1.spring官方指定了三种初始化回调方法
1.1、@PostConstruct、@PreDestory
1.2、实现 InitializingBean DisposableBean 接口
1.3、设置init-method和destory-method
三种方式的优先级从高到低
在spring官方文档里面有明确指出
Multiple lifecycle mechanisms configured for the same bean, with
different initialization methods, are called as follows:
Methods annotated with @PostConstruct
afterPropertiesSet() as defined by the InitializingBean callback interface
A custom configured init() method
Destroy methods are called in the same order:
Methods annotated with @PreDestroy
destroy() as defined by the DisposableBean callback interface
A custom configured destroy() method
2.三种配置初始化bean的方式，调用时机不同，对于注解，是在spring生命周期中第七个bean后置处理器调用的时候，由CommonAnnotationBeanPostProcessor的postProcessBeforeInitialization()方法来处理
而剩下的两种是在 第七个bean后置处理器执行之后的下一行代码中执行的
3.@PostConstruct和@PreDestroy源码
3.1、在spring生命周期中，第三个后置处理器方法：org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#applyMergedBeanDefinitionPostProcessors；在CommonAnnotationBeanPostProcessor的postProcessMergedBeanDefinition方法，调用了buildLifecycleMetadata()方法，有一段比较重要的代码：
do {
final List<LifecycleElement> currInitMethods = new ArrayList<>();
final List<LifecycleElement> currDestroyMethods = new ArrayList<>();
ReflectionUtils.doWithLocalMethods(targetClass, method -> {
//如果目标类中的方法，添加了@PostConstruct注解，就添加到currInitMethods
if (this.initAnnotationType != null && method.isAnnotationPresent(this.initAnnotationType)) {
LifecycleElement element = new LifecycleElement(method);
currInitMethods.add(element);
if (logger.isTraceEnabled()) {
logger.trace("Found init method on class [" + clazz.getName() + "]: " + method);
}
}
//如果目标类的方法，添加了@PreDestory注解，就添加到currDestroyMethod中
if (this.destroyAnnotationType != null && method.isAnnotationPresent(this.destroyAnnotationType)) {
currDestroyMethods.add(new LifecycleElement(method));
if (logger.isTraceEnabled()) {
logger.trace("Found destroy method on class [" + clazz.getName() + "]: " + method);
}
}
});
initMethods.addAll(0, currInitMethods);
destroyMethods.addAll(currDestroyMethods);
targetClass = targetClass.getSuperclass();
}
while (targetClass != null && targetClass != Object.class);
这里，最终会根据initMethods和destroyMethods来初始化一个LifecycleMetadata对象，然后再把new出来的元对象，存到了一个map中 lifecycleMetadataCache
private LifecycleMetadata findLifecycleMetadata(Class<?> clazz) {
if (this.lifecycleMetadataCache == null) {
// Happens after deserialization, during destruction...
return buildLifecycleMetadata(clazz);
}
// Quick check on the concurrent map first, with minimal locking.
LifecycleMetadata metadata = this.lifecycleMetadataCache.get(clazz);
if (metadata == null) {
synchronized (this.lifecycleMetadataCache) {
metadata = this.lifecycleMetadataCache.get(clazz);
if (metadata == null) {
metadata = buildLifecycleMetadata(clazz);
this.lifecycleMetadataCache.put(clazz, metadata);
}
return metadata;
}
}
return metadata;
}
3.2、在spring生命周期的第七个后置处理器方法中，org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor#postProcessBeforeInitialization
会尝试先从lifecycleMetadataCache中根据class，获取到元对象，然后调用元对象的 invokeInitMethods()方法
public void invokeInitMethods(Object target, String beanName) throws Throwable {
Collection<LifecycleElement> checkedInitMethods = this.checkedInitMethods;
Collection<LifecycleElement> initMethodsToIterate =
(checkedInitMethods != null ? checkedInitMethods : this.initMethods);
if (!initMethodsToIterate.isEmpty()) {
boolean debug = logger.isDebugEnabled();
for (LifecycleElement element : initMethodsToIterate) {
if (debug) {
logger.debug("Invoking init method on bean '" + beanName + "': " + element.getMethod());
}
element.invoke(target);
}
}
}
这里的element.invoke()就是调用bean中添加了@PostConstruct注解的方法(method.invoke())
@PreDestory注解的原理是一样的，只是调用时机是在调用annotationConfigApplicationContext.close()方法的时候，会调用
org.springframework.beans.factory.annotation.InitDestroyAnnotationBeanPostProcessor#postProcessBeforeDestruction
4.剩下的两种初始化方法，执行处理逻辑，是在一块代码中
//在调用第七个后置处理器的下一行，是这个代码
try {
invokeInitMethods(beanName, wrappedBean, mbd);
}
protected void invokeInitMethods(String beanName, final Object bean, @Nullable RootBeanDefinition mbd)
throws Throwable { //判断当前bean是否是 InitializingBean接口的实现类，如果是，判断是否实现了 afterPropertiesSet() 方法
boolean isInitializingBean = (bean instanceof InitializingBean);
if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
if (logger.isTraceEnabled()) {
logger.trace("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
}
if (System.getSecurityManager() != null) {
try {
AccessController.doPrivileged((PrivilegedExceptionAction<Object>) () -> {
((InitializingBean) bean).afterPropertiesSet();
return null;
}, getAccessControlContext());
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
((InitializingBean) bean).afterPropertiesSet();
}
} //如果是在@Bean中配置了init-method和destroy-method，那执行的是这里的方法
if (mbd != null && bean.getClass() != NullBean.class) {
String initMethodName = mbd.getInitMethodName();
if (StringUtils.hasLength(initMethodName) &&
!(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
!mbd.isExternallyManagedInitMethod(initMethodName)) {
invokeCustomInitMethod(beanName, bean, mbd);
}
}
}
对于在@Bean注解指定init-method方式的，是在将class扫描到beanDefinition的时候，会对@Bean这种方式的bean进行处理
loadBeanDefinitionsForBeanMethod(beanMethod);在这里会设置bean的initMethod
spring扫描bean，将class扫描到beanDefinitionMap中有三种方式；
@ComponentScan注解
@Import
importBeanDefinitionRegistrar
importSelector
@Bean
5.对于销毁方法，还没仔细研究，后面了解之后，再做补充：
public void destroy() {
//这里是处理@PreDestroy注解的销毁方法
if (!CollectionUtils.isEmpty(this.beanPostProcessors)) {
for (DestructionAwareBeanPostProcessor processor : this.beanPostProcessors) {
processor.postProcessBeforeDestruction(this.bean, this.beanName);
}
}
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 {
//这里是处理 DisposableBean 接口实现类中destroy()销毁方法的逻辑
((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);
}
}
}
//这是处理@Bean中 destory-method这种方式的销毁方法
if (this.destroyMethod != null) {
invokeCustomDestroyMethod(this.destroyMethod);
}
else if (this.destroyMethodName != null) {
Method methodToInvoke = determineDestroyMethod(this.destroyMethodName);
if (methodToInvoke != null) {
invokeCustomDestroyMethod(ClassUtils.getInterfaceMethodIfPossible(methodToInvoke));
}
}
}