Spring源码解密之默认标签的解析

public class DefaultBeanDefinitionDocumentReader implements BeanDefinitionDocumentReader {
 private void parseDefaultElement(Element ele,BeanDefinitionParserDelegate delegate) {
 // import 标签解析
 if (delegate.nodeNameEquals(ele,IMPORT_ELEMENT)) {
 importBeanDefinitionResource(ele);
 }
 // alias 标签解析
 else if (delegate.nodeNameEquals(ele,ALIAS_ELEMENT)) {
 processAliasRegistration(ele);
 }
 // bean 标签解析
 else if (delegate.nodeNameEquals(ele,BEAN_ELEMENT)) {
 processBeanDefinition(ele,delegate);
 }
 // import 标签解析
 else if (delegate.nodeNameEquals(ele,NESTED_BEANS_ELEMENT)) {
 // beans标签解析 递归方式 
 doRegisterBeanDefinitions(ele);
 }
 }
}

protected void processBeanDefinition(Element ele,BeanDefinitionParserDelegate delegate) {
 // 委托BeanDefinitionDelegate类的parseBeanDefinitionElement方法进行元素解析
 BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
 if (bdHolder != null) {
 // 当返回的bdHolder不为空的情况下若存在默认标签的子节点下再有自定义属性，还需要再次对自定义标签进行解析
 bdHolder = delegate.decorateBeanDefinitionIfRequired(ele,bdHolder);
 try {
 // 解析完成后需要对解析后的bdHolder进行注册，注册操作委托给了BeanDefinitionReaderUtils的registerBeanDefinition方法
 BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder,getReaderContext().getRegistry());
 }
 catch (BeanDefinitionStoreException ex) {
 getReaderContext().error("Failed to register bean definition with name '" +
 bdHolder.getBeanName() + "'",ele,ex);
 }
 // 最后发出响应事件，通知相关监听器这个bean已经被加载
 getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
 }
}

public class BeanDefinitionParserDelegate {
 @Nullable
 public BeanDefinitionHolder parseBeanDefinitionElement(Element ele,@Nullable BeanDefinition containingBean) {
 // 获取Bean标签的ID属性
 String id = ele.getAttribute(ID_ATTRIBUTE);
 // 获取Bean标签的Name属性
 String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);
 List<String> aliases = new ArrayList<>();
 if (StringUtils.hasLength(nameAttr)) {
 // 将name属性的值通过,; 进行分割 转为字符串数字(即在配置文件中如配置多个name 在此做处理)
 String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr,MULTI_VALUE_ATTRIBUTE_DELIMITERS);
 aliases.addAll(Arrays.asList(nameArr));
 }
 String beanName = id;
 // 如果ID为空 使用配置的第一个name属性作为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");
 }
 }
 if (containingBean == null) {
 // 校验beanName和aliases的唯一性
 // 内部核心为使用usedNames集合保存所有已经使用了的beanName和alisa
 checkNameUniqueness(beanName,aliases,ele);
 }
 // 进一步解析其他所有属性到GenericBeanDefinition对象中
 AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele,beanName,containingBean);
 if (beanDefinition != null) {
 // 如果bean没有指定beanName 那么使用默认规则为此Bean生成beanName
 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;
 }
 }
 String[] aliasesArray = StringUtils.toStringArray(aliases);
 // 将信息封装到BeanDefinitionHolder对象中
 return new BeanDefinitionHolder(beanDefinition,aliasesArray);
 }
 return null;
 }
}

@Nullable
public AbstractBeanDefinition parseBeanDefinitionElement(
 Element ele,@Nullable BeanDefinition containingBean) {
 this.parseState.push(new BeanEntry(beanName));
 // 获取Bean标签的class属性
 String className = null;
 if (ele.hasAttribute(CLASS_ATTRIBUTE)) {
 className = ele.getAttribute(CLASS_ATTRIBUTE).trim();
 }
 // 获取Bean标签的parent属性
 String parent = null;
 if (ele.hasAttribute(PARENT_ATTRIBUTE)) {
 parent = ele.getAttribute(PARENT_ATTRIBUTE);
 }
 try {
 // 创建用于承载属性的AbstractBeanDefinition
 AbstractBeanDefinition bd = createBeanDefinition(className,parent);
 // 获取bean标签各种属性
 parseBeanDefinitionAttributes(ele,containingBean,bd);
 // 解析description标签
 bd.setDescription(DomUtils.getChildElementValueByTagName(ele,DESCRIPTION_ELEMENT));
 // 解析meta标签
 parseMetaElements(ele,bd);
 // 解析lookup-method标签
 parseLookupOverrideSubElements(ele,bd.getMethodOverrides());
 // 解析replaced-method标签
 parseReplacedMethodSubElements(ele,bd.getMethodOverrides());
 // 解析constructor-arg标签
 parseConstructorArgElements(ele,bd);
 // 解析property标签
 parsePropertyElements(ele,bd);
 // 解析qualifier标签
 parseQualifierElements(ele,bd);
 bd.setResource(this.readerContext.getResource());
 bd.setSource(extractSource(ele));

 return bd;
 }
 catch (ClassNotFoundException ex) {
 error("Bean class [" + className + "] not found",ex);
 }
 catch (NoClassDefFoundError err) {
 error("Class that bean class [" + className + "] depends on not found",err);
 }
 catch (Throwable ex) {
 error("Unexpected failure during bean definition parsing",ex);
 }
 finally {
 this.parseState.pop();
 }
 return null;
}

// BeanDefinitionParserDelegate.java
protected AbstractBeanDefinition createBeanDefinition(@Nullable String className,@Nullable String parentName)
 throws ClassNotFoundException {
 return BeanDefinitionReaderUtils.createBeanDefinition(
 parentName,className,this.readerContext.getBeanClassLoader());
}
public class BeanDefinitionReaderUtils {
 public static AbstractBeanDefinition createBeanDefinition(
 @Nullable String parentName,@Nullable String className,@Nullable ClassLoader classLoader) throws ClassNotFoundException {
 GenericBeanDefinition bd = new GenericBeanDefinition();
 // parentName可能为空
 bd.setParentName(parentName);
 // 如果classLoader不为空 
 // 则使用传入的classLoader同一虚拟机加载类对象 否则只记录classLoader
 if (className != null) {
 if (classLoader != null) {
 bd.setBeanClass(ClassUtils.forName(className,classLoader));
 }
 else {
 bd.setBeanClassName(className);
 }
 }
 return bd;
 }
}

public class BeanDefinitionParserDelegate {
 public AbstractBeanDefinition parseBeanDefinitionAttributes(Element ele,@Nullable BeanDefinition containingBean,AbstractBeanDefinition bd) {
 // ...省略详细代码，该部分代码主要就是通过 if else 判断是否含有指定的属性，如果有就 bd.set(attribute);
 return bd;
 }
}
```

``` java
// BeanDefinitionParserDelegate.java
@Nullable
public BeanDefinitionHolder parseBeanDefinitionElement(Element ele,@Nullable BeanDefinition containingBean) {
 // ...
 return new BeanDefinitionHolder(beanDefinition,aliasesArray);
}

public class BeanDefinitionReaderUtils {
 public static void registerBeanDefinition(
 BeanDefinitionHolder definitionHolder,BeanDefinitionRegistry registry)
 throws BeanDefinitionStoreException {
 // 使用 beanName 做唯一标识符
 String beanName = definitionHolder.getBeanName();
 // 注册bean的核心代码
 registry.registerBeanDefinition(beanName,definitionHolder.getBeanDefinition());
 // 为bean注册所有的别名
 String[] aliases = definitionHolder.getAliases();
 if (aliases != null) {
 for (String alias : aliases) {
 registry.registerAlias(beanName,alias);
 }
 }
 }
}

public class DefaultListableBeanFactory {
 @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 {
 // 注册前的最后一次校验，这里的校验不同于XML文件校验
 // 主要是对于AbstractBeanDefinition属性中的methodOverrides校验
 // 校验methodOverrides是否与工厂方法并存或者methodOverrides对于的方法根本不存在
 ((AbstractBeanDefinition) beanDefinition).validate();
 }
 catch (BeanDefinitionValidationException ex) {
 throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(),"Validation of bean definition failed",ex);
 }
 }
 BeanDefinition oldBeanDefinition;
 // 获取缓存中的 beanDefinition
 oldBeanDefinition = this.beanDefinitionMap.get(beanName);
 if (oldBeanDefinition != null) {
 // 如果缓存中存在 判断是否允许覆盖
 if (!isAllowBeanDefinitionOverriding()) {
 throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(),"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 + "]");
 }
 }
 // 如果允许覆盖，保存beanDefinition到beanDefinitionMap中
 this.beanDefinitionMap.put(beanName,beanDefinition);
 }
 else {
 // 判断是否已经开始创建bean
 if (hasBeanCreationStarted()) {
 // Cannot modify startup-time collection elements anymore (for stable iteration)
 synchronized (this.beanDefinitionMap) {
 // 保存beanDefinition到beanDefinitionMap中
 this.beanDefinitionMap.put(beanName,beanDefinition);
 // 更新已经注册的beanName
 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 {
 // 还没开始创建bean
 this.beanDefinitionMap.put(beanName,beanDefinition);
 this.beanDefinitionNames.add(beanName);
 this.manualSingletonNames.remove(beanName);
 }
 this.frozenBeanDefinitionNames = null;
 }
 if (oldBeanDefinition != null || containsSingleton(beanName)) {
 // 重置beanName对应的缓存
 resetBeanDefinition(beanName);
 }
 }
}

public class SimpleAliasRegistry {
 /** Map from alias to canonical name */
 private final Map<String,String> aliasMap = new ConcurrentHashMap<>(16);
 public void registerAlias(String name,String alias) {
 Assert.hasText(name,"'name' must not be empty");
 Assert.hasText(alias,"'alias' must not be empty");
 if (alias.equals(name)) {
  // 如果beanName与alias相同的话不记录alias 并删除对应的alias
  this.aliasMap.remove(alias);
 } else {
  String registeredName = this.aliasMap.get(alias);
  if (registeredName != null) {
  if (registeredName.equals(name)) {
   // 如果别名已经注册过并且指向的name和当前name相同 不做任何处理
   return;
  }
  // 如果alias不允许被覆盖则抛出异常
  if (!allowAliasOverriding()) {
   throw new IllegalStateException("Cannot register alias '" + alias + "' for name '" + name + "': It is already registered for name '" + registeredName + "'.");
  }
  }
  // 校验循环指向依赖 如A->B B->C C->A则出错
  checkForAliasCircle(name,alias);
  this.aliasMap.put(alias,name);
 }
 }
}

protected void checkForAliasCircle(String name,String alias) {
 if (hasAlias(alias,name)) {
 throw new IllegalStateException("Cannot register alias '" + alias +
 "' for name '" + name + "': Circular reference - '" +
 name + "' is a direct or indirect alias for '" + alias + "' already");
 }
}
public boolean hasAlias(String name,String alias) {
 for (Map.Entry<String,String> entry : this.aliasMap.entrySet()) {
 String registeredName = entry.getValue();
 if (registeredName.equals(name)) {
 String registeredAlias = entry.getKey();
 return (registeredAlias.equals(alias) || hasAlias(registeredAlias,alias));
 }
 }
 return false;
}

//DefaultBeanDefinitionDocumentReader.java
protected void processBeanDefinition(Element ele,BeanDefinitionParserDelegate delegate) {
 // Send registration event.
 getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
}

public class DefaultBeanDefinitionDocumentReader {
 protected void processAliasRegistration(Element ele) {
 // 获取 alisa 标签 name 属性
 String name = ele.getAttribute(NAME_ATTRIBUTE);
 // 获取 alisa 标签 alias 属性
 String alias = ele.getAttribute(ALIAS_ATTRIBUTE);
 boolean valid = true;
 if (!StringUtils.hasText(name)) {
  getReaderContext().error("Name must not be empty",ele);
  valid = false;
 } if (!StringUtils.hasText(alias)) {
  getReaderContext().error("Alias must not be empty",ele);
  valid = false;
 }
 if (valid) {
  try {
  // 进行别名注册
  getReaderContext().getRegistry().registerAlias(name,alias);
  } catch (Exception ex) {
  getReaderContext().error("Failed to register alias '" + alias +
   "' for bean with name '" + name + "'",ex);
  }
  // 别名注册后告知监听器做相应处理
  getReaderContext().fireAliasRegistered(name,alias,extractSource(ele));
 }
 }
}

public class DefaultBeanDefinitionDocumentReader {
 protected void importBeanDefinitionResource(Element ele) {
 // 获取import标签的resource属性
 String location = ele.getAttribute(RESOURCE_ATTRIBUTE);
 // 如果不存在则不做任何处理
 if (!StringUtils.hasText(location)) {
  getReaderContext().error("Resource location must not be empty",ele);
  return;
 }
 // 解析占位符属性 格式如"${user.dir}"
 location = getReaderContext().getEnvironment().resolveRequiredPlaceholders(location);
 Set<Resource> actualResources = new LinkedHashSet<>(4);
 // 判断资源是绝对路径还是相对路径
 boolean absoluteLocation = false;
 try {
  absoluteLocation = ResourcePatternUtils.isUrl(location) || ResourceUtils.toURI(location).isAbsolute();
 } catch (URISyntaxException ex) {
  // cannot convert to an URI,considering the location relative
  // unless it is the well-known Spring prefix "classpath*:"
 }
 
 // 如果是绝对路径则直接根据地址加载对应的配置文件
 if (absoluteLocation) {
  try {
  int importCount = getReaderContext().getReader().loadBeanDefinitions(location,actualResources);
  if (logger.isDebugEnabled()) {
   logger.debug("Imported " + importCount + " bean definitions from URL location [" + location + "]");
  }
  } catch (BeanDefinitionStoreException ex) {
  getReaderContext().error("Failed to import bean definitions from URL location [" + location + "]",ex);
  }
 } else {
  try {
  int importCount;
  // 根据相对路径加载资源
  Resource relativeResource = getReaderContext().getResource().createRelative(location);
  if (relativeResource.exists()) {
   importCount = getReaderContext().getReader().loadBeanDefinitions(relativeResource);
   actualResources.add(relativeResource);
  } else {
   String baseLocation = getReaderContext().getResource().getURL().toString();
   importCount = getReaderContext().getReader().loadBeanDefinitions(StringUtils.applyRelativePath(baseLocation,location),actualResources);
  }
  if (logger.isDebugEnabled()) {
   logger.debug("Imported " + importCount + " bean definitions from relative location [" + location + "]");
  }
  } catch (IOException ex) {
  getReaderContext().error("Failed to resolve current resource location",ex);
  } catch (BeanDefinitionStoreException ex) {
  getReaderContext().error("Failed to import bean definitions from relative location [" + location + "]",ex);
  }
 }
 // 解析后进行监听器激活处理
 Resource[] actResArray = actualResources.toArray(new Resource[actualResources.size()]);
 getReaderContext().fireImportProcessed(location,actResArray,extractSource(ele));
 }
}