spring系列3-后置处理器

Spring对bean的赋值, 注入其它组件, 生命周期注解功能,@Async等等功能，在低层都是通过底层对BeanPostProcessor也就是后置处理器的支持实现的。下面看几个常见的后置处理器，具体的spring源码将在以后分析。

1.ApplicationContextAware

类实现了ApplicationContextAware接口，可以取得上下文ApplicationContex，用于自己业务操作。

public class A implements ApplicationContextAware{
private ApplicationContext applicationContext;
public A(){
}
@PostConstruct
public void init(){
System.out.println("....@PostConstruct........");
}
@PreDestroy
public void destory(){
System.out.println("....@PreDestroy......");
}
public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
//将applicationContext传进来,可以拿到
System.out.println(".....applicationContext........");
this.applicationContext = applicationContext;
}
}

ApplicationContextAware对应的后置处理器是ApplicationContextAwareProcessor。我们稍微看下源码：

//在类的属性设置完成之后，调用
public Object postProcessBeforeInitialization(final Object bean, String beanName) throws BeansException {
AccessControlContext acc = null;
//权限控制
if (System.getSecurityManager() != null &&
(bean instanceof EnvironmentAware || bean instanceof EmbeddedValueResolverAware ||
bean instanceof ResourceLoaderAware || bean instanceof ApplicationEventPublisherAware ||
bean instanceof MessageSourceAware || bean instanceof ApplicationContextAware)) {
acc = this.applicationContext.getBeanFactory().getAccessControlContext();
}
if (acc != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
//核心
invokeAwareInterfaces(bean);
return null;
}, acc);
}
else {
//核心
invokeAwareInterfaces(bean);
}
return bean;
}

private void invokeAwareInterfaces(Object bean) {
if (bean instanceof Aware) {
if (bean instanceof EnvironmentAware) {
((EnvironmentAware) bean).setEnvironment(this.applicationContext.getEnvironment());
}
if (bean instanceof EmbeddedValueResolverAware) {
((EmbeddedValueResolverAware) bean).setEmbeddedValueResolver(this.embeddedValueResolver);
}
if (bean instanceof ResourceLoaderAware) {
((ResourceLoaderAware) bean).setResourceLoader(this.applicationContext);
}
if (bean instanceof ApplicationEventPublisherAware) {
((ApplicationEventPublisherAware) bean).setApplicationEventPublisher(this.applicationContext);
}
if (bean instanceof MessageSourceAware) {
((MessageSourceAware) bean).setMessageSource(this.applicationContext);
}
if (bean instanceof ApplicationContextAware) {
//调用实现类的setApplicationContext方法
//applicationContext是在构造该后置处理器时候传入的
((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);
}
}
}

2.BeanValidationPostProcessor

BeanValidationPostProcessor用来做数据校验的，比如对加了@Validated类按照JSR提供的校验注解（比如@Null）进行校验。源码重点如下：

	public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
if (!this.afterInitialization) {
//类初始化前进行校验
doValidate(bean);
}
return bean;
}

	public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
if (this.afterInitialization) {
//类初始化后进行校验
doValidate(bean);
}
return bean;
}

3.InitDestroyAnnotationBeanPostProcessor

后置处理器是对@PostConstruct, @PreDestroy进行处理。关键源码如下：

	public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
//找到初始化方法比如，@PostConstruct注解的方法
LifecycleMetadata metadata = findLifecycleMetadata(bean.getClass());
try {
//通过反射注入
metadata.invokeInitMethods(bean, beanName);
}
catch (InvocationTargetException ex) {
throw new BeanCreationException(beanName, "Invocation of init method failed", ex.getTargetException());
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Failed to invoke init method", ex);
}
return bean;
}

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()) {
for (LifecycleElement element : initMethodsToIterate) {
if (logger.isTraceEnabled()) {
logger.trace("Invoking init method on bean '" + beanName + "': " + element.getMethod());
}
//反射调用，target为目标对象bean
element.invoke(target);
}
}
}

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