我有一个Spring Boot 1.5.x项目,其中一些@Component依赖于其他@Component,并且最终沿着依赖链,可以使用@ConditionalOnProperty完全启用或禁用某些@Component.

我正在使用@ConditionalOnBean来避免实例化依赖于因缺少属性而未实例化的其他@Component的@Component.

但是,它只适用于直接依赖,而不适用于传递依赖,但我无法理解为什么.

让我试着用一个简单的例子来解释.

考虑MyServices.kt：

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,具体如下：

**************************
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.

以下是自动配置的结果：

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application.yml：

service:
  a: true
  b: false
  c: true

然后一切正常,只有一个ServiceA实例被实例化,而没有创建ServiceB和ServiceC bean.

与@Bean而不是@Component相同的行为按预期工作.

MyBeans.kt：

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)

使用application.yml：

bean:
  a: false
  b: true
  c: true

我没有实例化BeanA,BeanB或BeanC类型的bean.

以下是自动配置的结果：

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)

我已经设置了一个带有测试的样本repo来重现：https://github.com/gotson/spring-transitive