【spring源码系列】之【Bean的销毁】

2022年3月7日 38点热度 0条评论 来源: 小猪爸爸


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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bean
			exposedObject = 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bean
			registerDisposableBeanIfNecessary(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<BeanPostProcessor> postProcessors, @Nullable AccessControlContext acc) {

		......

		this.beanPostProcessors = filterPostProcessors(postProcessors, bean);
	}
private List<DestructionAwareBeanPostProcessor> filterPostProcessors(List<BeanPostProcessor> processors, Object bean) {
		List<DestructionAwareBeanPostProcessor> 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: InitDestroyAnnotationBeanPostProcessor
		if (!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<Object>) () -> {
						((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-methodbean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 {
    
    @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:

	<bean id="wzj"  class="com.wzj.bean.Wzj" factory-method="factoryMethod" destroy-method="destroyMethod"/>

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。

    原文作者:小猪爸爸
    原文地址: https://www.cnblogs.com/father-of-little-pig/p/15962244.html
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