注解说明
@PostConstruct，@PreDestroy是Java规范JSR-250引入的注解，定义了对象的创建和销毁工作，同一期规范中还有注解@Resource，Spring也支持了这些注解。
在Spring中，@PostConstruct，@PreDestroy注解的解析是通过BeanPostProcessor实现的，具体的解析类是org.springframework.context.annotation.CommonAnnotationBeanPostProcessor，其父类是org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor，Spring官方说明了该类对JSR-250中@PostConstruct，@PreDestroy，@Resource注解的支持。
Spring's org.springframework.context.annotation.CommonAnnotationBeanPostProcessor supports the JSR-250 javax.annotation.PostConstruct and javax.annotation.PreDestroy annotations out of the box, as init annotation and destroy annotation, respectively. Furthermore, it also supports the javax.annotation.Resource annotation for annotation-driven injection of named beans.
调用过程
具体过程是，IOC容器先解析各个组件的定义信息，解析到@PostConstruct，@PreDestroy的时候，定义为生命周期相关的方法，组装组件的定义信息等待初始化；在创建组件时，创建组件并且属性赋值完成之后，在执行各类初始化方法之前，从容器中找出所有BeanPostProcessor的实现类，其中包括InitDestroyAnnotationBeanPostProcessor，执行所有BeanPostProcessor的postProcessBeforeInitialization方法，在InitDestroyAnnotationBeanPostProcessor中就是找出被@PostConstruct修饰的方法的定义信息，并执行被@PostConstruct标记的方法。
调用分析
@PostConstruct的调用链如下：
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(String, Object, RootBeanDefinition)初始化流程中，先执行org.springframework.beans.factory.config.BeanPostProcessor.postProcessBeforeInitialization(Object, String)方法，然后再执行初始化方法：
protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) { if (System.getSecurityManager() != null) { AccessController.doPrivileged(new PrivilegedAction<Object>() { @Override public Object run() { invokeAwareMethods(beanName, bean); return null; } }, getAccessControlContext()); } else { invokeAwareMethods(beanName, bean); } Object wrappedBean = bean; if (mbd == null || !mbd.isSynthetic()) { // 在执行初始化方法之前：先执行org.springframework.beans.factory.config.BeanPostProcessor.postProcessBeforeInitialization(Object, String)方法 wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName); } try { //执行InitializingBean的初始化方法和init-method指定的初始化方法 invokeInitMethods(beanName, wrappedBean, mbd); } catch (Throwable ex) { throw new BeanCreationException( (mbd != null ? mbd.getResourceDescription() : null), beanName, "Invocation of init method failed", ex); } if (mbd == null || !mbd.isSynthetic()) { wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName); } return wrappedBean; }
org.springframework.beans.factory.config.BeanPostProcessor.postProcessBeforeInitialization(Object, String)的说明如下：
Apply this BeanPostProcessor to the given new bean instance before any bean initialization callbacks (like InitializingBean's afterPropertiesSet or a custom init-method). The bean will already be populated with property values. The returned bean instance may be a wrapper around the original.
调用时机：在组件创建完属性复制完成之后，调用组件初始化方法之前；
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyBeanPostProcessorsBeforeInitialization(Object, String)的具体流程如下。
@Override public Object applyBeanPostProcessorsBeforeInitialization(Object existingBean, String beanName) throws BeansException { Object result = existingBean; for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) { //遍历所有BeanPostProcessor的实现类，执行BeanPostProcessor的postProcessBeforeInitialization //在InitDestroyAnnotationBeanPostProcessor中的实现是找出@PostConstruct标记的方法的定义信息，并执行 result = beanProcessor.postProcessBeforeInitialization(result, beanName); if (result == null) { return result; } } return result; }
@PreDestroy调用链如下：
@PreDestroy是通过org.springframework.beans.factory.config.DestructionAwareBeanPostProcessor.postProcessBeforeDestruction(Object, String)被调用（InitDestroyAnnotationBeanPostProcessor实现了该接口），该方法的说明如下：
Apply this BeanPostProcessor to the given bean instance before its destruction. Can invoke custom destruction callbacks.
Like DisposableBean's destroy and a custom destroy method, this callback just applies to singleton beans in the factory (including inner beans).
调用时机：该方法在组件的销毁之前调用；
org.springframework.beans.factory.support.DisposableBeanAdapter.destroy()的执行流程如下：
@Override public void destroy() { if (!CollectionUtils.isEmpty(this.beanPostProcessors)) { //调用所有DestructionAwareBeanPostProcessor的postProcessBeforeDestruction方法 for (DestructionAwareBeanPostProcessor processor : this.beanPostProcessors) { processor.postProcessBeforeDestruction(this.bean, this.beanName); } } if (this.invokeDisposableBean) { if (logger.isDebugEnabled()) { logger.debug("Invoking destroy() on bean with name '" + this.beanName + "'"); } try { if (System.getSecurityManager() != null) { AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() { @Override public Object run() throws Exception { ((DisposableBean) bean).destroy(); return null; } }, acc); } else { //调用DisposableBean的销毁方法 ((DisposableBean) bean).destroy(); } } catch (Throwable ex) { String msg = "Invocation of destroy method failed on bean with name '" + this.beanName + "'"; if (logger.isDebugEnabled()) { logger.warn(msg, ex); } else { logger.warn(msg + ": " + ex); } } } //调用自定义的销毁方法 if (this.destroyMethod != null) { invokeCustomDestroyMethod(this.destroyMethod); } else if (this.destroyMethodName != null) { Method methodToCall = determineDestroyMethod(); if (methodToCall != null) { invokeCustomDestroyMethod(methodToCall); } } }
所以是先调用DestructionAwareBeanPostProcessor的postProcessBeforeDestruction(@PreDestroy标记的方法被调用)，再是DisposableBean的destory方法，最后是自定义销毁方法。
