加载过程： 通过 ResourceLoader和其子类DefaultResourceLoader完成资源文件位置定位，实现从类路径，文件系统，url等方式定位功能，完成定位后得到Resource对象，再交给BeanDefinitionReader，它再委托给
BeanDefinitionParserDelegate完成bean的解析并得到BeanDefinition对象，然后通过registerBeanDefinition方法进行注册，IOC容器内ibu维护了一个HashMap来保存该BeanDefinition对象，Spring中的BeanDefinition其实就是我们用的JAVABean。
BeanDefinition是一个接口，描述了一个bean实例，它具有属性值，构造函数参数值以及具体实现提供的更多信息。
在开始之前需要认真阅读和理解这个过程，有了这个过程，阅读源码难度就小了一半。
大多源码都进行了注释，有的是官方英文注释。
一个普通的bean配置文件，这里我要强调的是它里面的格式，因为解析标签的时候会用到。它有<beans>``<bean>``<import>``<alias>等标签，下文会对他们进行解析并翻译成BeanDefinition对象。
<beans>
<!-- this definition could be inside one beanRefFactory.xml file -->
<bean id="a.qualified.name.of.some.sort"
class="org.springframework.context.support.ClassPathXmlApplicationContext">
<property name="configLocation" value="org/springframework/web/context/beans1.xml"/>
</bean>
<!-- while the following two could be inside another, also on the classpath,
perhaps coming from another component jar -->
<bean id="another.qualified.name"
class="org.springframework.context.support.ClassPathXmlApplicationContext">
<property name="configLocation" value="org/springframework/web/context/beans1.xml"/>
<property name="parent" ref="a.qualified.name.of.some.sort"/>
</bean>
<alias name="another.qualified.name" alias="a.qualified.name.which.is.an.alias"/>
</beans>
加载资源的策略接口(策略模式)。
DefaultResourceLoader is a standalone implementation that is usable outside an ApplicationContext, also used by ResourceEditor
An ApplicationContext is required to provide this functionality, plus extended ResourcePatternResolver support.
public interface ResourceLoader {
/** Pseudo URL prefix for loading from the class path: "classpath:". */
String CLASSPATH_URL_PREFIX = ResourceUtils.CLASSPATH_URL_PREFIX;
// 返回一个Resource 对象 (明确配置文件位置的对象)
Resource getResource(String location);
// 返回ResourceLoader的ClassLoader
@Nullable
ClassLoader getClassLoader();
}
然后我们看看DefaultResourceLoader对于getResource()方法的实现。
public Resource getResource(String location) {
Assert.notNull(location, "Location must not be null");
for (ProtocolResolver protocolResolver : this.protocolResolvers) {
Resource resource = protocolResolver.resolve(location, this);
if (resource != null) {
return resource;
}
}
// 如果location 以 / 开头
if (location.startsWith("/")) {
return getResourceByPath(location);
}
// 如果location 以classpath: 开头
else if (location.startsWith(CLASSPATH_URL_PREFIX)) {
return new ClassPathResource(location.substring(CLASSPATH_URL_PREFIX.length()), getClassLoader());
}
else {
try {
// Try to parse the location as a URL...
URL url = new URL(location);
return (ResourceUtils.isFileURL(url) ? new FileUrlResource(url) : new UrlResource(url));
}
catch (MalformedURLException ex) {
// No URL -> resolve as resource path.
return getResourceByPath(location);
}
}
}
可以看到，它判断了三种情况：/ classpath: url格式匹配， 然后调用相对应的处理方法，我只分析classpath:，因为这是最常用的。所以看一看ClassPathResource实现：
public ClassPathResource(String path, @Nullable ClassLoader classLoader) {
Assert.notNull(path, "Path must not be null");
String pathToUse = StringUtils.cleanPath(path);
if (pathToUse.startsWith("/")) {
pathToUse = pathToUse.substring(1);
}
this.path = pathToUse;
this.classLoader = (classLoader != null ? classLoader : ClassUtils.getDefaultClassLoader());
}
看了上面的代码，意味着你配置静态资源文件路径的时候，不用纠结classpath:后面用不用写/,因为如果写了它会给你过滤掉。
那url如何定位的呢?
跟踪getResourceByPath(location)方法:
@Override
protected Resource getResourceByPath(String path) {
if (path.startsWith("/")) {
path = path.substring(1);
}
// 这里使用文件系统资源对象来定义bean文件
return new FileSystemResource(path);
}
好了，很明显…跑偏了，因为我们想要的是xml文件及路径的解析，不过还好，换汤不换药。下文中会涉及到。
回到正题，我们在使用spring手动加载bean.xml的时候，用到：
ApplicationContext applicationContext = new ClassPathXmlApplicationContext("bean.xml");
那就从
ClassPathXmlApplicationContext开始：
这个类里面只有构造方法(多个)和一个getConfigResources()方法，构造方法最终都统一打到下面这个构造方法中(Spring源码经常这样，适配器模式):
public ClassPathXmlApplicationContext(
String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
throws BeansException {
// 动态的确定用哪个加载器去加载 配置文件
1.super(parent);
// 告诉读取器 配置文件在哪里， 定位加载配置文件
2.setConfigLocations(configLocations);
// 刷新
if (refresh) {
// 在创建IOC容器前，如果容器已经存在，则需要把已有的容器摧毁和关闭，以保证refresh
//之后使用的是新的IOC容器
3.refresh();
}
}
注意: 这个类非常关键，我认为它定义了一个xml加载bean的一个Life Cycle:
我标记的1. 2. 3. 对应后面的方法x, 方便阅读。
先深入了解下setConfigLocations(configLocations);方法:
// 解析Bean定义资源文件的路径，处理多个资源文件字符串数组
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++) {
// resolvePath 为同一个类中将字符串解析为路径的方法
this.configLocations[i] = resolvePath(locations[i]).trim();
}
}
else {
this.configLocations = null;
}
}
然后我们继续上面看
ClassPathXmlApplicationContext的refresh()方法:
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// 为refresh 准备上下文
prepareRefresh();
// 通知子类去刷新 Bean工厂
ConfigurableListableBeanFactory beanFactory = obtAInFreshBeanFactory();
// 用该 上下文来 准备bean工厂
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();
}
}
}
**注:**下面的方法全都是围绕refresh()里深入阅读，该方法套的很深，下面的阅读可能会引起不适。
然后看看refresh()方法中的obtainFreshBeanFactory()方法:
// 调用--刷新bean工厂
protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
// 委派模式：父类定义了refreshBeanFactory方法，具体实现调用子类容器
refreshBeanFactory();
return getBeanFactory();
}
然后看obtainFreshBeanFactory()的 refreshBeanFactory()方法
// 刷新bean工厂
protected final void refreshBeanFactory() throws BeansException {
// 如果存在容器，就先销毁并关闭
if (hasBeanFactory()) {
destroyBeans();
closeBeanFactory();
}
try {
// 创建IOC容器
DefaultListableBeanFactory beanFactory = createBeanFactory();
beanFactory.setSerializationId(getId());
// 对容器进行初始化
customizeBeanFactory(beanFactory);
// 调用载入Bean定义的方法，(使用了委派模式)
loadBeanDefinitions(beanFactory);
synchronized (this.beanFactoryMonitor) {
this.beanFactory = beanFactory;
}
}
catch (IOException ex) {
throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
}
}
然后再跟进refreshBeanFactory() 的loadBeanDefinitions()方法：
通过 XmlBeanDefinitionReader 加载 BeanDefinition
// 通过 XmlBeanDefinitionReader 加载 BeanDefinition
@Override
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
// Create a new XmlBeanDefinitionReader for the given BeanFactory.
// 为beanFactory 创建一个新的 XmlBeanDefinitionReader
XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
// Configure the bean definition reader with this context's
// resource loading environment.
beanDefinitionReader.setEnvironment(this.getEnvironment());
// 为 Bean读取器设置Spring资源加载器 (因为祖父类是ResourceLoader的子类，所以也是ResourceLoader)
beanDefinitionReader.setResourceLoader(this);
// 为 Bean读取器设置SAX xml解析器DOM4J
beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));
// Allow a subclass to provide custom initialization of the reader,
// then proceed with actually loading the bean definitions.
// 初始化 BeanDefinition读取器
initBeanDefinitionReader(beanDefinitionReader);
// 真正加载 bean定义
loadBeanDefinitions(beanDefinitionReader);
}
再跟进loadBeanDefinitions(
DefaultListableBeanFactory beanFactory)方法中的loadBeanDefinitions(XmlBeanDefinitionReader reader)方法:
XMLBean读取器加载BeanDefinition 资源
// XMLBean读取器加载Bean 定义资源
protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
// 获取Bean定义资源的定位
Resource[] configResources = getConfigResources();
if (configResources != null) {
// XMLBean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) {
// XMLBean读取器调用其父类 AbstractBeanDefinitionReader 读取定位的Bean定义资源
reader.loadBeanDefinitions(configLocations);
}
}
@Override
public int loadBeanDefinitions(Resource... resources) throws BeanDefinitionStoreException {
Assert.notNull(resources, "Resource array must not be null");
int count = 0;
//
for (Resource resource : resources) {
count += loadBeanDefinitions(resource);
}
return count;
}
再跟下去loadBeanDefinitions(): 这只是一个抽象方法，找到XmlBeanDefinitionReader子类的实现：
@Override
public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
return loadBeanDefinitions(new EncodedResource(resource));
}
再深入loadBeanDefinitions:
通过明确的xml文件加载bean
// 通过明确的xml文件加载bean
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 {
// 将资源文件转为InputStream的IO流
InputStream inputStream = encodedResource.getResource().getInputStream();
try {
// 从流中获取 xml解析资源
InputSource inputSource = new InputSource(inputStream);
if (encodedResource.getEncoding() != null) {
// 设置编码
inputSource.setEncoding(encodedResource.getEncoding());
}
// 具体的读取过程
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();
}
}
}
再深入到doLoadBeanDefinitions():
真正开始加载 BeanDefinitions
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {
try {
// 将xml 文件转换为DOM对象
Document doc = doLoadDocument(inputSource, resource);
// 对bean定义解析的过程，该过程会用到 Spring的bean配置规则
int count = registerBeanDefinitions(doc, resource);
if (logger.isDebugEnabled()) {
logger.debug("Loaded " + count + " bean definitions from " + resource);
}
return count;
}
... ... ..
}
doLoadDocument()方法将流进行解析，返回一个Document对象:return builder.parse(inputSource);为了避免扰乱思路，这里的深入自己去完成。
还需要再深入到:registerBeanDefinitions()
注册 BeanDefinitions
public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
// 得到容器中注册的bean数量
int countBefore = getRegistry().getBeanDefinitionCount();
// 解析过程入口，这里使用了委派模式
documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
// 统计解析的bean数量
return getRegistry().getBeanDefinitionCount() - countBefore;
}
再深入registerBeanDefinitions()方法（该方法是委派模式的结果）:
@Override
public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
// 获得XML描述符
this.readerContext = readerContext;
doRegisterBeanDefinitions(doc.getDocumentElement());
}
再深入doRegisterBeanDefinitions(doc.getDocumentElement());：
真正开始注册 BeanDefinitions :
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;
}
}
}
// 在bean解析定义之前，进行自定义解析，看是否是用户自定义标签
preProcessXml(root);
// 开始进行解析bean定义的document对象
parseBeanDefinitions(root, this.delegate);
// 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;
}
接下来看parseBeanDefinitions(root, this.delegate);:
document的根元素开始进行解析翻译成BeanDefinitions
// 从document的根元素开始进行解析翻译成BeanDefinitions
protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
// bean定义的document对象使用了spring默认的xml命名空间
if (delegate.isDefaultNamespace(root)) {
// 获取bean定义的document对象根元素的所有字节点
NodeList nl = root.getChildNodes();
for (int i = 0; i < nl.getLength(); i++) {
Node node = nl.item(i);
// 获得document节点是xml元素节点
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)) {
// 使用spring的bean规则解析元素 节点
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 {
delegate.parseCustomElement(root);
}
}
private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
// 解析 <import> 标签元素，并进行导入解析
if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
importBeanDefinitionResource(ele);
}
// alias
else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
processAliasRegistration(ele);
}
// bean
else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
processBeanDefinition(ele, delegate);
}
// beans
else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
// recurse
doRegisterBeanDefinitions(ele);
}
}
importBeanDefinitionResource(ele);``processAliasRegistration(ele);``processBeanDefinition(ele, delegate);这三个方法里分别展示了标签解析的详细过程。
这下看到了，它其实使用DOM4J来解析import bean alias等标签，然后递归标签内部直到拿到所有属性并封装到BeanDefition对象中。比如说processBeanDefinition方法:
给我一个element 解析成 BeanDefinition
// 给我一个element 解析成 BeanDefinition
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
// 真正解析过程
BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
if (bdHolder != null) {
bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
try {
// Register the final decorated instance.
// 注册： 将db注册到ioc，委托模式
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));
}
}
继续深入registerBeanDefinition():
注册BeanDefinitions 到 bean 工厂
// 注册BeanDefinitions 到 bean 工厂
// definitionHolder ： bean定义，包含了 name和aliases
// registry： 注册到的bean工厂
public static void registerBeanDefinition(
BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
throws BeanDefinitionStoreException {
// Register bean definition under primary name.
String beanName = definitionHolder.getBeanName();
// 真正注册
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);
}
}
}
再深入
registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
注册BeanDefinitions 到IOC容器
注意:该方法所在类是接口，我们查看的是
DefaultListableBeanFactory.java所实现的该方法。
// 实现BeanDefinitionRegistry接口,注册BeanDefinitions
@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");
// 校验是否是 AbstractBeanDefinition)
if (beanDefinition instanceof AbstractBeanDefinition) {
try {
// 标记 beanDefinition 生效
((AbstractBeanDefinition) beanDefinition).validate();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Validation of bean definition failed", ex);
}
}
// 判断beanDefinitionMap 里是否已经有这个bean
BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
//如果没有这个bean
if (existingDefinition != null) {
//如果不允许bd 覆盖已注册的bean， 就抛出异常
if (!isAllowBeanDefinitionOverriding()) {
throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
}
// 如果允许覆盖， 则同名的bean， 注册的覆盖先注册的
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 + "]");
}
}
// 注册到容器，beanDefinitionMap 就是个容器
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 {
// Still in startup registration phase
this.beanDefinitionMap.put(beanName, beanDefinition);
this.beanDefinitionNames.add(beanName);
this.manualSingletonNames.remove(beanName);
}
this.frozenBeanDefinitionNames = null;
}
if (existingDefinition != null || containsSingleton(beanName)) {
resetBeanDefinition(beanName);
}
}
这个方法中对所需要加载的bean进行校验，没有问题的话就put到beanDefinitionMap中，beanDefinitionMap其实就是IOC.这样我们的Bean就被加载到IOC容器中了。
