SpringBootAnnotations
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:
1
2
3
4
5
6
7
@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:
1
2
3
@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:
1
2
3
4
5
@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:
1
2
3
4
5
@Bean
@ConditionalOnBean
(name =
"dataSource"
)
LocalContainerEntityManagerFactoryBean entityManagerFactory() {
// ...
}
4.3. @ConditionalOnProperty
With this annotation, we can make conditions on the values of properties:
1
2
3
4
5
6
7
8
@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:
1
2
3
4
@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:
1
2
3
4
@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:
1
2
3
4
5
@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:
1
2
3
4
@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.
As usual, the examples are available over on GitHub.
以上是 SpringBootAnnotations 的全部内容， 来源链接： utcz.com/z/513015.html
