一、简介：
学习过Spring框架的人一定都会听过Spring的IoC(控制反转) 、DI(依赖注入)这两个概念，对于初学Spring的人来说，总觉得IoC 、DI这两个概念是模糊不清的，最近学习了Spring源码，有了新的理解，今天带大家谈谈我对Spring Ioc的理解。
如图：
实际上我们将我们的类全部注入到IOC这个容器中，类的创建销毁等全部交由IOC处理。Spring核心思想就是IOC思想。
拿掉IOC容器可以看出来，各个类之间没有任何关系，因为我们对类的使用都是通过容器去获取的，因此起到了解耦的作用。
二、简述Spring整体处理流程
接下来先讲解Spring一个大概的流程处理过程。
读取XML － 解析XML封装成DOC对象 － DOC对象转换成Spring beanDefinition对象 - 转换成metaData对象 - 获取metaData对象开始创建bean
大家一定要多注意beanDefinition、metaData 这些类，他们在Spring源码中出现的次数非常多。
beanDefinition：是解析XML封装的一些数据。
metaData：是对类元数据的封装。
三、BeanDefinition注册过程
下面从源码上给大家跟一下BeanDefinition的注册过程。建议大家也都根据我的注释，也从源码上一个一个的跟一下。
// ClassPathXmlApplicationContext.java
public ClassPathXmlApplicationContext(
String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
throws BeansException {
super(parent);
//创建解析器，解析configLocations
setConfigLocations(configLocations);
if (refresh) {
// 核心方法
refresh();
}
}
// AbstractApplicationContext.java
/*
* 该方法是spring容器初始化的核心方法。是spring容器初始化的核心流程，是一个典型的父类模板设计模式的运用
* 根据不同的上下文对象，会调到不同的上下文对象子类方法中
*
* 核心上下文子类有：
* ClassPathXmlApplicationContext
* FileSystemXmlApplicationContext
* AnnotationConfigApplicationContext
* EmbeddedWebApplicationContext(springboot) 里面会用到钩子方法启动内嵌tomcat
* */
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
//为容器初始化做准备
// Prepare this context for refreshing.
prepareRefresh();
/*
非常重要
1、创建BeanFactory对象
* 2、xml解析
* 传统标签解析：bean、import等
* 自定义标签解析 如：<context:component-scan base-package="com.xiangxue.jack"/>
* 自定义标签解析流程：
* a、根据当前解析标签的头信息找到对应的namespaceUri
* b、加载spring所以jar中的spring.handlers文件。并建立映射关系
* c、根据namespaceUri从映射关系中找到对应的实现了NamespaceHandler接口的类
* d、调用类的init方法，init方法是注册了各种自定义标签的解析类
* e、根据namespaceUri找到对应的解析类，然后调用paser方法完成标签解析
*
* 3、把解析出来的xml标签封装成BeanDefinition对象
* */
// 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) {
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();
}
}
}@Override
protected final void refreshBeanFactory() throws BeansException {
//如果BeanFactory不为空，则清除BeanFactory和里面的实例
if (hasBeanFactory()) {
destroyBeans();
closeBeanFactory();
}
try {
//创建DefaultListableBeanFactory
DefaultListableBeanFactory beanFactory = createBeanFactory();
beanFactory.setSerializationId(getId());
//设置是否可以循环依赖 allowCircularReferences
//是否允许使用相同名称重新注册不同的bean实现.
customizeBeanFactory(beanFactory);
//解析xml，并把xml中的标签封装成BeanDefinition对象
loadBeanDefinitions(beanFactory);
synchronized (this.beanFactoryMonitor) {
this.beanFactory = beanFactory;
}
}
catch (IOException ex) {
throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
}
}@Override
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
// Create a new XmlBeanDefinitionReader for the given BeanFactory.
//创建xml的解析器，这里是一个委托模式
XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
// Configure the bean definition reader with this context's
// resource loading environment.
beanDefinitionReader.setEnvironment(this.getEnvironment());
//这里传一个this进去，因为ApplicationContext是实现了ResourceLoader接口的
beanDefinitionReader.setResourceLoader(this);
beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));
// Allow a subclass to provide custom initialization of the reader,
// then proceed with actually loading the bean definitions.
initBeanDefinitionReader(beanDefinitionReader);
//主要看这个方法 重要程度 5
loadBeanDefinitions(beanDefinitionReader);
}protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
Resource[] configResources = getConfigResources();
if (configResources != null) {
reader.loadBeanDefinitions(configResources);
}
//获取需要加载的xml配置文件
String[] configLocations = getConfigLocations();
if (configLocations != null) {
reader.loadBeanDefinitions(configLocations);
}
}// AbstractBeanDefinitionReader.java
@Override
public int loadBeanDefinitions(String... locations) throws BeanDefinitionStoreException {
Assert.notNull(locations, "Location array must not be null");
int count = 0;
//配置文件有多个，加载多个配置文件
for (String location : locations) {
count += loadBeanDefinitions(location);
}
return count;
}public int loadBeanDefinitions(String location, @Nullable Set<Resource> actualResources) throws BeanDefinitionStoreException {
ResourceLoader resourceLoader = getResourceLoader();
if (resourceLoader == null) {
throw new BeanDefinitionStoreException(
"Cannot load bean definitions from location [" + location + "]: no ResourceLoader available");
}
if (resourceLoader instanceof ResourcePatternResolver) {
// Resource pattern matching available.
try {
//把字符串类型的xml文件路径，形如：classpath*:user/**/*-context.xml,转换成Resource对象类型，其实就是用流
//的方式加载配置文件，然后封装成Resource对象，不重要，可以不看
Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
//主要看这个方法
int count = loadBeanDefinitions(resources);
if (actualResources != null) {
Collections.addAll(actualResources, resources);
}
if (logger.isTraceEnabled()) {
logger.trace("Loaded " + count + " bean definitions from location pattern [" + location + "]");
}
return count;
}
catch (IOException ex) {
throw new BeanDefinitionStoreException(
"Could not resolve bean definition resource pattern [" + location + "]", ex);
}
}
else {
// Can only load single resources by absolute URL.
Resource resource = resourceLoader.getResource(location);
int count = loadBeanDefinitions(resource);
if (actualResources != null) {
actualResources.add(resource);
}
if (logger.isTraceEnabled()) {
logger.trace("Loaded " + count + " bean definitions from location [" + location + "]");
}
return count;
}
}public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
Assert.notNull(encodedResource, "EncodedResource must not be null");
if (logger.isTraceEnabled()) {
logger.trace("Loading XML bean definitions from " + encodedResource);
}
Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
if (currentResources == null) {
currentResources = new HashSet<>(4);
this.resourcesCurrentlyBeingLoaded.set(currentResources);
}
if (!currentResources.add(encodedResource)) {
throw new BeanDefinitionStoreException(
"Detected cyclic loading of " + encodedResource + " - check your import definitions!");
}
try {
//获取Resource对象中的xml文件流对象
InputStream inputStream = encodedResource.getResource().getInputStream();
try {
//InputSource是jdk中的sax xml文件解析对象
InputSource inputSource = new InputSource(inputStream);
if (encodedResource.getEncoding() != null) {
inputSource.setEncoding(encodedResource.getEncoding());
}
//主要看这个方法
return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
}
finally {
inputStream.close();
}
}
catch (IOException ex) {
throw new BeanDefinitionStoreException(
"IOException parsing XML document from " + encodedResource.getResource(), ex);
}
finally {
currentResources.remove(encodedResource);
if (currentResources.isEmpty()) {
this.resourcesCurrentlyBeingLoaded.remove();
}
}
}// 此时已经将XML解析封装为Document对象
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {
try {
//把inputSource 封装成Document文件对象，这是jdk的API
Document doc = doLoadDocument(inputSource, resource);
//主要看这个方法，根据解析出来的document对象，拿到里面的标签元素封装成BeanDefinition
int count = registerBeanDefinitions(doc, resource);
if (logger.isDebugEnabled()) {
logger.debug("Loaded " + count + " bean definitions from " + resource);
}
return count;
}
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);
}
}public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
//又来一记委托模式，BeanDefinitionDocumentReader委托这个类进行document的解析
BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
int countBefore = getRegistry().getBeanDefinitionCount();
//主要看这个方法，createReaderContext(resource) XmlReaderContext上下文，封装了XmlBeanDefinitionReader对象
documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
return getRegistry().getBeanDefinitionCount() - countBefore;
}@Override
public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
this.readerContext = readerContext;
//主要看这个方法，把root节点传进去
doRegisterBeanDefinitions(doc.getDocumentElement());
}protected void doRegisterBeanDefinitions(Element root) {
// Any nested <beans> elements will cause recursion in this method. In
// order to propagate and preserve <beans> 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);
// We cannot use Profiles.of(...) since profile expressions are not supported
// in XML config. See SPR-12458 for details.
if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
if (logger.isDebugEnabled()) {
logger.debug("Skipped XML bean definition file due to specified profiles [" + profileSpec +
"] not matching: " + getReaderContext().getResource());
}
return;
}
}
}
preProcessXml(root);
//主要看这个方法，标签具体解析过程
parseBeanDefinitions(root, this.delegate);
postProcessXml(root);
this.delegate = parent;
}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)) {
//默认标签解析
parseDefaultElement(ele, delegate);
}
else {
//自定义标签解析
delegate.parseCustomElement(ele);
}
}
}
}
else {
delegate.parseCustomElement(root);
}
}private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
//import标签解析 重要程度 1 ，可看可不看
if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
importBeanDefinitionResource(ele);
}
//alias标签解析 别名标签 重要程度 1 ，可看可不看
else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
processAliasRegistration(ele);
}
//bean标签，重要程度 5，必须看
else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
processBeanDefinition(ele, delegate);
}
else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
// recurse
doRegisterBeanDefinitions(ele);
}
}protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
//重点看这个方法，重要程度 5 ，解析document，封装成BeanDefinition
BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
if (bdHolder != null) {
//该方法功能不重要，设计模式重点看一下，装饰者设计模式，加上SPI设计思想
bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
try {
//完成document到BeanDefinition对象转换后，对BeanDefinition对象进行缓存注册
// 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));
}
}public static void registerBeanDefinition(
BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
throws BeanDefinitionStoreException {
// Register bean definition under primary name.
String beanName = definitionHolder.getBeanName();
//完成BeanDefinition的注册，重点看，重要程度 5
registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
//建立别名和 id的映射，这样就可以根据别名获取到id
// Register aliases for bean name, if any.
String[] aliases = definitionHolder.getAliases();
if (aliases != null) {
for (String alias : aliases) {
registry.registerAlias(beanName, alias);
}
}
}// 哇哇卡卡 终于跟到最后一个注册方法了
@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);
}
}
//先判断BeanDefinition是否已经注册
BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
if (existingDefinition != null) {
if (!isAllowBeanDefinitionOverriding()) {
throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
}
else if (existingDefinition.getRole() < beanDefinition.getRole()) {
// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
if (logger.isInfoEnabled()) {
logger.info("Overriding user-defined bean definition for bean '" + beanName +
"' with a framework-generated bean definition: replacing [" +
existingDefinition + "] with [" + beanDefinition + "]");
}
}
else if (!beanDefinition.equals(existingDefinition)) {
if (logger.isDebugEnabled()) {
logger.debug("Overriding bean definition for bean '" + beanName +
"' with a different definition: replacing [" + existingDefinition +
"] with [" + beanDefinition + "]");
}
}
else {
if (logger.isTraceEnabled()) {
logger.trace("Overriding bean definition for bean '" + beanName +
"' with an equivalent definition: replacing [" + existingDefinition +
"] 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<String> updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
updatedDefinitions.addAll(this.beanDefinitionNames);
updatedDefinitions.add(beanName);
this.beanDefinitionNames = updatedDefinitions;
if (this.manualSingletonNames.contains(beanName)) {
Set<String> updatedSingletons = new LinkedHashSet<>(this.manualSingletonNames);
updatedSingletons.remove(beanName);
this.manualSingletonNames = updatedSingletons;
}
}
}
else {
//把beanDefinition缓存到map中
// Still in startup registration phase
this.beanDefinitionMap.put(beanName, beanDefinition);
//把beanName放到beanDefinitionNames list中，这个list着重记住，bean实例化的时候需要用到
this.beanDefinitionNames.add(beanName);
this.manualSingletonNames.remove(beanName);
}
this.frozenBeanDefinitionNames = null;
}
if (existingDefinition != null || containsSingleton(beanName)) {
resetBeanDefinition(beanName);
}
}
上面就是一个个方法跟到，beanDefinition注册的过程。
建议大家一步一步跟下去看看。刚开始看起来比较费劲，建议多看几遍哦！
如有问题欢迎大家指出，谢谢！