spring初始过程

spring初始化 可以理解为IOC容器初始化的过程。
IOC: spring核心理念，即控制反转，将设计好的Bean教由框架管理，在需要的时候进行实例化
IOC容器的初始化：

一.代码形式：
实例化org.springframework.context.support.ClassPathXmlApplicationContext注入对应xml文件ClassPathXmlApplicationContext ctx=new ClassPathXmlApplicationContext("XXX.xml","XXX.xml");

public class ClassPathXmlApplicationContext extends AbstractXmlApplicationContext {

@Nullable
private Resource[] configResources;

public ClassPathXmlApplicationContext() {
}

public ClassPathXmlApplicationContext(ApplicationContext parent) {
super(parent);
}

public ClassPathXmlApplicationContext(String configLocation) throws BeansException {
this(new String[] {configLocation}, true, null);
}

public ClassPathXmlApplicationContext(String... configLocations) throws BeansException {
this(configLocations, true, null);
}

public ClassPathXmlApplicationContext(String[] configLocations, @Nullable ApplicationContext parent)
throws BeansException {

this(configLocations, true, parent);
}

public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh) throws BeansException {
this(configLocations, refresh, null);
}

public ClassPathXmlApplicationContext(
String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
throws BeansException {

super(parent);
setConfigLocations(configLocations);
if (refresh) {
refresh();
}
}

public ClassPathXmlApplicationContext(String path, Class<?> clazz) throws BeansException {
this(new String[] {path}, clazz);
}

public ClassPathXmlApplicationContext(String[] paths, Class<?> clazz) throws BeansException {
this(paths, clazz, null);
}

public ClassPathXmlApplicationContext(String[] paths, Class<?> clazz, @Nullable ApplicationContext parent)
throws BeansException {

super(parent);
Assert.notNull(paths, "Path array must not be null");
Assert.notNull(clazz, "Class argument must not be null");
this.configResources = new Resource[paths.length];
for (int i = 0; i < paths.length; i++) {
this.configResources[i] = new ClassPathResource(paths[i], clazz);
}
refresh();
}

@Override
@Nullable
protected Resource[] getConfigResources() {
return this.configResources;
}

}

通过ClassPathXmlApplicationContext构造器将配置文件写入实例，将参数统一后，交由该构造方法统一处理public ClassPathXmlApplicationContext(
String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
throws BeansException {

super(parent);
setConfigLocations(configLocations);
if (refresh) {
refresh();
}
}首先，调用父类容器的构造方法(super(parent)方法)为容器设置好Bean资源加载器。然后，再调用父类AbstractRefreshableConfigApplicationContext的setConfigLocations(configLocations)方法设置Bean定义资源文件的定位路径。最后调用AbstractApplicationContext父类的refresh()方法，该方法基本就是spring初始化核心方法

public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.调用容器准备刷新的方法，获取容器的当时时间，同时给容器设置同步标识 
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.告诉子类启动refreshBeanFactory()方法，Bean定义资源文件的载入从告诉子类启动refreshBeanFactory()方法，Bean定义资源文件的载入从
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.为BeanFactory配置容器特性，例如类加载器、事件处理器等
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.为容器的某些子类指定特殊的BeanPost事件处理器  
postProcessBeanFactory(beanFactory);

// Invoke factory processors registered as beans in the context.调用所有注册的BeanFactoryPostProcessor的Bean  
invokeBeanFactoryPostProcessors(beanFactory);

// Register bean processors that intercept bean creation.为BeanFactory注册BeanPost事件处理器.BeanPostProcessor是Bean后置处理器，用于监听容器触发的事件    
registerBeanPostProcessors(beanFactory);

// Initialize message source for this context.初始化信息源，和国际化相关.  
initMessageSource();

// Initialize event multicaster for this context.初始化容器事件传播器.  
initApplicationEventMulticaster();

// Initialize other special beans in specific context subclasses.调用子类的某些特殊Bean初始化方法
onRefresh();

// Check for listener beans and register them.为事件传播器注册事件监听器.
registerListeners();

// Instantiate all remaining (non-lazy-init) singletons.为事件传播器注册事件监听器.
finishBeanFactoryInitialization(beanFactory);

// Last step: publish corresponding event.为事件传播器注册事件监听器.
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}

// Destroy already created singletons to avoid dangling resources.销毁以创建的单态Bean 
destroyBeans();

// Reset 'active' flag.取消refresh操作，重置容器的同步标识.
cancelRefresh(ex);

// Propagate exception to caller.
throw ex;
}
finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}
}

refresh()方法主要为IoC容器Bean的生命周期管理提供条件，Spring IoC容器载入Bean定义资源文件从其子类容器的refreshBeanFactory()方法启动，所以整个refresh()中
“ConfigurableListableBeanFactory beanFactory =obtainFreshBeanFactory();”这句以后代码的都是注册容器的信息源和生命周期事件，载入过程就是从这句代码启动。
 refresh()方法的作用是：在创建IoC容器前，如果已经有容器存在，则需要把已有的容器销毁和关闭，以保证在refresh之后使用的是新建立起来的IoC容器。refresh的作用类似于对IoC容器的重启，在新建立好的容器中对容器进行初始化，对Bean定义资源进行载入

bstractApplicationContext的obtainFreshBeanFactory()方法调用子类容器的refreshBeanFactory()方法，启动容器载入Bean定义资源文件的过程，代码如下：
protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
refreshBeanFactory();
ConfigurableListableBeanFactory beanFactory = getBeanFactory();
if (logger.isDebugEnabled()) {
logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
}
return beanFactory;
}在该类中只抽象了refreshBeanFactory()方法 具体实现由子类AbstractRefreshableApplicationContext实现方法，代码如下：protected final void refreshBeanFactory() throws BeansException {
if (hasBeanFactory()) {//如果已经有容器，销毁容器中的bean，关闭容器
destroyBeans();
closeBeanFactory();
}
try {
DefaultListableBeanFactory beanFactory = createBeanFactory();
beanFactory.setSerializationId(getId());
customizeBeanFactory(beanFactory);
loadBeanDefinitions(beanFactory);
synchronized (this.beanFactoryMonitor) {
this.beanFactory = beanFactory;
}
}
catch (IOException ex) {
throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
}
}在这个抽象方法中，先判断BeanFactory是否存在，如果存在则先销毁beans并关闭beanFactory，接着创建DefaultListableBeanFactory，并调用loadBeanDefinitions(beanFactory)装载bean定义同样在该方法中loadBeanDefinitions()也只是抽象方法具体实现是由子类AbstractXmlApplicationContext实现protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
// Create a new XmlBeanDefinitionReader for the given BeanFactory.
//创建XmlBeanDefinitionReader，即创建Bean读取器，并通过回调设置到容器中去，容器使用该读取器读取Bean定义资源

XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

// Configure the bean definition reader with this context's
// resource loading environment.
//为Bean读取器设置Spring资源加载器，AbstractXmlApplicationContext的
//祖先父类AbstractApplicationContext继承DefaultResourceLoader，因此，容器本身也是一个资源加载器
beanDefinitionReader.setEnvironment(this.getEnvironment());
beanDefinitionReader.setResourceLoader(this);
//为Bean读取器设置SAX xml解析器
beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

}

// Allow a subclass to provide custom initialization of the reader,
// then proceed with actually loading the bean definitions.                //当Bean读取器读取Bean定义的Xml资源文件时，启用Xml的校验机制
initBeanDefinitionReader(beanDefinitionReader);                //Bean读取器真正实现加载的方法  
loadBeanDefinitions(beanDefinitionReader);
}最后一步调用真正的解析方法

protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
//获取Bean定义资源的定位
                Resource[] configResources = getConfigResources();
if (configResources != null) {
                        //Xml Bean读取器调用其父类AbstractBeanDefinitionReader读取定位  
                        //的Bean定义资源 
reader.loadBeanDefinitions(configResources);
}
                //如果子类中获取的Bean定义资源定位为空，则获取FileSystemXmlApplicationContext构造方法中setConfigLocations方法设置的资源  
String[] configLocations = getConfigLocations();
if (configLocations != null) {
                        //Xml Bean读取器调用其父类AbstractBeanDefinitionReader读取定位  
                        //的Bean定义资源  
reader.loadBeanDefinitions(configLocations);
}
}

Xml Bean读取器(XmlBeanDefinitionReader)
调用其父类AbstractBeanDefinitionReader的 reader.loadBeanDefinitions方法读取Bean定义资源。在其抽象父类AbstractBeanDefinitionReader中定义了载入过程(具体解析文件过程略，有时间在分析)
自此spring初始化完成。