0108Bean销毁-Bean生命周期详解-spring

Bean使用阶段，调用getBean()得到bean之后，根据需要，自行使用。

1 销毁Bean的几种方式

• 调用org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#destroyBean
• 调用org.springframework.beans.factory.config.ConfigurableBeanFactory#destroySingletons
• 调用ApplicationContext中的close方法，实际会调用起BeanFactory的destroySingletons方法

第一种为销毁单个bean，后两种为销毁容器中所有的单例beans。我们下面主要讲解第一种方式，后面等到讲解容器的时候在详细介绍。

2 Bean销毁逻辑

• 轮询beanPostProcessors列表，如果是DestructionAwareBeanPostProcessor这种类型的，会调用其内部的postProcessBeforeDestruction方法
• 如果bean实现了org.springframework.beans.factory.DisposableBean接口，会调用这个接口中的destroy方法
• 如果bean实现了java.lang.AutoCloseable接口，调用该接口中的close()方法，这个下面不详细讲解。
• 调用bean自定义的销毁方法

追踪下AbstractAutowireCapableBeanFactory#destroyBean方法，代码如下：

@Override
public void destroyBean(Object existingBean) {new DisposableBeanAdapter(existingBean, getBeanPostProcessorCache().destructionAware).destroy();
}

会继续调用DisposableBeanAdapter的destroy方法，代码如下：

@Overridepublic void destroy() {if (!CollectionUtils.isEmpty(this.beanPostProcessors)) {for (DestructionAwareBeanPostProcessor processor : this.beanPostProcessors) {processor.postProcessBeforeDestruction(this.bean, this.beanName);}}if (this.invokeDisposableBean) {// 省略日志记录try {((DisposableBean) this.bean).destroy();}// 省略异常处理}if (this.invokeAutoCloseable) {// 省略日志记录try {((AutoCloseable) this.bean).close();}// 省略异常处理}else if (this.destroyMethods != null) {for (Method destroyMethod : this.destroyMethods) {invokeCustomDestroyMethod(destroyMethod);}}else if (this.destroyMethodNames != null) {for (String destroyMethodName: this.destroyMethodNames) {Method destroyMethod = determineDestroyMethod(destroyMethodName);if (destroyMethod != null) {invokeCustomDestroyMethod(ClassUtils.getInterfaceMethodIfPossible(destroyMethod, this.bean.getClass()));}}}}

3 DestructionAwareBeanPostProcessor后置处理器

package org.springframework.beans.factory.config;import org.springframework.beans.BeansException;public interface DestructionAwareBeanPostProcessor extends BeanPostProcessor {/*** 销毁之前执行*/void postProcessBeforeDestruction(Object bean, String beanName) throws BeansException;/*** 是否要执行销毁*/default boolean requiresDestruction(Object bean) {return true;}}

3.1 CommonAnnotationBeanPostProcessor

其中这个接口有个重要的实现类

org.springframework.context.annotation.CommonAnnotationBeanPostProcessor

这个类中并没有postProcessBeforeDestruction方法，即它直接继承自起父类InitDestroyAnnotationBeanPostProcessor，代码如下：

public void postProcessBeforeDestruction(Object bean, String beanName) throws BeansException {LifecycleMetadata metadata = findLifecycleMetadata(bean.getClass());try {metadata.invokeDestroyMethods(bean, beanName);}// 省略。。。
}
public void invokeDestroyMethods(Object target, String beanName) throws Throwable {Collection<LifecycleElement> checkedDestroyMethods = this.checkedDestroyMethods;Collection<LifecycleElement> destroyMethodsToUse =(checkedDestroyMethods != null ? checkedDestroyMethods : this.destroyMethods);if (!destroyMethodsToUse.isEmpty()) {for (LifecycleElement element : destroyMethodsToUse) {if (logger.isTraceEnabled()) {logger.trace("Invoking destroy method on bean '" + beanName + "': " + element.getMethod());}element.invoke(target);}}
}

那么在detroMethods中有哪些销毁方法呢？我们看下上面findLifecycleMetadata方法，代码如下；

private LifecycleMetadata findLifecycleMetadata(Class<?> clazz) {if (this.lifecycleMetadataCache == null) {// Happens after deserialization, during destruction...return buildLifecycleMetadata(clazz);}// Quick check on the concurrent map first, with minimal locking.LifecycleMetadata metadata = this.lifecycleMetadataCache.get(clazz);if (metadata == null) {synchronized (this.lifecycleMetadataCache) {metadata = this.lifecycleMetadataCache.get(clazz);if (metadata == null) {metadata = buildLifecycleMetadata(clazz);this.lifecycleMetadataCache.put(clazz, metadata);}return metadata;}}return metadata;
}

• 判断lifecycleMetadataCache缓存为空
  • 为空，执行buildLifecycleMetadata方法
• 不为空，直接缓存获取

我们看下缓存为空的情况，即执行buildLifecycleMetadata方法，

private LifecycleMetadata buildLifecycleMetadata(final Class<?> clazz) {if (!AnnotationUtils.isCandidateClass(clazz, Arrays.asList(this.initAnnotationType, this.destroyAnnotationType))) {return this.emptyLifecycleMetadata;}List<LifecycleElement> initMethods = new ArrayList<>();List<LifecycleElement> destroyMethods = new ArrayList<>();Class<?> targetClass = clazz;do {final List<LifecycleElement> currInitMethods = new ArrayList<>();final List<LifecycleElement> currDestroyMethods = new ArrayList<>();ReflectionUtils.doWithLocalMethods(targetClass, method -> {if (this.initAnnotationType != null && method.isAnnotationPresent(this.initAnnotationType)) {LifecycleElement element = new LifecycleElement(method);currInitMethods.add(element);if (logger.isTraceEnabled()) {logger.trace("Found init method on class [" + clazz.getName() + "]: " + method);}}if (this.destroyAnnotationType != null && method.isAnnotationPresent(this.destroyAnnotationType)) {currDestroyMethods.add(new LifecycleElement(method));if (logger.isTraceEnabled()) {logger.trace("Found destroy method on class [" + clazz.getName() + "]: " + method);}}});initMethods.addAll(0, currInitMethods);destroyMethods.addAll(currDestroyMethods);targetClass = targetClass.getSuperclass();}while (targetClass != null && targetClass != Object.class);return (initMethods.isEmpty() && destroyMethods.isEmpty() ? this.emptyLifecycleMetadata :new LifecycleMetadata(clazz, initMethods, destroyMethods));
}

该方法用于收集bean的初始化方法和销毁方法：

• 遍历目标bean Class的方法，判断方法上注解是否是initAnnotationType类型，如果是加入初始化方法集合；如果方法注解是destroyAnnotationType类型，加入销毁方法集合。

• initAnnotationType和destroyAnnotationType类型，是在CommonAnnotationBeanPostProcessor类的构造方法里面设置的

  public CommonAnnotationBeanPostProcessor() {setOrder(Ordered.LOWEST_PRECEDENCE - 3);setInitAnnotationType(PostConstruct.class);setDestroyAnnotationType(PreDestroy.class);// java.naming module present on JDK 9+?if (jndiPresent) {this.jndiFactory = new SimpleJndiBeanFactory();}}
  

• 即destroyAnnotationType类型就是判断方法注解是否是@PreDestroy

CommonAnnotationBeanPostProcessor#postProcessBeforeDestruction方法中会调用bean中所有标注了@PreDestroy的方法

4 @PreDestroy注解

@PreDestroy注解主要用于在对象销毁前执行一些清理工作，比如释放资源或者关闭连接。

下面来做个测试，测试代码如下：

package com.gaogzhen.myspring.bean;import jakarta.annotation.PreDestroy;/*** 测试@PreDestory* @author gaogzhen*/
public class DestroyBean {public DestroyBean() {System.out.println("creat======");}@PreDestroypublic void testPreDestroy() {System.out.println("===preDestroy===");}
}
@Test
public void testPreDestroy() {DefaultListableBeanFactory factory = new DefaultListableBeanFactory();// 自定义DestructionAwareBeanPostProcessorfactory.addBeanPostProcessor((DestructionAwareBeanPostProcessor) (bean, beanName) -> System.out.println("准备销毁bean：" + beanName));// 默认CommonAnnotationBeanPostProcessorfactory.addBeanPostProcessor(new CommonAnnotationBeanPostProcessor());// 容器注入BeanDefinitionfactory.registerBeanDefinition("destroyBean", BeanDefinitionBuilder.genericBeanDefinition(DestroyBean.class).getBeanDefinition());// 触发所有单例bean初始化factory.preInstantiateSingletons();// 销毁指定beanfactory.destroySingleton("destroyBean");
}

测试结果：

creat======
准备销毁bean：destroyBean
===preDestroy===

5 DisposableBean接口

public interface DisposableBean {/*** 在bean被销毁时被调用*/void destroy() throws Exception;}

• 作用用于销毁前释放资源或者连接

这里不单独做测试，在下面#6一起测试下执行顺序。

6 自定义销毁方法

我们将通过AnnotationConfigApplicationContext来演示下，当bean被销毁时，@PreDestroy，DisposableBean以及自定义销毁方法的执行顺序。

示例代码如下6-1所示：

package com.gaogzhen.myspring.service;import jakarta.annotation.PreDestroy;
import org.springframework.beans.factory.DisposableBean;/*** 测试销毁相关的方法执行顺序* @author gaogzhen*/
public class ServiceDestroy implements DisposableBean {@Overridepublic void destroy() throws Exception {System.out.println("DisposableBean接口方法执行");}@PreDestroypublic void preDestroy() {System.out.println("@PreDestroy注解方法执行");}public void customDestroy() {System.out.println("自定义销毁方法执行");}
}public class TestDestroy {@Bean(destroyMethod = "customDestroy")public ServiceDestroy serviceDestroy() {return new ServiceDestroy();}@Testpublic void testDestroyOrder() {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();context.register(TestDestroy.class);System.out.println("准备启动容器");context.refresh();System.out.println("容器启动完毕");System.out.println("serviceDestroy: " + context.getBean(ServiceDestroy.class));System.out.println("准备关闭容器");context.close();System.out.println("容器关闭");}
}

测试结果：

准备启动容器
容器启动完毕
serviceDestroy: com.gaogzhen.myspring.service.ServiceDestroy@40e10ff8
准备关闭容器
@PreDestroy注解方法执行
DisposableBean接口方法执行
容器关闭

后记

如果小伙伴什么问题或者指教，欢迎交流。

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⭐️源代码仓库地址：https://gitee.com/gaogzhen/spring6-study

参考：

[1]Spring系列之Bean生命周期详解[CP/OL]