Spring生命周期注解之@PostConstruct，@PreDestroy

1. 注解说明

2. 调用过程

3. 调用分析

1. 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.

2. 调用过程

具体过程是，先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；

3. 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>() {@Overridepublic 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>() {@Overridepublic 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；