懒加载弹簧豆

Question

如果一个bean被延迟加载，那么延迟加载的bean中定义的所有bean都会被延迟加载吗？ （即使它们没有被定义@Lazy）

这里是测试项目： https://github.com/madhur/conditional-property-test/blob/master/src/main/java/com/example/demo/EventPublishService.java

我有这些豆子：

@Service
@ConditionalOnProperty(
        name = {"publish.feed.events"},
        havingValue = "true"
)
public class EventPublishService {
 
     @Autowired
     private KafkaPublisher kafkaPublisher;

}

//////////////////

@Service
public class EventService {

    @Autowired
    @Lazy
    private EventPublishService eventPublishService;

    @Value("${publish.feed.events:false}")
    private boolean isPublishEvents;
}

//////////////////////

@Component
public class KafkaPublisher {

    @Value("${kafka.producer.financial_feed.topic}")
    private String financeFeedTopic;
}

我的任何 application.properties 只有一个属性，

publish.feed.events=false

由于，我没有加载 EventPublishService bean（因为属性为 false），我希望依赖 bean KafkaPublisher 也不应该加载。但是，我在启动时收到错误，这意味着 KafkaPublisher bean 正在加载，即使 EventPublishService bean 没有被加载。

如何确保不加载 KafkaPublisher bean？ 因此，对于不需要 EventPublishService bean 的任何人来说，属性不应该是强制性的吗？

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'kafkaPublisher': Injection of autowired dependencies failed; nested exception is java.lang.IllegalArgumentException: Could not resolve placeholder 'kafka.producer.financial_feed.topic' in value "${kafka.producer.financial_feed.topic}"
        at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:379) ~[spring-beans-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1344) ~[spring-beans-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:578) ~[spring-beans-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:501) ~[spring-beans-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:317) ~[spring-beans-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:228) ~[spring-beans-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:315) ~[spring-beans-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199) ~[spring-beans-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:760) ~[spring-beans-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:869) ~[spring-context-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:550) ~[spring-context-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:759) [spring-boot-2.0.1.RELEASE.jar:2.0.1.RELEASE]
        at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:395) [spring-boot-2.0.1.RELEASE.jar:2.0.1.RELEASE]
        at org.springframework.boot.SpringApplication.run(SpringApplication.java:327) [spring-boot-2.0.1.RELEASE.jar:2.0.1.RELEASE]
        at org.springframework.boot.SpringApplication.run(SpringApplication.java:1255) [spring-boot-2.0.1.RELEASE.jar:2.0.1.RELEASE]
        at org.springframework.boot.SpringApplication.run(SpringApplication.java:1243) [spring-boot-2.0.1.RELEASE.jar:2.0.1.RELEASE]
        at com.example.demo.DemoApplication.main(DemoApplication.java:17) [main/:na] Caused by: java.lang.IllegalArgumentException: Could not resolve placeholder 'kafka.producer.financial_feed.topic' in value "${kafka.producer.financial_feed.topic}"
        at org.springframework.util.PropertyPlaceholderHelper.parseStringValue(PropertyPlaceholderHelper.java:172) ~[spring-core-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.util.PropertyPlaceholderHelper.replacePlaceholders(PropertyPlaceholderHelper.java:124) ~[spring-core-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.core.env.AbstractPropertyResolver.doResolvePlaceholders(AbstractPropertyResolver.java:237) ~[spring-core-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.core.env.AbstractPropertyResolver.resolveRequiredPlaceholders(AbstractPropertyResolver.java:211) ~[spring-core-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.context.support.PropertySourcesPlaceholderConfigurer.lambda$processProperties$0(PropertySourcesPlaceholderConfigurer.java:175) ~[spring-context-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.beans.factory.support.AbstractBeanFactory.resolveEmbeddedValue(AbstractBeanFactory.java:839) ~[spring-beans-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1086) ~[spring-beans-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1065) ~[spring-beans-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:584) ~[spring-beans-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:91) ~[spring-beans-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:373) ~[spring-beans-5.0.5.RELEASE.jar:5.0.5.RELEASE]
        ... 16 common frames omitted

Answer 1

作为stated in the doc，

在创建容器时会创建单例范围并设置为预实例化（默认）的 Bean

因此，不管你把@Lazy放在@Autowired上，bean都会在启动时创建只要bean本身没有定义为lazy：

@Component
public class KafkaPublisher {
    ...
}

如果你想延迟初始化这个bean，把@Lazy放在上面。如果你想让bean注入的过程变得懒惰，把@Lazy放在@Autowired上，就这么简单。

可以通过在@Configuration 类中声明它们并标记配置@Lazy 来延迟初始化一组相关bean。但是，请注意，一旦 @Configuration 类被初始化，所有声明的 bean 都将被初始化。

@Lazy
@Configuration
@ComponentScan(...) // will be lazily initialized with config
public class LazyConfiguration {

    @Bean
    public SomeBean beanName() { // will be lazily initialized with config
        return new SomeBean();
    } 

    @Bean
    public OtherBean beanName() { // will be lazily initialized with config
        return new OtherBean();
    }     
}

但是，当使用 @ComponentScan 时，请确保要延迟初始化的 bean（即 @Service、@Component 等）尚未被其他上下文扫描。如果是，请将它们作为排除项添加到扫描该上下文中。

---

出于历史目的，下面的过时答案（在 OP 完全改变问题之前）：

在 bean 中声明 bean 不是一个好习惯，但如果你将 @Configuration 标记为惰性，那么实际上其中的所有 bean 都将被惰性初始化，如 @Lazy 的 javadoc 中所述：

如果 @Configuration 类上存在 Lazy，则表示所有 @Configuration 中的 @Bean 方法应该被延迟初始化。

如果 "beans defined inside" 你实际上是指注入点，例如@Autowired，javadoc 也有答案：

懒惰的注解也可以放置在标记为的注入点上 Autowired 或 Inject：在这种情况下，它会导致创建一个 所有受影响依赖项的惰性解析代理，作为替代 使用 ObjectFactory 或 Provider。

然而，实际的自动装配 bean 将被急切地初始化（如果没有标记为 @Lazy 为好）。在这种情况下，注入本身是惰性的。

否则，请澄清你的意思。