consider the Notification Sender interface:

public interface NotificationService {

}

In this example, the SMSNotificationService and EmailNotificationService class is only loaded if a particular SpEL is enabled,

@Configuration
public class AppConfig {

    @Bean
    @ConditionalOnExpression("${notification.enabled:false} " +
            "&& ${notification.sms.enabled:false}")
    public NotificationService smsNotificationService()
    {
        return new SMSNotificationService();
    }

    @Bean
    @ConditionalOnExpression("${notification.enabled:false} " +
            "&& ${notification.email.enabled:false}")
    public NotificationService emailNotificationService()
    {
        return new EmailNotificationService();
    }
}

Here SMSNotificationService class will load because notification.enabled=true and notification.sms.enabled=true, but EmailNotificationService class will not load because notification.email.enabled=false. 

By appending :false to the properties we tell Spring to use false as a default value.


2. Using @ConditionalOnExpression on @Service class

Suppose we want to add another notification service – for example, a service that will allow us to send Twitter notifications.

To do this, we need to create another Notification Sender implementation:

@ConditionalOnExpression("${notification.enabled:false} " +
        "&& ${notification.twitter.enabled:false}")
@Service
public class TwitterNotificationService implements NotificationService{

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

Here TwitterNotificationService class will load because notification.enabled=true and notification.twitter.enabled=true. 

The following example creates a Spring Boot web application which uses @ConditionalOnExpression 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-conditionalonexpression-annotation-example</artifactId>
   <version>0.0.1-SNAPSHOT</version>
   <name>spring-boot-conditionalonexpression-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>


application.properties

notification.enabled=true
notification.sms.enabled=true
notification.email.enabled=false
notification.twitter.enabled=true


NotificationService.java

package com.knf.dev.demo.service;

public interface NotificationService {

}


EmailNotificationService.java

package com.knf.dev.demo.service;

public class EmailNotificationService implements NotificationService  {

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


SMSNotificationService.java

package com.knf.dev.demo.service;

public class SMSNotificationService implements NotificationService{

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


AppConfig.java

package com.knf.dev.demo.config;

import com.knf.dev.demo.service.EmailNotificationService;
import com.knf.dev.demo.service.NotificationService;
import com.knf.dev.demo.service.SMSNotificationService;
import org.springframework.boot.autoconfigure.condition.ConditionalOnExpression;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class AppConfig {

    @Bean
    @ConditionalOnExpression("${notification.enabled:false} " +
            "&& ${notification.sms.enabled:false}")
    public NotificationService smsNotificationService()
    {
        return new SMSNotificationService();
    }

    @Bean
    @ConditionalOnExpression("${notification.enabled:false} " +
            "&& ${notification.email.enabled:false}")
    public NotificationService emailNotificationService()
    {
        return new EmailNotificationService();
    }

}


TwitterNotificationService.java

package com.knf.dev.demo.service;

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

@ConditionalOnExpression("${notification.enabled:false} " +
        "&& ${notification.twitter.enabled:false}")
@Service
public class TwitterNotificationService implements NotificationService{

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


Run the application - Application.java

package com.knf.dev.demo;

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);

       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


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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