在上一篇文章中，我们主要讲了BeanFactory和ApplicationContext之间的关系，并且看了一部分的源码
本文将顺着上一篇文章的内容，浅谈 IoC容器初始化的过程。
IoC初始化大概可以分为三个部分：
IoC的容器初始化大体可以分为三个部分：定位、加载和注册
定位：读取资源配置，包括import 、url、classpath
加载：解析配置文件，组装beanDefinition
注册：把beanDefinition对象放入IoC容器中
整个流程的流程图如下：
https://www.processon.com/view/link/5b66825de4b0555b39d4205d
我们从ApplicationContext开始，以其实现类ClassPathXmlApplicationContext为例
其一构造函数为
public ClassPathXmlApplicationContext(
String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
throws BeansException {
super(parent);
setConfigLocations(configLocations);
if (refresh) {
refresh();
}
}
其中refresh()方法即为整个IoC容器初始化时最核心的方法，该方法顾名思义是刷新容器，并且触发容器的初始化过程。具体的方法在AbstractApplicationContext中定义：
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// 设置Spring容器的启动时间，撤销关闭状态，开启活跃状态。
//初始化属性源信息(Property)
// 验证环境信息里一些必须存在的属性
prepareRefresh();
// 单例创建 beanFactory 加载所有的xml配置的bean
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// 从Spring容器获取BeanFactory(Spring Bean容器)并进行相关的设置为后续的使用做准备
prepareBeanFactory(beanFactory);
try {
// 子类的后处理，不同容器不一样 大致有添加后处理器、忽略依赖接口
postProcessBeanFactory(beanFactory);
// 用默认的BeanFactoryPostProcessor加载其余的BeanFactoryPostProcessor
//再处理完成所有的BeanFactoryPostProcessor
invokeBeanFactoryPostProcessors(beanFactory);
// 将beanPostProcessor关联到容器
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
initMessageSource();
// 初始化事件发送器
initApplicationEventMulticaster();
// 每个子类可以重写的部分 比如说SpringBoot的容器在此创建了一个Tomcat的类
onRefresh();
// 关联所有的listener
registerListeners();
// 实例化BeanFactory中已经被注册但是未实例化的所有实例(懒加载的不需要实例化)。
//比如invokeBeanFactoryPostProcessors方法中根据各种注解解析出来的类，在这个时候都会被初始化。
//实例化的过程各种BeanPostProcessor开始起作用。
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
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.
destroyBeans();
// Reset 'active' flag.
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();
}
}
}
整个过程是由synchronized 锁住的，我们先来分析第二行，注释写的很清楚，这一行的作用就是告诉底层刷新beanFactory的。
// Tell the subclass to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
其中
protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
//销毁上一个BeanFactory，并初始化一个新的BeanFactory
refreshBeanFactory();
ConfigurableListableBeanFactory beanFactory = getBeanFactory();
if (logger.isDebugEnabled()) {
logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
}
return beanFactory;
}
refreshBeanFactory方法的定义在AbstractRefreshableApplicationContext中:
@Override
protected final void refreshBeanFactory() throws BeansException {
//如果存在beanFactory，则销毁
if (hasBeanFactory()) {
destroyBeans();
closeBeanFactory();
}
try {
//创建beanFactory并设置属性
DefaultListableBeanFactory beanFactory = createBeanFactory();
beanFactory.setSerializationId(getId());
customizeBeanFactory(beanFactory);
//读取beanDefinitions
loadBeanDefinitions(beanFactory);
synchronized (this.beanFactoryMonitor) {
this.beanFactory = beanFactory;
}
}
catch (IOException ex) {
throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
}
}
以xml配置为例，此处的loadBeanDefinitions 一路调用到了AbstractXmlApplicationContent中
/**
* 通过XmlBeanDefinitionReader 读取bean definition.
* @see org.springframework.beans.factory.xml.XmlBeanDefinitionReader
* @see #initBeanDefinitionReader
* @see #loadBeanDefinitions
*/
@Override
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
// 为给定的BeanFactory创建一个新的XmlBeanDefinitionReader.
XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
// 使用当前容器的资源读取环境(resource loading environment)
// 配置bean definition reader
beanDefinitionReader.setEnvironment(this.getEnvironment());
//为Bean读取器设置资源加载器，
// 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
// 所以，这个资源加载器由始至终都是容器自身
beanDefinitionReader.setResourceLoader(this);
beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));
// 初始化beandefinitionReader.
initBeanDefinitionReader(beanDefinitionReader);
//读取bean definitions
loadBeanDefinitions(beanDefinitionReader);
}
loadBeanDefinitions最终调用的是XmlBeanDefinitionReader中的doLoadBeanDefinition 方法。
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {
try {
//利用DocumentLoader加载对应的文档
Document doc = doLoadDocument(inputSource, resource);
// 注册给定DOM文档中包含的bean定义
return registerBeanDefinitions(doc, resource);
}
catch (BeanDefinitionStoreException ex) {
throw ex;
}
catch (SAXParseException ex) {
throw new XmlBeanDefinitionStoreException(resource.getDescription(),
"Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
}
catch (SAXException ex) {
throw new XmlBeanDefinitionStoreException(resource.getDescription(),
"XML document from " + resource + " is invalid", ex);
}
catch (ParserConfigurationException ex) {
throw new BeanDefinitionStoreException(resource.getDescription(),
"Parser configuration exception parsing XML from " + resource, ex);
}
catch (IOException ex) {
throw new BeanDefinitionStoreException(resource.getDescription(),
"IOException parsing XML document from " + resource, ex);
}
catch (Throwable ex) {
throw new BeanDefinitionStoreException(resource.getDescription(),
"Unexpected exception parsing XML document from " + resource, ex);
}
}
其registerBeanDefinition方法最终调用的是DefaultBeanDefintionDocumentReader中的doRegisterBeanDefinitions
/**
* Register each bean definition within the given root {@code elements will cause recursion in this method. In
// order to propagate and preserve default-* attributes correctly,
// keep track of the current (parent) delegate, which may be null. Create
// the new (child) delegate with a reference to the parent for fallback purposes,
// then ultimately reset this.delegate back to its original (parent) reference.
// this behavior emulates a stack of delegates without actually necessitating one.
BeanDefinitionParserDelegate parent = this.delegate;
this.delegate = createDelegate(getReaderContext(), root, parent);
if (this.delegate.isDefaultNamespace(root)) {
String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
if (StringUtils.hasText(profileSpec)) {
String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
if (logger.isInfoEnabled()) {
logger.info("Skipped XML bean definition file due to specified profiles [" + profileSpec +
"] not matching: " + getReaderContext().getResource());
}
return;
}
}
}
/*
在处理beanDefinition之前，允许通过自定义标签对xml进行拓展
*/
preProcessXml(root);
/**
处理beanDefinition
*/
parseBeanDefinitions(root, this.delegate);
/**
处理BeanDefinition之后，允许通过自定义标签对xml进行拓展
*/
postProcessXml(root);
this.delegate = parent;
}
其中，parseBeanDefinitions方法为真正处理beanDefinition的方法
protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
if (delegate.isDefaultNamespace(root)) {
NodeList nl = root.getChildNodes();
for (int i = 0; i < nl.getLength(); i++) {
Node node = nl.item(i);
if (node instanceof Element) {
Element ele = (Element) node;
if (delegate.isDefaultNamespace(ele)) {
//解析默认元素
parseDefaultElement(ele, delegate);
}
else {
//解析自定义元素
delegate.parseCustomElement(ele);
}
}
}
}
else {
delegate.parseCustomElement(root);
}
}
其中，parseDefaultElement方法:
private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
importBeanDefinitionResource(ele);
}
else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
processAliasRegistration(ele);
}
else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
processBeanDefinition(ele, delegate);
}
else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
// recurse
doRegisterBeanDefinitions(ele);
}
}
我们这里解析的是bean，因此进入processBeanDefinition方法
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
if (bdHolder != null) {
bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
try {
// Register the final decorated instance.
BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
}
catch (BeanDefinitionStoreException ex) {
getReaderContext().error("Failed to register bean definition with name '" +
bdHolder.getBeanName() + "'", ele, ex);
}
// Send registration event.
getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
}
}
之后进入的是BeanDefinitionUtils的 registerBeanDefinition方法
public static void registerBeanDefinition(
BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
throws BeanDefinitionStoreException {
// Register bean definition under primary name.
String beanName = definitionHolder.getBeanName();
registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
// Register aliases for bean name, if any.
String[] aliases = definitionHolder.getAliases();
if (aliases != null) {
for (String alias : aliases) {
registry.registerAlias(beanName, alias);
}
}
}
最后的最后，我们又到了DefaultListableBeanFactory之中，相信看过我上一篇博客的人应该对这个已经很熟悉了
@Override
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
throws BeanDefinitionStoreException {
Assert.hasText(beanName, "Bean name must not be empty");
Assert.notNull(beanDefinition, "BeanDefinition must not be null");
if (beanDefinition instanceof AbstractBeanDefinition) {
try {
((AbstractBeanDefinition) beanDefinition).validate();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Validation of bean definition failed", ex);
}
}
BeanDefinition oldBeanDefinition;
oldBeanDefinition = this.beanDefinitionMap.get(beanName);
if (oldBeanDefinition != null) {
if (!isAllowBeanDefinitionOverriding()) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
"': There is already [" + oldBeanDefinition + "] bound.");
}
else if (oldBeanDefinition.getRole() < beanDefinition.getRole()) {
// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
if (this.logger.isWarnEnabled()) {
this.logger.warn("Overriding user-defined bean definition for bean '" + beanName +
"' with a framework-generated bean definition: replacing [" +
oldBeanDefinition + "] with [" + beanDefinition + "]");
}
}
else if (!beanDefinition.equals(oldBeanDefinition)) {
if (this.logger.isInfoEnabled()) {
this.logger.info("Overriding bean definition for bean '" + beanName +
"' with a different definition: replacing [" + oldBeanDefinition +
"] with [" + beanDefinition + "]");
}
}
else {
if (this.logger.isDebugEnabled()) {
this.logger.debug("Overriding bean definition for bean '" + beanName +
"' with an equivalent definition: replacing [" + oldBeanDefinition +
"] with [" + beanDefinition + "]");
}
}
this.beanDefinitionMap.put(beanName, beanDefinition);
}
else {
if (hasBeanCreationStarted()) {
// Cannot modify startup-time collection elements anymore (for stable iteration)
synchronized (this.beanDefinitionMap) {
this.beanDefinitionMap.put(beanName, beanDefinition);
List updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
updatedDefinitions.addAll(this.beanDefinitionNames);
updatedDefinitions.add(beanName);
this.beanDefinitionNames = updatedDefinitions;
if (this.manualSingletonNames.contains(beanName)) {
Set updatedSingletons = new LinkedHashSet<>(this.manualSingletonNames);
updatedSingletons.remove(beanName);
this.manualSingletonNames = updatedSingletons;
}
}
}
else {
// Still in startup registration phase
this.beanDefinitionMap.put(beanName, beanDefinition);
this.beanDefinitionNames.add(beanName);
this.manualSingletonNames.remove(beanName);
}
this.frozenBeanDefinitionNames = null;
}
if (oldBeanDefinition != null || containsSingleton(beanName)) {
resetBeanDefinition(beanName);
}
}
这一段代码中的核心，就是以下代码：
synchronized (this.beanDefinitionMap) {
this.beanDefinitionMap.put(beanName, beanDefinition);
List updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
updatedDefinitions.addAll(this.beanDefinitionNames);
updatedDefinitions.add(beanName);
this.beanDefinitionNames = updatedDefinitions;
if (this.manualSingletonNames.contains(beanName)) {
Set updatedSingletons = new LinkedHashSet<>(this.manualSingletonNames);
updatedSingletons.remove(beanName);
this.manualSingletonNames = updatedSingletons;
}
}
其中, beanDefinitionMap就是所谓的IoC容器，其具体定义为：
/** Map of bean definition objects, keyed by bean name */
//传说中的IoC容器
private final Map beanDefinitionMap = new ConcurrentHashMap<>(256);
BeanDefinition就是类的具体信息。