Spring容器中的init和destroy方法
Spring Bean的生命周期大致如下:
Spring提供了定义好的Spring Bean生命周期内的init和destroy方法，实现InitializingBean和DisposableBean方法即可。同时，Spring也支持自定义的初始化和销毁方法。
基于xml配置的方式，在配置bean的时候，我们可以指定init-method和destroy-method方法。在完成org.springframework.beans.factory.InitializingBean#afterPropertiesSet方法调用后，会调用配置的init-method方法。同理,在容器关闭的时候，会调用配置的destroy-method方法。
基于注解的方法，Spring支持JSR-250规范(为Java SE和Java EE平台中的通用语义概念开发注释,核心还是out-of-the-box开箱即用,主要是指javax.annotation包下的几个注解)。其中包括javax.annotation.PostConstruct、javax.annotation.PreDestroy和javax.annotation.Resource。在Spring生命周期中，PostConstruct和PreDestroy可以标记Spring bean的初始化/销毁动作。其实现类是org.springframework.context.annotation.CommonAnnotationBeanPostProcessor。
三种不同的指定定初始化/销毁动作的方式：
- 实现
InitializingBean/DisposableBean接口，在afterPropertiesSet/destroy方法中定义初始化/销毁动作。 - 通过xml中
<bean />定义的init-method和destroy-method方法，指定初始化/销毁动作。 - 通过
@PostConstruct/@PreDestroy注解指定初始化/销毁动作。
根据Spring Bean生命周期的示意图我们可以知道，如果同时配置了三种方式，那么:
@PostConstruct>InitializingBean#afterPropertiesSet>init-method>@PreDestroy>DisposableBean#destroy>destroy-method
InitializingBean/DisposableBean接口 和 init-method/destroy-method方法
基于接口形式的定制的初始化/销毁动作。由BeanFactory调用，可以参考官方源码上的javadoc。
public interface InitializingBean {
/**
* Invoked by a BeanFactory after it has set all bean properties supplied
* (and satisfied BeanFactoryAware and ApplicationContextAware).
* <p>This method allows the bean instance to perform initialization only
* possible when all bean properties have been set and to throw an
* exception in the event of misconfiguration.
* @throws Exception in the event of misconfiguration (such
* as failure to set an essential property) or if initialization fails.
*/
void afterPropertiesSet() throws Exception;
}
Spring默认使用的BeanFactoryDefaultListableBeanFactory,其继承自AbstractAutowireCapableBeanFactory，AbstractAutowireCapableBeanFactory主要负责Bean的实例化,属性注入，调用初始化方法(包括BeanNameAware/BeanClassLoaderAware/BeanFactoryAware/BeanPostProcessor/InitializingBean等方法)等。
AbstractAutowireCapableBeanFactory#invokeInitMethods
protected void invokeInitMethods(String beanName, final Object bean, RootBeanDefinition mbd)
throws Throwable {
//判断是否实现了 InitializingBean 接口
boolean isInitializingBean = (bean instanceof InitializingBean);
if (isInitializingBean && (mbd == null || !mbd.isExternallyManagedInitMethod("afterPropertiesSet"))) {
if (logger.isDebugEnabled()) {
logger.debug("Invoking afterPropertiesSet() on bean with name '" + beanName + "'");
}
if (System.getSecurityManager() != null) { //如果启用了启用Java安全管理器
try {
AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
@Override
public Object run() throws Exception {
//调用afterPropertiesSet方法
((InitializingBean) bean).afterPropertiesSet();
return null;
}
}, getAccessControlContext());
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
//调用afterPropertiesSet方法
((InitializingBean) bean).afterPropertiesSet();
}
}
if (mbd != null) {
String initMethodName = mbd.getInitMethodName();
//是否指定自定义的`init-method`方法
if (initMethodName != null && !(isInitializingBean && "afterPropertiesSet".equals(initMethodName)) &&
!mbd.isExternallyManagedInitMethod(initMethodName)) {
//如果指定了`init-method`方法，反射调用指定的`init-method`方法
invokeCustomInitMethod(beanName, bean, mbd);
}
}
}
使用支持@PostConstruct和@PreDestroy注解的BeanPostProcessor
基于xml配置的话，如果配置了<context:annotation-config />，会隐式地向Spring容器注册CommonAnnotationBeanPostProcessor,AutowiredAnnotationBeanPostProcessor,RequiredAnnotationBeanPostProcessor等注解。下面是官方xml的schema描述:
<xsd:element name="annotation-config">
<xsd:annotation>
<xsd:documentation><![CDATA[
Activates various annotations to be detected in bean classes: Spring's @Required and
@Autowired, as well as JSR 250's @PostConstruct, @PreDestroy and @Resource (if available),
JAX-WS's @WebServiceRef (if available), EJB3's @EJB (if available), and JPA's
@PersistenceContext and @PersistenceUnit (if available). Alternatively, you may
choose to activate the individual BeanPostProcessors for those annotations.
Note: This tag does not activate processing of Spring's @Transactional or EJB3's
@TransactionAttribute annotation. Consider the use of the <tx:annotation-driven>
tag for that purpose.
]]></xsd:documentation>
</xsd:annotation>
</xsd:element>
值得一提的是，如果配置了<context:component-scan />的话，就可以不用配置<context:annotation-config />了，因为component-scan内部包含了annotation-config的功能。
<xsd:element name="component-scan">
<xsd:annotation>
<xsd:documentation><![CDATA[
Scans the classpath for annotated components that will be auto-registered as
Spring beans. By default, the Spring-provided @Component, @Repository,
@Service, and @Controller stereotypes will be detected.
Note: This tag implies the effects of the 'annotation-config' tag, activating @Required,
@Autowired, @PostConstruct, @PreDestroy, @Resource, @PersistenceContext and @PersistenceUnit
annotations in the component classes, which is usually desired for autodetected components
(without external configuration). Turn off the 'annotation-config' attribute to deactivate
this default behavior, for example in order to use custom BeanPostProcessor definitions
for handling those annotations.
Note: You may use placeholders in package paths, but only resolved against system
properties (analogous to resource paths). A component scan results in new bean definition
being registered; Spring's PropertyPlaceholderConfigurer will apply to those bean
definitions just like to regular bean definitions, but it won't apply to the component
scan settings themselves.
]]></xsd:documentation>
基于注解的形式，可以参考AnnotationConfigUtils#registerAnnotationConfigProcessors(BeanDefinitionRegistry, Object)方法，CommonAnnotationBeanPostProcessor通过这个此方法被注册到spring容器中
容易踩坑的地方
@PostConstruct和@PreDestroy注解属于Java EE，而从JDK 9开始，Java EE被@Deprecated，并在JDK 11中被正式移除。使用这些注解，必须另外进行引入相关依赖。
<dependency>
<groupId>javax.annotation</groupId>
<artifactId>javax.annotation-api</artifactId>
<version>1.3.2</version>
</dependency>