Top 10 Spring Boot Conditional Annotations

• @ConditionalOnBean: This annotation is used to specify that a bean should only be created if another bean of a specific type is already present in the application context.

@Configuration
 public class MyAutoConfiguration {

     @ConditionalOnBean
     @Bean
     public MyService myService() {
         ...
     }

 }

• @ConditionalOnMissingBean: This annotation is used to specify that a bean should only be created if another bean of a specific type is not already present in the application context.

@Configuration
 public class MyAutoConfiguration {

     @ConditionalOnMissingBean
     @Bean
     public MyService myService() {
         ...
     }

 }

• @ConditionalOnClass: This annotation is used to specify that a bean should only be created if a certain class is present on the classpath.

@Configuration
public class MyConfiguration {

    @Bean
    @ConditionalOnClass({ org.springframework.jdbc.datasource.DriverManagerDataSource.class })
    public MyBean myBean() {
        return new MyBean();
    }

    public static class MyBean {
        public void doSomething() {
            System.out.println("MyBean is doing something.");
        }
    }
}

• @ConditionalOnMissingClass: This annotation is the opposite of @ConditionalOnClass. It specifies that a component should be created only if a specified class is missing from the classpath.

@Configuration
public class MyConfiguration {

    @Bean
    @ConditionalOnMissingClass("com.example.MyLibrary")
    public MyBean myBean() {
        return new MyBean();
    }
}

• @ConditionalOnProperty: This annotation is used to specify that a bean should only be created if a certain property is set to a specific value.

@Configuration
public class MyFeatureConfiguration {

    @Bean
    @ConditionalOnProperty(
        name = "myapp.feature.enabled",
        havingValue = "true",
        matchIfMissing = true
    )
    public MyFeatureService myFeatureService() {
        return new MyFeatureService();
    }
}

• @ConditionalOnResource: This annotation is used to specify that a bean should only be created if a certain resource is present.

@Configuration
@ConditionalOnResource(resources = "myconfig.properties")
public class MyConfig {

    @Bean
    public MyService myService() {
        return new MyService();
    }
}

• @ConditionalOnWebApplication: This annotation is used to specify that a bean should only be created if the application is a web application.

@Configuration
@ConditionalOnWebApplication(type = ConditionalOnWebApplication.Type.SERVLET)
public class MyServletConfig {

    @RestController
    static class MyController {
        @GetMapping("/hello")
        public String hello() {
            return "Hello from Servlet-based application!";
        }
    }
}

Now, the MyServletConfig configuration class and its associated controller will only be active when your Spring Boot application is configured as a Servlet-based web application.

• @ConditionalOnNotWebApplication: Similar to @ConditionalOnWebApplication, but creates components when the application is not a web application.

@Configuration
@ConditionalOnNotWebApplication
public class MyNonWebConfig {

    @Bean
    public MyNonWebComponent myNonWebComponent() {
        return new MyNonWebComponent();
    }

    static class MyNonWebComponent {
        void doSomething() {
            System.out.println("Doing something for non-web applications.");
        }
    }
}

• @ConditionalOnJava : This annotation in Spring Boot allows you to conditionally enable or disable a bean or configuration class based on the version of the Java Runtime Environment (JRE) being used. You can specify the desired Java version or version range as a condition for the bean to be created.

@Configuration
public class MyConfiguration {

    @Bean
    @ConditionalOnJava(range = ConditionalOnJava.Range.EQUAL_OR_NEWER, value = ConditionalOnJava.JavaVersion.EIGHT)
    public MyBean myBean() {
        return new MyBean();
    }

    public static class MyBean {
        public void doSomething() {
            System.out.println("MyBean is doing something.");
        }
    }
}

This bean should be created if the Java version is equal to or newer than Java 8.

• @ConditionalOnExpression: This annotation in Spring Boot allows you to conditionally enable or disable a bean or configuration class based on a custom SpEL (Spring Expression Language) expression. If the specified expression evaluates to true, the bean or configuration class is enabled; otherwise, it’s disabled.