撸了今年阿里、头条和美团的面试，我有一个重要发现.......>>> 
初始化项目
- spring配置文件如下
- 启动项目
ApplicationContext applicationContext = new ClassPathXmlApplicationContext("spring-application.xml");
Object student = applicationContext.getBean("student");
System.out.println(student);
源码分析启动过程
- 跳转到
ClassPathXmlApplicationContext构造方法
public ClassPathXmlApplicationContext(
String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
throws BeansException {
super(parent);
setConfigLocations(configLocations);
if (refresh) {
refresh();
}
}
- 执行
super(parent);跳转到AbstractApplicationContext
public AbstractApplicationContext(@Nullable ApplicationContext parent) {
this();
setParent(parent);
}
this();方法初始化一个PathMatchingResourcePatternResolver类并将AbstractApplicationContext做完参数初始化进去。构造资源加载器。
- 执行
setConfigLocations(configLocations)跳转到AbstractRefreshableConfigApplicationContext
public void setConfigLocations(@Nullable String... locations) {
if (locations != null) {
Assert.noNullElements(locations, "Config locations must not be null");
this.configLocations = new String[locations.length];
for (int i = 0; i < locations.length; i++) {
this.configLocations[i] = resolvePath(locations[i]).trim();
}
}
else {
this.configLocations = null;
}
}
保存资源文件。多个资源文件路径之间可以是用” , ; \t\n”分割
- 执行
refresh();方法跳转到AbstractApplicationContext
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
//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();
//告诉子类启动 refreshBeanFactory()方法，Bean 定义资源文件的载入从
//子类的 refreshBeanFactory()方法启动
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// 为 BeanFactory 配置容器特性，例如类加载器、事件处理器等
prepareBeanFactory(beanFactory);
try {
// 为容器的某些子类指定特殊的 BeanPost 事件处理器
postProcessBeanFactory(beanFactory);
// 调用所有注册的 BeanFactoryPostProcessor 的 Bean
invokeBeanFactoryPostProcessors(beanFactory);
//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);
// 初始化信息源，和国际化相关.
initMessageSource();
// 初始化容器事件传播器.
initApplicationEventMulticaster();
//调用子类的某些特殊 Bean 初始化方法.
onRefresh();
// 为事件传播器注册事件监听器.
registerListeners();
// 初始化单例Bean中配置lazy-init为false的
finishBeanFactoryInitialization(beanFactory);
// 初始化容器的生命周期事件处理器，并发布容器的生命周期事件
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}
//销毁已创建的 Bean
destroyBeans();
// /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();
}
}
}
AbstractApplicationContext类的obtainFreshBeanFactory()方法调用子类容器AbstractRefreshableApplicationContext类的refreshBeanFactory()方法，启动容器载入Bean定义资源文件。
@Override
protected final void refreshBeanFactory() throws BeansException {
if (hasBeanFactory()) {
destroyBeans();
closeBeanFactory();
}
try {
//创建bean工厂
DefaultListableBeanFactory beanFactory = createBeanFactory();
beanFactory.setSerializationId(getId());
customizeBeanFactory(beanFactory);
//加载bean
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。
- 执行
loadBeanDefinitions(beanFactory)最后跳转到XmlBeanDefinitionReader类下面的方法
@Override
public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
//将读入的 XML 资源进行特殊编码处理
return loadBeanDefinitions(new EncodedResource(resource));
}
- 进入到重载方法
public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
try {
//将资源文件转为IO流
InputStream inputStream = encodedResource.getResource().getInputStream();
try {
InputSource inputSource = new InputSource(inputStream);
if (encodedResource.getEncoding() != null) {
inputSource.setEncoding(encodedResource.getEncoding());
}
//读取IO流
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();
}
}
}
- 真正加载xml文件的代码
doLoadBeanDefinitions(inputSource, encodedResource.getResource())
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource) throws BeanDefinitionStoreException {
try {
//将XML文件转换为DOM对象，解析过程由documentLoader实现
Document doc = doLoadDocument(inputSource, resource);
//启动对Bean定义解析的详细过程
return registerBeanDefinitions(doc, resource);
}
//...
}
- 注册解析bean
//按照 Spring 的 Bean 语义要求将 Bean 定义资源解析并转换为容器内部数据结构
public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
//得到 BeanDefinitionDocumentReader 来对 xml 格式的 BeanDefinition 解析
BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
//获得容器中注册的 Bean 数量
int countBefore = getRegistry().getBeanDefinitionCount();
//解析过程入口，这里使用了委派模式，BeanDefinitionDocumentReader 只是个接口,
//具体的解析实现过程有实现类 DefaultBeanDefinitionDocumentReader 完成
documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
//统计解析的 Bean 数量
return getRegistry().getBeanDefinitionCount() - countBefore;
}
- 执行
documentReader.registerBeanDefinitions(doc, createReaderContext(resource))
@Override
public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
this.readerContext = readerContext;
logger.debug("Loading bean definitions");
//获得 Document 的根元素
Element root = doc.getDocumentElement();
doRegisterBeanDefinitions(root);
}
- 执行
doRegisterBeanDefinitions(root)
protected void doRegisterBeanDefinitions(Element root) {
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;
}
}
}
//在解析Bean定义之前,进行自定义的解析,增强解析过程的可扩展性
preProcessXml(root);
//从 Document 的根元素开始进行 Bean 定义的 Document 对象
parseBeanDefinitions(root, this.delegate);
//在解析Bean定义之后,进行自定义的解析,增加解析过程的可扩展性
postProcessXml(root);
this.delegate = parent;
}
这里主要是生成一个BeanDefinitionParserDelegate delegate类，里面定义了bean标签的各种配置信息
- 执行
parseBeanDefinitions(root, this.delegate)
protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
if (delegate.isDefaultNamespace(root)) {
//获取Bean定义的Document对象根元素的所有子节点
NodeList nl = root.getChildNodes();
for (int i = 0; i < nl.getLength(); i++) {
Node node = nl.item(i);
//获取的元素是xml节点
if (node instanceof Element) {
Element ele = (Element) node;
//Bean定义的Document的元素节点使用的是 Spring 默认的 XML 命名空间
if (delegate.isDefaultNamespace(ele)) {
//解析元素标签
parseDefaultElement(ele, delegate);
}
else {
//没有使用Spring默认的XML命名空间，则使用用户自定义的解析规则解析元素节点
delegate.parseCustomElement(ele);
}
}
}
}
else {
//没有使用Spring默认的XML命名空间，则使用用户自定义的解析规则解析元素节点
delegate.parseCustomElement(root);
}
}
- 执行
parseDefaultElement(ele, delegate)
private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
//如果元素节点是导入元素，进行导入解析
if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
importBeanDefinitionResource(ele);
}
//如果元素节点是别名元素，进行别名解析
else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
processAliasRegistration(ele);
}
//如果元素节点是普通的元素，进行解析
else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
processBeanDefinition(ele, delegate);
}
//如果元素节点是元素，解析里面的子标签
else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
// recurse
doRegisterBeanDefinitions(ele);
}
}
- 执行
processBeanDefinition(ele, delegate)解析bean
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
//BeanDefinitionHolder是对BeanDefinition的封装
BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
if (bdHolder != null) {
bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
try {
//向spring容器注册解析的beanDefinition
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));
}
}
- 执行
delegate.parseBeanDefinitionElement(ele)生成bdHolder
@Nullable
public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, @Nullable BeanDefinition containingBean) {
//获取bean标签的id和name属性
String id = ele.getAttribute(ID_ATTRIBUTE);
String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);
//获取name属性的值，这里可以看出标签的name可以配置多个并用“,; ”分割
List aliases = new ArrayList<>();
if (StringUtils.hasLength(nameAttr)) {
String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, MULTI_VALUE_ATTRIBUTE_DELIMITERS);
aliases.addAll(Arrays.asList(nameArr));
}
//如果标签id属性为空，则赋值id为第一个别名的值
String beanName = id;
if (!StringUtils.hasText(beanName) && !aliases.isEmpty()) {
beanName = aliases.remove(0);
if (logger.isDebugEnabled()) {
logger.debug("No XML 'id' specified - using '" + beanName +
"' as bean name and " + aliases + " as aliases");
}
}
//校验当前的beanName和aliases是否已经存在
//如果不存在放到delegate的Set usedNames属性中
if (containingBean == null) {
checkNameUniqueness(beanName, aliases, ele);
}
//解析标签中的其他属性
AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);
if (beanDefinition != null) {
if (!StringUtils.hasText(beanName)) {
try {
if (containingBean != null) {
beanName = BeanDefinitionReaderUtils.generateBeanName(
beanDefinition, this.readerContext.getRegistry(), true);
} else {
beanName = this.readerContext.generateBeanName(beanDefinition);
// Register an alias for the plain bean class name, if still possible,
// if the generator returned the class name plus a suffix.
// This is expected for Spring 1.2/2.0 backwards compatibility.
String beanClassName = beanDefinition.getBeanClassName();
if (beanClassName != null &&
beanName.startsWith(beanClassName) && beanName.length() > beanClassName.length() &&
!this.readerContext.getRegistry().isBeanNameInUse(beanClassName)) {
aliases.add(beanClassName);
}
}
if (logger.isDebugEnabled()) {
logger.debug("Neither XML 'id' nor 'name' specified - " +
"using generated bean name [" + beanName + "]");
}
} catch (Exception ex) {
error(ex.getMessage(), ele);
return null;
}
}
//最后生成一个BeanDefinitionHolder对象
String[] aliasesArray = StringUtils.toStringArray(aliases);
return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);
}
return null;
}
- 执行
parseBeanDefinitionElement(ele, beanName, containingBean)
@Nullable
public AbstractBeanDefinition parseBeanDefinitionElement(
Element ele, String beanName, @Nullable BeanDefinition containingBean) {
//创建一个BeanEntry并赋值其唯一属性beanDefinitionName为beanName
//同时将其放入到delegate的parseState的state中，state是一个Stack
this.parseState.push(new BeanEntry(beanName));
//解析class标签
String className = null;
if (ele.hasAttribute(CLASS_ATTRIBUTE)) {
className = ele.getAttribute(CLASS_ATTRIBUTE).trim();
}
//解析parent标签
String parent = null;
if (ele.hasAttribute(PARENT_ATTRIBUTE)) {
parent = ele.getAttribute(PARENT_ATTRIBUTE);
}
try {
//根据class和parent属性值创建 BeanDefinition
AbstractBeanDefinition bd = createBeanDefinition(className, parent);
//对当前的元素中配置的一些属性进行解析和设置，如配置的单态,延迟加载(singleton,;lazy-init)属性等
parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);
bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));
//对元素的 meta(元信息)属性解析
parseMetaElements(ele, bd);
//对元素的 lookup-Method 属性解析
parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
//对元素的 replaced-Method 属性解析
parseReplacedMethodSubElements(ele, bd.getMethodOverrides());
//解析元素的构造方法设置
parseConstructorArgElements(ele, bd);
//解析元素的设置
parsePropertyElements(ele, bd);
//解析元素的 qualifier 属性
parseQualifierElements(ele, bd);
//为当前解析的 Bean 设置所需的资源和依赖对象
bd.setResource(this.readerContext.getResource());
bd.setSource(extractSource(ele));
return bd;
} catch (ClassNotFoundException ex) {
error("Bean class [" + className + "] not found", ele, ex);
} catch (NoClassDefFoundError err) {
error("Class that bean class [" + className + "] depends on not found", ele, err);
} catch (Throwable ex) {
error("Unexpected failure during bean definition parsing", ele, ex);
}
finally {
this.parseState.pop();
}
return null;
}
- 最后解析完毕,注册
BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
@Override
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
throws BeanDefinitionStoreException {
//...
// Still in startup registration phase
this.beanDefinitionMap.put(beanName, beanDefinition);
this.beanDefinitionNames.add(beanName);
this.manualSingletonNames.remove(beanName);
//...
}