Spring 标签加载bean的原理是怎样的
Spring管理bean：
1.从web.xml里面配置的ContextLoaderListener开始。
2.ContextLoaderListener继承ContextLoader
3.执行ContextLoaderListener的contextInitialized方法，获得servletContext.
4.把servletContext传入ContextLoader的initWebApplicationContext(ServletContext servletContext)方法并执行。
5.通过ContextLoader.createWebApplicationContext(servletContext)获得this.context（WebApplicationContext）实例。
默认情况下这个context就是XmlWebApplicationContext。
这个XmlWebApplicationContext继承一个AbstractApplicationContext类。
再执行ContextLoader.configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac, ServletContext sc)方法，
执行AbstractApplicationContext的.refresh();方法
[java] view plain copy
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
}
catch (BeansException ex) {
logger.warn("Exception encountered during context initialization - cancelling refresh attempt", ex);
// Destroy already created singletons to avoid dangling resources.
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}
}
- 这个方法完成了WebApplicationContext里面的beanfactory的初始化和bean载入，beanfactorypostprocessor的调用，beanpostprocessor的注册，ApplicationEvent的监听和注册,non-lazy-init的bean的初始化。
换言之，已经把该准备的都准备好了，只需要有请求来获取bean，就根据情况或返回已经初始化的bean或进行bean的Instantiation 和 Initialization。
