我有一个Spring Boot 1.5.x
项目,其中某些@Component
依赖于其他@Component
,最终沿着依赖关系链,可以使用@Component
完全启用或禁用某些@ConditionalOnProperty
。
我正在使用@ConditionalOnBean
来避免实例化依赖于其他@Component
的@Component
,这些其他properties
由于缺少MyServices.kt
而尚未实例化。
但是,它仅适用于直接依赖关系,不适用于传递依赖关系,但是我不明白为什么。
让我尝试用一个简单的例子来解释。
考虑到application.yml
:
private val logger = KotlinLogging.logger {}
class MyServices
@ConditionalOnProperty("service.a")
@Service
class ServiceA {
init {
logger.info { "A SERVICE" }
}
}
@ConditionalOnBean(ServiceA::class)
@ConditionalOnProperty("service.b")
@Service
class ServiceB(
private val serviceA: ServiceA
) {
init {
logger.info { "B SERVICE depends on $serviceA" }
}
}
@ConditionalOnBean(ServiceB::class)
@ConditionalOnProperty("service.c")
@Service
class ServiceC(
private val serviceB: ServiceB
) {
init {
logger.info { "C Service depends on $serviceB" }
}
}
使用以下
application.yml
:service:
a: false
b: true
c: true
然后Spring在启动时崩溃,并显示以下内容:
**************************
APPLICATION FAILED TO START
***************************
Description:
Parameter 0 of constructor in org.gotson.transitivebeandependencies.ServiceC required a bean of type 'org.gotson.transitivebeandependencies.ServiceB' that could not be found.
Action:
Consider defining a bean of type 'org.gotson.transitivebeandependencies.ServiceB' in your configuration.
这是自动配置的结果:
Positive matches:
ServiceC matched:
- @ConditionalOnProperty (service.c) matched (OnPropertyCondition)
- @ConditionalOnBean (types: org.gotson.transitivebeandependencies.ServiceB; SearchStrategy: all) found bean 'serviceB' (OnBeanCondition)
Negative matches:
ServiceA:
Did not match:
- @ConditionalOnProperty (service.a) found different value in property 'service.a' (OnPropertyCondition)
ServiceB:
Did not match:
- @ConditionalOnBean (types: org.gotson.transitivebeandependencies.ServiceA; SearchStrategy: all) did not find any beans (OnBeanCondition)
Matched:
- @ConditionalOnProperty (service.b) matched (OnPropertyCondition)
但是,使用以下
ServiceA
:service:
a: true
b: false
c: true
然后一切正常,仅实例化
ServiceB
实例,而未创建ServiceC
和@Bean
Bean。与
@Component
而不是MyBeans.kt
相同的行为可以正常工作。application.yml
:private val logger = KotlinLogging.logger {}
@Configuration
class MyBeans {
@ConditionalOnProperty("bean.a")
@Bean
fun beanA(): BeanA {
logger.info { "A BEAN" }
return BeanA("beanA")
}
@ConditionalOnBean(BeanA::class)
@ConditionalOnProperty("bean.b")
@Bean
fun beanB(beanA: BeanA): BeanB {
logger.info { "B BEAN depends on $beanA" }
return BeanB("beanB")
}
@ConditionalOnBean(BeanB::class)
@ConditionalOnProperty("bean.c")
@Bean
fun beanC(beanB: BeanB): BeanC {
logger.info { "C BEAN depends on $beanB" }
return BeanC("beanC")
}
}
data class BeanA(val name: String)
data class BeanB(val name: String)
data class BeanC(val name: String)
用
BeanA
:bean:
a: false
b: true
c: true
我没有实例化类型为
BeanB
,BeanC
或ojit_code的bean。这是自动配置的结果:
Negative matches:
MyBeans#beanA:
Did not match:
- @ConditionalOnProperty (bean.a) found different value in property 'bean.a' (OnPropertyCondition)
MyBeans#beanB:
Did not match:
- @ConditionalOnBean (types: org.gotson.transitivebeandependencies.BeanA; SearchStrategy: all) did not find any beans (OnBeanCondition)
Matched:
- @ConditionalOnProperty (bean.b) matched (OnPropertyCondition)
MyBeans#beanC:
Did not match:
- @ConditionalOnBean (types: org.gotson.transitivebeandependencies.BeanB; SearchStrategy: all) did not find any beans (OnBeanCondition)
Matched:
- @ConditionalOnProperty (bean.c) matched (OnPropertyCondition)
我已经设置了一个样本仓库,其中包含要重现的测试:https://github.com/gotson/spring-transitive
最佳答案
@ConditionalOnBean
是Bean注册阶段检查,因此,需要概述ApplicationContext
中有效使用哪些bean。可以使用常规的@Bean
以标准方式注册Bean,从而公开与该方法的返回类型相同的目标类型。您可能还拥有带有更复杂逻辑的FactoryBean
,这可能导致我们不得不处理异国情调的设置。
无论如何,订购是关键。如果要使Bean类型匹配正常工作,则必须按给定的顺序处理配置类。如果您有一个C1
配置类,仅当bean A
可用并且该bean由B
贡献时,才贡献bean C2
,则C2
必须首先运行。
Spring Boot的解析阶段分为两个步骤:首先,我们解析所有用户的配置。完成后,我们将解析自动配置的bean定义。自动配置本身是有序的(使用@AutoConfigureBefore
,@AutoConfigureAfter
)。这样,我们可以保证,如果您将@ConditionalOnBean
放入自动配置中,则会按照用户配置的预期进行处理。而且,如果您依靠其他自动配置提供的功能,则可以使用这些注释轻松订购它。
您的设置完全可以避免订购,因此如果订购正确,则可以使用,如果订购不正确,则可以使用。 @ConditionalOnBean
的Javadoc显然是states that
如果您想了解更多信息,可以参加3小时的大学类(class),即on youtube这个话题。
关于java - 使用@ConditionalOnBean的Spring Boot可传递的@Component依赖项,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/50286863/