Spring Boot @ConditionalOnMissingBean Annotation Example

In this section we will learn about @ConditionalOnMissingBean Annotation.

You can use @ConditionalOnMissingBean if you want to load a bean only if another bean doesn’t exist in the application context.

@Bean
@ConditionalOnMissingBean
public NotificationService emailNotificationService()
{
    return new EmailNotificationService();
}

This example loads the EmailNotificationService into the application context if there is no other NotificationService exist in application context. 

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

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("EmailNotificationService Constructor");
    }
}

SMSNotificationService.java

package com.knf.dev.demo.service;

public class SMSNotificationService implements NotificationService{

    public SMSNotificationService() {
        System.out.println("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.ConditionalOnMissingBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class AppConfig {

    @Bean
    public NotificationService smsNotificationService()
    {
        return new SMSNotificationService();
    }

    @Bean
    @ConditionalOnMissingBean
    public NotificationService emailNotificationService()
    {
        return new EmailNotificationService();
    }
}

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:

SMSNotificationService Constructor

Download Source Code

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

More related topics,

Spring Core Annotations

• Spring @Qualifier Annotation Example

• Spring @Autowired Annotation Example

• Spring @Value Annotation Example

• Spring @Scope Annotation Example

• Spring @Bean Annotation Example

• Spring @Primary Annotation Example

• Spring @ImportResource Annotation Example

• Spring @LookUp Annotation Example

• Spring @Lazy Annotation Example

• Spring @PropertySource Annotation Example

• Spring @DependsOn Annotation Example

Spring Web Annotations

• Spring @RequestHeader Annotation Example 

• Spring @RestController Annotation Example

• Spring @RequestParam Annotation Example

• Spring @PathVariable Annotation Example

• Spring @ResponseBody Annotation Example

• Spring @RequestBody Annotation Example

• Spring @ControllerAdvice Annotation Example 

• Spring @ModelAttribute Annotation Example 

• Spring @ResponseStatus Annotation Example

• Spring @GetMapping, @PostMapping, @PutMapping, @DeleteMapping, and @PatchMapping Annotations Example

• Spring @RequestMapping Annotation Example

Spring Boot Annotations

• Spring Boot @ConditionalOnProperty Annotation Example

• Spring Boot @ConditionalOnResource Annotation Example