@SpringBootApplication encapsulates @Configuration, @EnableAutoConfiguration, and @ComponentScan annotations with their default attributes.
@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 {}
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.
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 {
//...
}
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() {
// ...
}
3. @ConditionalOnProperty
With this annotation, we can make conditions on the values of properties:
@Bean
@ConditionalOnProperty(
name = "usemysql",
havingValue = "local"
)
DataSource dataSource() {
// ...
}
4. @ConditionalOnResource
We can make Spring to use a definition only when a specific resource is present:
@ConditionalOnResource(resources = "classpath:mysql.properties")
Properties additionalProperties() {
// ...
}
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() {
// ...
}
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() {
// ...
}
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() {
//...
}