【SpringBoot 】注解

参考： https://www.baeldung.com/spring-boot-annotations

@SpringBootApplication

mark the main class of a Spring Boot application:

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

@EnableAutoConfiguration

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

we have to use this annotation with @Configuration:

@Configuration
@EnableAutoConfiguration
class VehicleFactoryConfig {}

 

条件注解

@ConditionalOnClass

当某个特定类存在于类路径时，才会被创建； 通常用于处理可选的依赖

@Configuration
class Appconfig{

    @Bean
    @ConditionalOnClass(name = "com.example.SomeClass") 
    public MyService myService(){
        return new MyServiceImpl()
    }
}

@ConditionalOnMissingClass

当某个特定类不存在于类路径时，才会被创建； 通常用于处理可选的依赖

@Configuration
class Appconfig{

    @Bean
    @ConditionalOnMissingClass(name = "com.example.SomeClass") 
    public MyService myService(){
        return new MyServiceImpl()
    }
}

@ConditionalOnBean

当某个或某些特定的Bean存在于Spring容器中，当前bean才被创建；场景：一个服务需要另外一个服务作为依赖，且希望只有另外一个服务存在时，当前服务才会被创建

@Configuration
class MySQLAutoconfiguration {

    @Bean
    @ConditionalOnBean(Datasource.class) 
    public MyService myService(){
        return new MyServiceImpl()
    }
}

 @ConditionalOnMissingBean

当某个或某些特定Bean不存在Spring容器中，才创建。 通常创建默认的Bean，当用户没有定义自己的Bean时

@Configuration
class MySQLAutoconfiguration {

    @Bean
    @ConditionalOnBean(Datasource.class) 
    public MyService myService(){
        return new DefaultMyService()
    }
}

@ConditionalOnProperty

@Bean
@ConditionalOnProperty(name = "my.service.enabled",havingValue = "true")
  public MyService myService(){
        return new DefaultMyService()
  }
}

@ConditionalOnResource

某个特定的资源存在

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

@ConditionalOnWebApplication and @ConditionalOnNotWebApplication

当应用是（或不是）一个web应用程序，bean才会创建

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

@ConditionalExpression

要求：一个SPEL表达式结果=true

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