文章目录
- 前言
- 一、Bean 的生成过程：
- 1.1 创建过程：
- 1.1.1 doGetBean 创建bean
- 1.1.2 单例bean 创建getSingleton：
- 1.1.3 单例bean 创建createBean：
- 1.1.4 单例bean 创建doCreateBean：
- 1.1.5 bean 实例化和初始化之后factoryBean 处理
- 1.2 bean 的销毁：
- 1.2.1 spring 容器关闭，通过close 关闭容器：
- 1.2.2 销毁所有的单例 bean 通过 destroyBeans下destroySingletons：
- 二、扩展点
- 2.1 Bean 的实例化之前：
- 2.2 Bean 的实例化之后属性注入之前：
- 2.3 Bean 的属性注入后初始化：
- 2.4 所有Bean实例化和初始化之后：
- 2.5 Bean 的销毁：
- 总结
前言
spring在启动时完成了对需要生成bean 的BeanDefinition 定义，在finishBeanFactoryInitialization(beanFactory) 对bean 进行生成，bean的生成过程中，spring 也提供一些扩展点供开发者使用，如在bean 的实例化之前，实例化之后，初始化之前，初始化之后，所有的bean 都完成实例化和初始化之后 均可以进行扩展。
一、Bean 的生成过程：
1.1 创建过程：
循环遍历所有的BeanDefinition 的map ， 合并父类的bean 定义 得到一个新的 RootBeanDefinition，如果没有父类则直接 生成一个新的RootBeanDefinition ，放入到 Map<String, RootBeanDefinition> mergedBeanDefinitions map 中去，后续使用新的 RootBeanDefinition 进行bean 的创建；
注意BeanDefinition 合并的属性值：
如果子类没有的属性父亲有 则使用父亲的，但是如果是子类也有这个属性则进行覆盖；
1.1.1 doGetBean 创建bean
AbstractBeanFactory.doGetBean 去创建单例，原型 等bean
protected <T> T doGetBean(String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly) throws BeansException {
// bean 名称获取 如果 name 以& 开头 最终也会去掉 & 符号，如果传入的bean name 是别名，则通过别名的
// 的 map 获取bean的真正name并返回
String beanName = this.transformedBeanName(name);
// 从3级缓存获取bean
Object sharedInstance = this.getSingleton(beanName);
Object beanInstance;
if (sharedInstance != null && args == null) {
// 从缓存中获取到了bean 则处理后进行返回
if (this.logger.isTraceEnabled()) {
if (this.isSingletonCurrentlyInCreation(beanName)) {
this.logger.trace("Returning eagerly cached instance of singleton bean '" + beanName + "' that is not fully initialized yet - a consequence of a circular reference");
} else {
this.logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
}
}
beanInstance = this.getObjectForBeanInstance(sharedInstance, name, beanName, (RootBeanDefinition)null);
} else {
// 创建bean
// 检查bean 是否正在创建
if (this.isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// 如果本身的beanfactory 中没有这个 bean 的 BeanDefinition
// 如果有父的beanfactory 则尝试从父的beanfactory 获取bean 的 BeanDefinition
BeanFactory parentBeanFactory = this.getParentBeanFactory();
if (parentBeanFactory != null && !this.containsBeanDefinition(beanName)) {
String nameToLookup = this.originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory)parentBeanFactory).doGetBean(nameToLookup, requiredType, args, typeCheckOnly);
}
if (args != null) {
return parentBeanFactory.getBean(nameToLookup, args);
}
if (requiredType != null) {
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
return parentBeanFactory.getBean(nameToLookup);
}
// 标识这个bean 正在被创建
if (!typeCheckOnly) {
this.markBeanAsCreated(beanName);
}
StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate").tag("beanName", name);
try {
if (requiredType != null) {
beanCreation.tag("beanType", requiredType::toString);
}
// 获取这个bean 合并后的 BeanDefinition
RootBeanDefinition mbd = this.getMergedLocalBeanDefinition(beanName);
// 检查是否是抽象的
this.checkMergedBeanDefinition(mbd, beanName, args);
// 获取 DependsOn 注解
String[] dependsOn = mbd.getDependsOn();
String[] prototypeInstance;
if (dependsOn != null) {
// 先创建DependsOn 注解的bean
prototypeInstance = dependsOn;
int var13 = dependsOn.length;
for(int var14 = 0; var14 < var13; ++var14) {
String dep = prototypeInstance[var14];
// 是否 DependsOn 的bean 是否依赖了 当前正在创建的bean 出现循环依赖则抛异常
if (this.isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
// 添加 map 某个bean 被哪些bean 依赖了
// private final Map<String, Set<String>> dependentBeanMap = new ConcurrentHashMap(64);
this.registerDependentBean(dep, beanName);
try {
// 去获取 被依赖的bean
this.getBean(dep);
} catch (NoSuchBeanDefinitionException var31) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "'" + beanName + "' depends on missing bean '" + dep + "'", var31);
}
}
}
if (mbd.isSingleton()) {
// bean是单例，创建单例bean 并放入到单例池中
sharedInstance = this.getSingleton(beanName, () -> {
try {
return this.createBean(beanName, mbd, args);
} catch (BeansException var5) {
this.destroySingleton(beanName);
throw var5;
}
});
beanInstance = this.getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
} else if (mbd.isPrototype()) {
// 如果是原型bean 则直接创建后进行返回，不放人单例池中
prototypeInstance = null;
Object prototypeInstance;
try {
// 当前bean 正在被创建标识
this.beforePrototypeCreation(beanName);
prototypeInstance = this.createBean(beanName, mbd, args);
} finally {
// 移除标识
this.afterPrototypeCreation(beanName);
}
beanInstance = this.getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
} else {
// 其它bean 的作用域,bean 获取，如果session ，request 等如 @RequestScope
String scopeName = mbd.getScope();
if (!StringUtils.hasLength(scopeName)) {
throw new IllegalStateException("No scope name defined for bean '" + beanName + "'");
}
Scope scope = (Scope)this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, () -> {
this.beforePrototypeCreation(beanName);
Object var4;
try {
var4 = this.createBean(beanName, mbd, args);
} finally {
this.afterPrototypeCreation(beanName);
}
return var4;
});
beanInstance = this.getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
} catch (IllegalStateException var30) {
throw new ScopeNotActiveException(beanName, scopeName, var30);
}
}
} catch (BeansException var32) {
beanCreation.tag("exception", var32.getClass().toString());
beanCreation.tag("message", String.valueOf(var32.getMessage()));
this.cleanupAfterBeanCreationFailure(beanName);
throw var32;
} finally {
beanCreation.end();
}
}
return this.adaptBeanInstance(name, beanInstance, requiredType);
}
1.1.2 单例bean 创建getSingleton：
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
synchronized(this.singletonObjects) {
// 先尝试从单例池周中获取bean
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName, "Singleton bean creation not allowed while singletons of this factory are in destruction (Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (this.logger.isDebugEnabled()) {
this.logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
// 记录单例bean 正在被创建
this.beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = this.suppressedExceptions == null;
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet();
}
try {
// 执行 lambda 表达式 调用 createBean 方法 创建一个对象
singletonObject = singletonFactory.getObject();
newSingleton = true;
} catch (IllegalStateException var16) {
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
throw var16;
}
} catch (BeanCreationException var17) {
BeanCreationException ex = var17;
if (recordSuppressedExceptions) {
Iterator var8 = this.suppressedExceptions.iterator();
while(var8.hasNext()) {
Exception suppressedException = (Exception)var8.next();
ex.addRelatedCause(suppressedException);
}
}
throw ex;
} finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
// 移除改单例bean 正在被创建标识
this.afterSingletonCreation(beanName);
}
// 如果单例bean 创建成功，则添加进入单例池中
if (newSingleton) {
this.addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}
1.1.3 单例bean 创建createBean：
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {
if (this.logger.isTraceEnabled()) {
this.logger.trace("Creating instance of bean '" + beanName + "'");
}
RootBeanDefinition mbdToUse = mbd;
// 加载class 通过默认的appclassLoader 类加载器 加载到这个类 然后设置beanClass
Class<?> resolvedClass = this.resolveBeanClass(mbd, beanName, new Class[0]);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
try {
mbdToUse.prepareMethodOverrides();
} catch (BeanDefinitionValidationException var9) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(), beanName, "Validation of method overrides failed", var9);
}
Object beanInstance;
try {
// bean 实例化前做的工作
beanInstance = this.resolveBeforeInstantiation(beanName, mbdToUse);
if (beanInstance != null) {
// 实例化前 由自定义的业务 返回了改对象 则直接返回改对象
return beanInstance;
}
} catch (Throwable var10) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName, "BeanPostProcessor before instantiation of bean failed", var10);
}
try {
// 创建bean
beanInstance = this.doCreateBean(beanName, mbdToUse, args);
if (this.logger.isTraceEnabled()) {
this.logger.trace("Finished creating instance of bean '" + beanName + "'");
}
// 返回bean
return beanInstance;
} catch (ImplicitlyAppearedSingletonException | BeanCreationException var7) {
throw var7;
} catch (Throwable var8) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", var8);
}
}
1.1.4 单例bean 创建doCreateBean：
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException {
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = (BeanWrapper)this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
// 推断构造方法实例化对象
instanceWrapper = this.createBeanInstance(beanName, mbd, args);
}
Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
synchronized(mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
// 处理合并好了的 BeanDefinition 后置处理器, 对 @Autowired ne @Value @Resource 进行预处理
// 在bean 的实例化之后，做工作，如在BeanDefinition 定义初始化方法
this.applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
} catch (Throwable var17) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Post-processing of merged bean definition failed", var17);
}
mbd.postProcessed = true;
}
}
boolean earlySingletonExposure = mbd.isSingleton() && this.allowCircularReferences && this.isSingletonCurrentlyInCreation(beanName);
// 如果这个bean 可以提前暴露
if (earlySingletonExposure) {
if (this.logger.isTraceEnabled()) {
this.logger.trace("Eagerly caching bean '" + beanName + "' to allow for resolving potential circular references");
}
// 获取提前暴露出来的对象 并将beanfactory 放入到第3级缓存中
this.addSingletonFactory(beanName, () -> {
return this.getEarlyBeanReference(beanName, mbd, bean);
});
}
Object exposedObject = bean;
try {
// 属性注入
this.populateBean(beanName, mbd, instanceWrapper);
// 执行bean 的初始化方法 如实现 InitializingBean 接口调用 afterPropertiesSet 方法
exposedObject = this.initializeBean(beanName, exposedObject, mbd);
} catch (Throwable var18) {
if (var18 instanceof BeanCreationException && beanName.equals(((BeanCreationException)var18).getBeanName())) {
throw (BeanCreationException)var18;
}
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", var18);
}
if (earlySingletonExposure) {
Object earlySingletonReference = this.getSingleton(beanName, false);
if (earlySingletonReference != null) {
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
} else if (!this.allowRawInjectionDespiteWrapping && this.hasDependentBean(beanName)) {
String[] dependentBeans = this.getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet(dependentBeans.length);
String[] var12 = dependentBeans;
int var13 = dependentBeans.length;
for(int var14 = 0; var14 < var13; ++var14) {
String dependentBean = var12[var14];
if (!this.removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
if (!actualDependentBeans.isEmpty()) {
throw new BeanCurrentlyInCreationException(beanName, "Bean with name '" + beanName + "' has been injected into other beans [" + StringUtils.collectionToCommaDelimitedString(actualDependentBeans) + "] in its raw version as part of a circular reference, but has eventually been wrapped. This means that said other beans do not use the final version of the bean. This is often the result of over-eager type matching - consider using 'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}
try {
// bean 销毁逻辑赋值 destroy 方法,容器关闭时，销毁销毁bean 的过程中调用
this.registerDisposableBeanIfNecessary(beanName, bean, mbd);
return exposedObject;
} catch (BeanDefinitionValidationException var16) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", var16);
}
}
1.1.5 bean 实例化和初始化之后factoryBean 处理
通过 getObjectForBeanInstance 方法对factoryBean 进行特殊处理
protected Object getObjectForBeanInstance(Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
if (BeanFactoryUtils.isFactoryDereference(name)) {
// 如果 传递进来的原始的name 以& 开头则认为是一个factory 的bean
if (beanInstance instanceof NullBean) {
return beanInstance;
} else if (!(beanInstance instanceof FactoryBean)) {
// 判断是否真的是factory 的bean 如果发现不是则抛出异常 针对业务本身getbean（“&xxxx”） 情况
throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass());
} else {
if (mbd != null) {
mbd.isFactoryBean = true;
}
return beanInstance;
}
} else if (!(beanInstance instanceof FactoryBean)) {
// 普通的bean 则直接返回
return beanInstance;
} else {
// 针对业务本身getbean（“xxxx”） 情况 虽然不以& 开头但是本身确实是factory的bean
Object object = null;
if (mbd != null) {
mbd.isFactoryBean = true;
} else {
object = this.getCachedObjectForFactoryBean(beanName);
}
// 从缓存中没有获取到bean
if (object == null) {
FactoryBean<?> factory = (FactoryBean)beanInstance;
if (mbd == null && this.containsBeanDefinition(beanName)) {
mbd = this.getMergedLocalBeanDefinition(beanName);
}
boolean synthetic = mbd != null && mbd.isSynthetic();
// 最终调用改bean 对应类的object 方法获取bean
object = this.getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
}
1.2 bean 的销毁：
1.2.1 spring 容器关闭，通过close 关闭容器：
protected void doClose() {
if (this.active.get() && this.closed.compareAndSet(false, true)) {
if (this.logger.isDebugEnabled()) {
this.logger.debug("Closing " + this);
}
if (!NativeDetector.inNativeImage()) {
LiveBeansView.unregisterApplicationContext(this);
}
try {
// 发布容器关闭事件
this.publishEvent((ApplicationEvent)(new ContextClosedEvent(this)));
} catch (Throwable var3) {
this.logger.warn("Exception thrown from ApplicationListener handling ContextClosedEvent", var3);
}
if (this.lifecycleProcessor != null) {
try {
this.lifecycleProcessor.onClose();
} catch (Throwable var2) {
this.logger.warn("Exception thrown from LifecycleProcessor on context close", var2);
}
}
// 销毁所有的单例 bean
this.destroyBeans();
this.closeBeanFactory();
this.onClose();
if (this.earlyApplicationListeners != null) {
this.applicationListeners.clear();
this.applicationListeners.addAll(this.earlyApplicationListeners);
}
this.active.set(false);
}
}
1.2.2 销毁所有的单例 bean 通过 destroyBeans下destroySingletons：
public void destroySingletons() {
if (this.logger.isTraceEnabled()) {
this.logger.trace("Destroying singletons in " + this);
}
synchronized(this.singletonObjects) {
this.singletonsCurrentlyInDestruction = true;
}
String[] disposableBeanNames;
synchronized(this.disposableBeans) {
disposableBeanNames = StringUtils.toStringArray(this.disposableBeans.keySet());
}
for(int i = disposableBeanNames.length - 1; i >= 0; --i) {
// 遍历销毁每个单例bean 然后执行bean 的销毁方法
this.destroySingleton(disposableBeanNames[i]);
}
this.containedBeanMap.clear();
this.dependentBeanMap.clear();
this.dependenciesForBeanMap.clear();
// 单例池清空
this.clearSingletonCache();
}
// 单例池清空
protected void clearSingletonCache() {
synchronized(this.singletonObjects) {
this.singletonObjects.clear();
this.singletonFactories.clear();
this.earlySingletonObjects.clear();
this.registeredSingletons.clear();
this.singletonsCurrentlyInDestruction = false;
}
}
二、扩展点
2.1 Bean 的实例化之前：
在创建bean 的过程中通过 resolveBeforeInstantiation 方法调用
@Nullable
protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
Object bean = null;
if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
if (!mbd.isSynthetic() && this.hasInstantiationAwareBeanPostProcessors()) {
// 这个是否实现了InstantiationAwareBeanPostProcessor
Class<?> targetType = this.determineTargetType(beanName, mbd);
if (targetType != null) {
// 执行实例化之前的方法，遍历所以实现了InstantiationAwareBeanPostProcessor 接口的方法
// 如果得到了返回值 当前类的实例化对象（业务自定义处理），则直接返回，否则遍历完成还没有则返回null
bean = this.applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
if (bean != null) {
// 如果这个bean 进行了实例化，执行初始化之后方法
bean = this.applyBeanPostProcessorsAfterInitialization(bean, beanName);
}
}
}
mbd.beforeInstantiationResolved = bean != null;
}
return bean;
}
InstantiationAwareBeanPostProcessor是Spring的bean后置处理器，在bean实例化过程中的一些重要阶段提供了扩展点。这个接口继承自BeanPostProcessor，添加了实例化过程中的三个方法:
-
postProcessBeforeInstantiation: 此方法在Spring开始实例化bean之前调用，允许我们在Spring的默认实例化逻辑之前提供自定义实例化逻辑。如果这个方法返回一个非空对象，那么Spring会使用这个对象作为bean，而不再进行默认的实例化流程。 -
postProcessAfterInstantiation: 此方法在Spring实例化bean之后调用，允许我们在属性注入（依赖注入）前进行一些自定义操作。如果它返回false，那么Spring将跳过这个bean的属性注入步骤。 -
postProcessPropertyValues: 此方法在Spring进行依赖注入（属性注入）之前调用，允许我们对将要注入的属性做一些自定义处理。例如，我们可以修改这些待注入的属性。
2.2 Bean 的实例化之后属性注入之前：
在populateBean bean 属性注入的方法中通过 bp.postProcessAfterInstantiation 执行：
if (!mbd.isSynthetic() && this.hasInstantiationAwareBeanPostProcessors()) {
// 获取所有实现了InstantiationAwareBeanPostProcessor 接口进行遍历
Iterator var4 = this.getBeanPostProcessorCache().instantiationAware.iterator();
while(var4.hasNext()) {
InstantiationAwareBeanPostProcessor bp = (InstantiationAwareBeanPostProcessor)var4.next();
// 调用bean 的实例化之后的方法
if (!bp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
return;
}
}
}
postProcessAfterInstantiation: 此方法在Spring实例化bean之后调用，允许我们在属性注入（依赖注入）前进行一些自定义操作。如果它返回false，那么Spring将跳过这个bean的属性注入步骤；
2.3 Bean 的属性注入后初始化：
initializeBean 方法处理bean 在属性注入之后的逻辑
protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
if (System.getSecurityManager() != null) {
AccessController.doPrivileged(() -> {
this.invokeAwareMethods(beanName, bean);
return null;
}, this.getAccessControlContext());
} else {
this.invokeAwareMethods(beanName, bean);
}
Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic()) {
// 初始化之前调用 postProcessBeforeInitialization
wrappedBean = this.applyBeanPostProcessorsBeforeInitialization(bean, beanName);
}
try {
// 初始化方法调用 isInitializingBean 接口实现的 afterPropertiesSet 方法
this.invokeInitMethods(beanName, wrappedBean, mbd);
} catch (Throwable var6) {
throw new BeanCreationException(mbd != null ? mbd.getResourceDescription() : null, beanName, "Invocation of init method failed", var6);
}
if (mbd == null || !mbd.isSynthetic()) {
// 初始化之前调用 postProcessAfterInitialization
wrappedBean = this.applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}
2.4 所有Bean实例化和初始化之后：
SmartInitializingSingleton 是 Spring 中的一个接口，主要的作用是对所有单例 bean（包括懒加载的 bean）进行回调。在 Spring 容器全局范围内，所有的单例 bean 都初始化完成后，它才会回调。
它只包含一个方法 afterSingletonsInstantiated，这个方法在所有单例 Bean 都准备就绪之后调用。这个接口通常用于在所有 Bean初始化完成后进行一些额外的校验或者预处理。相比@PostConstruct注解，该接口的回调更晚一些，所有单例bean都初始化完毕之后才调用。
可以使用 SmartInitializingSingleton 来确保 Spring 上下文中所需的所有依赖关系都已经设置完毕，或者你可以仅仅用它来检查应用的一致性。
2.5 Bean 的销毁：
bean 销毁逻辑赋值 destroy 方法,容器关闭时，销毁销毁bean 的过程中调用；
在Spring框架中，我们可以在Bean的销毁阶段插入自己的逻辑，这可以通过以下几种方式实现：
-. DisposableBean：该接口提供一个destroy方法，Spring容器在销毁bean时会调用这个方法，我们可以在这个方法中添加自定义的销毁逻辑。
-. destroy-method：在xml配置文件中使用<bean>标签定义一个bean时，可以为destroy-method属性指定一个方法名。Spring容器在销毁bean时会调用该方法。
@PreDestroy：在bean的一个方法上添加此注解，Spring容器在销毁bean时会调用该方法。这是一种基于注解的方式。
-. DestructionAwareBeanPostProcessor接口：装备了postProcessBeforeDestruction方法，当bean在销毁之前会调用这个方法。
总结
spring 通过BeanDefinition map 进行遍历创建bean ，在bean 的实例化之前和实例化之后可以通过 实现InstantiationAwareBeanPostProcessor接口 来进行扩展，在bean 的初始中可以通过实现 BeanPostProcessor 接口扩展初始化前和后，通过实现InitializingBean 接口扩展初始化业务，在说的bean 都完成实例化和初始化之后 实现 SmartInitializingSingleton 扩展，在容器关闭时实现 DisposableBean 接口扩展bean的销毁；
