- Bean 加载原理
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。
什么是BeanDefinition对象
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。
注重理解加载过程
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。中文是主线(本文也主要也是过一遍主线)，想要面面俱到需要自己再去摸索。
1. bean.xml
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>2. ResourceLoader.java
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。
触发bean加载
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类开始深入：
3. ClassPathXmlApplicationContext.java
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(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 LifeCycle:
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super() 方法完成类加载器的指定。
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setConfigLocations(configLocations);方法对配置文件进行定位和解析，拿到Resource对象。
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refresh();方法对标签进行解析拿到BeanDefition对象，在通过校验后将其注册到IOC容器。(主要研究该方法)
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:
方法2. setConfigLocations(configLocations)
// 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:
方法3. refresh()
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:
方法3.1 obtainFreshBeanFactory()
// 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
方法3.1.1 refreshBeanFactory()
// 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：
方法3.1.1.1 loadBeanDefinitions()
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(DefaultListableBeanFactorybeanFactory)方法中的 loadBeanDefinitions(XmlBeanDefinitionReaderreader)方法:
方法3.1.1.1.1 loadBeanDefinitions()
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: returnbuilder.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 + "]"); } ��Ұ,Ұ�� } 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。