本文給大家講解Spring中@DependsOn注解的作用及實現(xiàn)原理！

官方文檔解釋

Beans on which the current bean depends. Any beans specified are guaranteed to be created by the container before this bean. Used infrequently in cases where a bean does not explicitly depend on another through properties or constructor arguments, but rather depends on the side effects of another bean's initialization.
A depends-on declaration can specify both an initialization-time dependency and, in the case of singleton beans only, a corresponding destruction-time dependency. Dependent beans that define a depends-on relationship with a given bean are destroyed first, prior to the given bean itself being destroyed. Thus, a depends-on declaration can also control shutdown order.
May be used on any class directly or indirectly annotated with org.springframework.stereotype.Component or on methods annotated with Bean.
Using DependsOn at the class level has no effect unless component-scanning is being used. If a DependsOn-annotated class is declared via XML, DependsOn annotation metadata is ignored, and <bean depends-on="..."/> is respected instead.

@DependsOn注解的作用

org.springframework.context.annotation.DependsOn
該注解的屬性是一個字符串數(shù)組，數(shù)組的元素是每個依賴的bean的名稱。

@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface DependsOn {

	String[] value() default {};

}

@DependsOn注解主要用于指定當前bean所依賴的beans。任何被指定依賴的bean都由Spring保證在當前bean之前創(chuàng)建。在少數(shù)情況下，bean不是通過屬性或構造函數(shù)參數(shù)顯式依賴于另一個bean，但卻需要要求另一個bean優(yōu)先完成初始化，則可以使用@DependsOn這個注解。

@DependsOn既可以指定初始化依賴順序，也可以指定bean相應的銷毀執(zhí)行順序（僅在單例bean的情況下）。

可用于任何直接或間接帶@Component注解的bean或在用@Bean注釋的方法上。
如果使用的是xml配置，則需要使用<bean dependens on=“…”/>標簽.

簡單描述就是@DependsOn可以控制bean的創(chuàng)建、初始化（InitializingBean）、銷毀方法執(zhí)行順序。

示例：假如有三個Bean類叫Aaa、Bbb、Ccc分別實現(xiàn)了如下兩個接口。
org.springframework.beans.factory.InitializingBean
org.springframework.beans.factory.DisposableBean

Ccc通過@DependsOn指定依賴bean創(chuàng)建的順序為Bbb > Aaa

@DependsOn({"bbb","ccc"})
@Service
public class Aaa implements InitializingBean, DisposableBean {
 private static final Logger logger = LoggerFactory.getLogger(Aaa.class);

 public Aaa() {
  logger.info(this.getClass().getName() + " Construction");
 }

 @Override
 public void afterPropertiesSet() throws Exception {
  logger.info(this.getClass().getName() + " afterPropertiesSet");
 }

 @Override
 public void destroy() throws Exception {
  logger.info(this.getClass().getName() + " destroy");
 }
}

Bbb Ccc類實現(xiàn)如下

@Service
public class Bbb implements InitializingBean, DisposableBean {
 //實現(xiàn)和Aaa相同
}
@Service
public class Ccc implements InitializingBean, DisposableBean {
 //實現(xiàn)和Aaa相同
}

那么初始順序如下: bbb --> ccc --> aaa

而銷毀方法執(zhí)行順序正好相反如下: aaa --> ccc --> bbb

@DependsOn注解的實現(xiàn)原理

Spring在啟動時掃描到一個bean，會封裝成一個BeanDefinition，如果是AnnotatedBeanDefinition則解析類上的注解信息，發(fā)現(xiàn)@DependsOn注解，則讀取value值，調(diào)用BeanDefinition#setDependsOn保存。
源碼見ClassPathBeanDefinitionScanner#doScan，AnnotationConfigUtils#processCommonDefinitionAnnotations(AnnotatedBeanDefinition, AnnotatedTypeMetadata)

創(chuàng)建bean時，也就是調(diào)用AbstractBeanFactory#doGetBean時，會獲取這些被依賴的beanName，按照數(shù)組順序，再調(diào)用AbstractBeanFactory#getBean(beanName)來優(yōu)先創(chuàng)建被依賴的bean，從而達到控制依賴順序。

除此之外，在創(chuàng)建bean時，還會調(diào)用AbstractBeanFactory#registerDisposableBeanIfNecessary來向Spring中注冊帶有銷毀方法的bean，源碼見DefaultSingletonBeanRegistry#registerDisposableBean，內(nèi)部通過LinkedHashMap保存。key為bean名稱。進程退出時，會逆序調(diào)用銷毀方法。
源碼見DefaultSingletonBeanRegistry#destroySingletons

public void destroySingletons() {
	if (logger.isTraceEnabled()) {
		logger.trace("Destroying singletons in " + this);
	}
	synchronized (this.singletonObjects) {
		this.singletonsCurrentlyInDestruction = true;
	}

	String[] disposableBeanNames;
	synchronized (this.disposableBeans) {
		disposableBeanNames = StringUtils.toStringArray(this.disposableBeans.keySet());
	}
	for (int i = disposableBeanNames.length - 1; i >= 0; i--) {
		destroySingleton(disposableBeanNames[i]);
	}

	this.containedBeanMap.clear();
	this.dependentBeanMap.clear();
	this.dependenciesForBeanMap.clear();

	clearSingletonCache();
}

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