【spring源码系列】之【Bean的销毁】

1. 概述

Bean的销毁是Bean的生命周期中最后一步，比如在Tomcat等容器关闭的时候会调用Bean的销毁方法，下面逐步分析。

2. 源码分析

在bean创建完成后，就会对这个bean注册一个销毁的Adapter对象，

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)throws BeanCreationException {......if (instanceWrapper == null) {//创建对象实例instanceWrapper = createBeanInstance(beanName, mbd, args);}Object bean = instanceWrapper.getWrappedInstance();Class beanType = instanceWrapper.getWrappedClass();if (beanType != NullBean.class) {mbd.resolvedTargetType = beanType;}......try {// 属性赋值populateBean(beanName, mbd, instanceWrapper);// 初始化beanexposedObject = initializeBean(beanName, exposedObject, mbd);}catch (Throwable ex) {if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {throw (BeanCreationException) ex;}else {throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);}}......// Register bean as disposable.try {// 注册销毁的beanregisterDisposableBeanIfNecessary(beanName, bean, mbd);}catch (BeanDefinitionValidationException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);}return exposedObject;}

registerDisposableBeanIfNecessary 方法中 disposableBeans 集合负责收集需要销毁的 bean 。

protected void registerDisposableBeanIfNecessary(String beanName, Object bean, RootBeanDefinition mbd) {AccessControlContext acc = (System.getSecurityManager() != null ? getAccessControlContext() : null);if (!mbd.isPrototype() && requiresDestruction(bean, mbd)) {if (mbd.isSingleton()) {// Register a DisposableBean implementation that performs all destruction// work for the given bean: DestructionAwareBeanPostProcessors,// DisposableBean interface, custom destroy method.// 注册销毁bean的DisposableBeanAdapter对象registerDisposableBean(beanName,new DisposableBeanAdapter(bean, beanName, mbd, getBeanPostProcessors(), acc));}  ......

public void registerDisposableBean(String beanName, DisposableBean bean) {synchronized (this.disposableBeans) {this.disposableBeans.put(beanName, bean);}}

DisposableBeanAdapter 对象就是负责 bean 销毁的类，这个类中收集 bean 是否实现了 DisposableBean 接口

class DisposableBeanAdapter implements DisposableBean, Runnable, Serializable

是否配置 destroy-method 属性，过滤了 DestructionAwareBeanPostProcessor 类型的接口，如下图所示：

public DisposableBeanAdapter(Object bean, String beanName, RootBeanDefinition beanDefinition,List postProcessors, @Nullable AccessControlContext acc) {......this.beanPostProcessors = filterPostProcessors(postProcessors, bean);}

private List filterPostProcessors(List processors, Object bean) {List filteredPostProcessors = null;if (!CollectionUtils.isEmpty(processors)) {filteredPostProcessors = new ArrayList(processors.size());for (BeanPostProcessor processor : processors) {if (processor instanceof DestructionAwareBeanPostProcessor) {DestructionAwareBeanPostProcessor dabpp = (DestructionAwareBeanPostProcessor) processor;if (dabpp.requiresDestruction(bean)) {filteredPostProcessors.add(dabpp);}}}}return filteredPostProcessors;

然后 bean 是在什么时候被销毁呢，在 tomcat 关闭的时候就会调用到 servlet 中的销毁方法，具体是通过类 ContextLoaderListener.java 中的 contextDestroyed 方法，通过 closeWebApplicationContext 方法一直往下找此为 servlet 规范的使用，一路往下调用。

最终会进入 DisposableBeanAdapter 类中的 destroy ，方法该方法就会根据前面的收集进行调用。

public void destroy() {// 处理@PreDestroy注解的beanpostProcessor实现类: InitDestroyAnnotationBeanPostProcessorif (!CollectionUtils.isEmpty(this.beanPostProcessors)) {for (DestructionAwareBeanPostProcessor processor : this.beanPostProcessors) {processor.postProcessBeforeDestruction(this.bean, this.beanName);}}// 处理实现DisposableBean接口的bean的销毁if (this.invokeDisposableBean) {if (logger.isTraceEnabled()) {logger.trace("Invoking destroy() on bean with name '" + this.beanName + "'");}try {if (System.getSecurityManager() != null) {AccessController.doPrivileged((PrivilegedExceptionAction) () -> {((DisposableBean) this.bean).destroy();return null;}, this.acc);}else {((DisposableBean) this.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);}}}// 处理在配置文件中的bean配置了destroy-method的bean的销毁if (this.destroyMethod != null) {invokeCustomDestroyMethod(this.destroyMethod);}else if (this.destroyMethodName != null) {Method methodToInvoke = determineDestroyMethod(this.destroyMethodName);if (methodToInvoke != null) {invokeCustomDestroyMethod(ClassUtils.getInterfaceMethodIfPossible(methodToInvoke));}}}

销毁bean的顺序是如下：

1）判断是否需要处理 @PreDestroy 注解的 bean ，如果需要，则通过 beanpostProcessor 实现类 InitDestroyAnnotationBeanPostProcessor 处理；

2）判断是否需要处理实现 DisposableBean 接口的 bean 的销毁；

3）判断是否需要处理配置文件中的 bean 配置了 destroy-method 的 bean 的销毁。

3. 案例演示

定义 Bean ，同时加入销毁对应的三种方法；

/** * @Author: wzj * @Date: 2021/7/2 11:32 * @Desc: */public class Wzj implements DisposableBean {    public static Wzj factoryMethod() { CQ cq = new CQ(); SC sc = new SC(); return new Wzj(sc, cq);    }    @PreDestroy    public void close() { System.out.println("通过 @PreDestroy：销毁实例wzj");    }    public void destroyMethod() { System.out.println("通过配置文件配置destroy-method：销毁实例wzj");    }    @Override    public void destroy() { System.out.println("通过DisposableBean接口：销毁实例wzj");    }

配置文件如下：

测试类：

/** * @Author: wzj * @Date: 2021/3/30 15:08 * @Desc: 测试类 */@RunWith(SpringJUnit4ClassRunner.class)@ContextConfiguration(locations = {"classpath:spring.xml"})public class TestSpring {    @Autowired    private ApplicationContext applicationContext;    @Test    public void testDestroy() { ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("spring.xml"); Wzj wzj = (Wzj)applicationContext.getBean("wzj"); applicationContext.getBeanFactory().destroyBean("wzj");      }

结果:

有人可能会问，为何 Bean 可以多次销毁，其实 Bean 的销毁并不是真正意义上的销毁 Bean ，而是在销毁前执行销毁方法，可能包含关闭数据库连接、关闭网络请求等逻辑操作，而后真正的销毁是由 Spring 容器执行关闭，其内部 Bean 也就自然而然消失了， Bean 销毁是发生在 Spring 容器关闭过程中的。