Spring Boot Annotations

1. Overview

Spring Boot made configuring Spring easier with its auto-configuration feature.

In this quick tutorial, we’ll explore the annotations from the org.springframework.boot.autoconfigure and org.springframework.boot.autoconfigure.condition packages.

2. @SpringBootApplication

We use this annotation to mark the main class of a Spring Boot application:

@SpringBootApplication
class VehicleFactoryApplication {

    public static void main(String[] args) {
        SpringApplication.run(VehicleFactoryApplication.class, args);
    }
}

@SpringBootApplication encapsulates @Configuration, @EnableAutoConfiguration, and @ComponentScan annotations with their default attributes.

3. @EnableAutoConfiguration

@EnableAutoConfiguration, as its name says, enables auto-configuration. It means that Spring Boot looks for auto-configuration beans on its classpath and automatically applies them.

Note, that we have to use this annotation with @Configuration:

@Configuration
@EnableAutoConfiguration
class VehicleFactoryConfig {}

4. Auto-Configuration Conditions

Usually, when we write our custom auto-configurations, we want Spring to use them conditionally. We can achieve this with the annotations in this section.

We can place the annotations in this section on @Configuration classes or @Bean methods.

In the next sections, we’ll only introduce the basic concept behind each condition. For further information, please visit this article.

4.1. @ConditionalOnClass and @ConditionalOnMissingClass

Using these conditions, Spring will only use the marked auto-configuration bean if the class in the annotation’s argument is present/absent:

@Configuration
@ConditionalOnClass(DataSource.class)
class MySQLAutoconfiguration {
    //...
}

4.2. @ConditionalOnBean and @ConditionalOnMissingBean

We can use these annotations when we want to define conditions based on the presence or absence of a specific bean:

@Bean
@ConditionalOnBean(name = "dataSource")
LocalContainerEntityManagerFactoryBean entityManagerFactory() {
    // ...
}

4.3. @ConditionalOnProperty

With this annotation, we can make conditions on the values of properties:

@Bean
@ConditionalOnProperty(
    name = "usemysql", 
    havingValue = "local"
)
DataSource dataSource() {
    // ...
}

4.4. @ConditionalOnResource

We can make Spring to use a definition only when a specific resource is present:

@ConditionalOnResource(resources = "classpath:mysql.properties")
Properties additionalProperties() {
    // ...
}

4.5. @ConditionalOnWebApplication and @ConditionalOnNotWebApplication

With these annotations, we can create conditions based on if the current application is or isn’t a web application:

@ConditionalOnWebApplication
HealthCheckController healthCheckController() {
    // ...
}

4.6. @ConditionalExpression

We can use this annotation in more complex situations. Spring will use the marked definition when the SpEL expression is evaluated to true:

@Bean
@ConditionalOnExpression("${usemysql} && ${mysqlserver == 'local'}")
DataSource dataSource() {
    // ...
}

4.7. @Conditional

For even more complex conditions, we can create a class evaluating the custom condition. We tell Spring to use this custom condition with @Conditional:

@Conditional(HibernateCondition.class)
Properties additionalProperties() {
    //...
}

5. Conclusion

In this article, we saw an overview of how can we fine-tune the auto-configuration process and provide conditions for custom auto-configuration beans.