如何利用Spring把元素解析成BeanDefinition对象

public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {
    
    String SCOPE_SINGLETON = ConfigurableBeanFactory.SCOPE_SINGLETON;   // singleton作用域
    String SCOPE_PROTOTYPE = ConfigurableBeanFactory.SCOPE_PROTOTYPE;   // prototype作用域

    
    int ROLE_APPLICATION = 0;   // 表明BeanDefinition是应用程序的主要部分。通常对应于用户定义的Bean
    int ROLE_SUPPORT = 1;   // 表明BeanDefinition是支持大部分配置。通常是外部的ComponentDefinition
    int ROLE_INFRASTRUCTURE = 2;    // 表明BeanDefinition提供了一个完全后台角色，与最终用户无关。这个提示是在注册bean时使用的，这些bean完全是组件定义的内部工作的一部分
    
    String getParentName();     
    void setParentName(String parentName);
    
    String getBeanClassName();      // 注意：这并不一定是运行时真正的类名，如果有一个子定义覆盖/继承父类的名称。
    void setBeanClassName(String beanClassName);    // 注意：类名可以在bean工厂的后处理器中进行修改，通常用它的解析变量替换原来的类名

    
    String getFactoryBeanName();
    void setFactoryBeanName(String factoryBeanName);
    String getFactoryMethodName();
    void setFactoryMethodName(String factoryMethodName);    // 注意：该方法由指定工厂Bean调用或者作为本地bean类的静态方法

    
    String getScope();
    void setScope(String scope);
    
    boolean isLazyInit();
    void setLazyInit(boolean lazyInit);
    
    String[] getDependsOn();
    void setDependsOn(String... dependsOn);
    
    boolean isAutowireCandidate
    void setAutowireCandidate(boolean autowireCandidate);

    
    boolean isPrimary();
    void setPrimary(boolean primary);

    
    ConstructorArgumentValues getConstructorArgumentValues();
    
    MutablePropertyValues getPropertyValues();
    
    boolean isSingleton();
    boolean isPrototype();
    boolean isAbstract();
    
    int getRole();
    
    String getDescription();
    String getResourceDescription();
    
    BeanDefinition getOriginatingBeanDefinition();
}

	
	public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, BeanDefinition containingBean) {
		//获取id
		String id = ele.getAttribute(ID_ATTRIBUTE);
		//获取name属性；例如：<bean id="simpleProducer" name="aliases1,;aliases2,;aliases3" > 这样就设置了多个别名了
		String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);
		List<String> aliases = new ArrayList<String>();
		if (StringUtils.hasLength(nameAttr)) {
		//解析别名；上面的例子就解析成了  三个别名了aliases1,;aliases2,;aliases3
			String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, MULTI_VALUE_ATTRIBUTE_DELIMITERS);
			aliases.addAll(Arrays.asList(nameArr));
		}
		String beanName = id;
		//如果没有设置id，但是设置了name ；那么就把name别名中的第一个当做 bean的名字；并且从别名中remove掉
		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) {
			
			checkNameUniqueness(beanName, aliases, ele);
		}
		
		AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);
		if (beanDefinition != null) {
			//如果没有设置 id 也没有设置name属性；则spring生成一个beanName
			if (!StringUtils.hasText(beanName)) {
				try {
					//TODO... 这个containingBean还不知道是干啥用的
					if (containingBean != null) {
						//自动生成beanName;生成规则我就不赘述了
						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);
			//
			
			return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);
		}

		return null;
	}

	
	public AbstractBeanDefinition parseBeanDefinitionElement(
			Element ele, String beanName, BeanDefinition containingBean) {
		
		this.parseState.push(new BeanEntry(beanName));
		//获取element中有 class属性；
		String className = null;
		if (ele.hasAttribute(CLASS_ATTRIBUTE)) {
			className = ele.getAttribute(CLASS_ATTRIBUTE).trim();
		}

		try {
			//获取parent属性
			String parent = null;
			if (ele.hasAttribute(PARENT_ATTRIBUTE)) {
				parent = ele.getAttribute(PARENT_ATTRIBUTE);
			}
			
			AbstractBeanDefinition bd = createBeanDefinition(className, parent);
			
			parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);
			//设置description值；例如<description>test</description>
			bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));
			
			parseMetaElements(ele, bd);
			
			parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
			
			parseReplacedMethodSubElements(ele, bd.getMethodOverrides());
			//
			
			parseConstructorArgElements(ele, bd);
			
			parsePropertyElements(ele, bd);
			
			parseQualifierElements(ele, bd);

			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;
	}

	 //解析bean的属性值
	public AbstractBeanDefinition parseBeanDefinitionAttributes(Element ele, String beanName,
			BeanDefinition containingBean, AbstractBeanDefinition bd) {
		
		if (ele.hasAttribute(SINGLETON_ATTRIBUTE)) {
			error("Old 1.x 'singleton' attribute in use - upgrade to 'scope' declaration", ele);
		}
		
		else if (ele.hasAttribute(SCOPE_ATTRIBUTE)) {
			bd.setScope(ele.getAttribute(SCOPE_ATTRIBUTE));
		}
		
		else if (containingBean != null) {
			// Take default from containing bean in case of an inner bean definition.
			bd.setScope(containingBean.getScope());
		}
		//设置 abstract属性，默认是 false
		if (ele.hasAttribute(ABSTRACT_ATTRIBUTE)) {
			bd.setAbstract(TRUE_VALUE.equals(ele.getAttribute(ABSTRACT_ATTRIBUTE)));
		}
		//如果当前元素没有设置 lazyInit 懒加载；则去 this.defaults.getLazyInit()；这个defaults是上一篇分析过的；整个xml文件全局的默认值；
		String lazyInit = ele.getAttribute(LAZY_INIT_ATTRIBUTE);
		if (DEFAULT_VALUE.equals(lazyInit)) {
			lazyInit = this.defaults.getLazyInit();
		}
		bd.setLazyInit(TRUE_VALUE.equals(lazyInit));
		// 设置注入方式，有 byName byType 等等，如果当前没有设置，则取this.defaults.getAutowire()的值；
		//然后将其转换为对应的int类型；例如byName是int AUTOWIRE_BY_NAME = 1;
		String autowire = ele.getAttribute(AUTOWIRE_ATTRIBUTE);
		bd.setAutowireMode(getAutowireMode(autowire));
		//这个dependency-check字面意义是依赖检查的类型；具体一会分析；如果当前没有设置，则取this.defaults.getDependencyCheck()的值；TODO...
		String dependencyCheck = ele.getAttribute(DEPENDENCY_CHECK_ATTRIBUTE);
		bd.setDependencyCheck(getDependencyCheck(dependencyCheck));
		
		if (ele.hasAttribute(DEPENDS_ON_ATTRIBUTE)) {
			String dependsOn = ele.getAttribute(DEPENDS_ON_ATTRIBUTE);
			bd.setDependsOn(StringUtils.tokenizeToStringArray(dependsOn, MULTI_VALUE_ATTRIBUTE_DELIMITERS));
		}
		//这个autowireCandidate不是很清楚  回头分析TODo..
		String autowireCandidate = ele.getAttribute(AUTOWIRE_CANDIDATE_ATTRIBUTE);
		if ("".equals(autowireCandidate) || DEFAULT_VALUE.equals(autowireCandidate)) {
			String candidatePattern = this.defaults.getAutowireCandidates();
			if (candidatePattern != null) {
				String[] patterns = StringUtils.commaDelimitedListToStringArray(candidatePattern);
				bd.setAutowireCandidate(PatternMatchUtils.simpleMatch(patterns, beanName));
			}
		}
		else {
			bd.setAutowireCandidate(TRUE_VALUE.equals(autowireCandidate));
		}
		//设置primary 装配时当出现多个Bean候选者时，被标记为Primary的Bean将作为首选者，否则将抛出异常 ；默认为false
		//https://blog.csdn.net/qq_16055765/article/details/78833260
		if (ele.hasAttribute(PRIMARY_ATTRIBUTE)) {
			bd.setPrimary(TRUE_VALUE.equals(ele.getAttribute(PRIMARY_ATTRIBUTE)));
		}
		//设置初始化方法
		if (ele.hasAttribute(INIT_METHOD_ATTRIBUTE)) {
			String initMethodName = ele.getAttribute(INIT_METHOD_ATTRIBUTE);
			if (!"".equals(initMethodName)) {
				bd.setInitMethodName(initMethodName);
			}
		}
		else {
			//如果 当前元素没有设置 init_method 属性，则判断 xml全局配置有没有设置；this.defaults是全局设置
			if (this.defaults.getInitMethod() != null) {
				bd.setInitMethodName(this.defaults.getInitMethod());
				bd.setEnforceInitMethod(false);
			}
		}
		//与上面初始化无二
		if (ele.hasAttribute(DESTROY_METHOD_ATTRIBUTE)) {
			String destroyMethodName = ele.getAttribute(DESTROY_METHOD_ATTRIBUTE);
			bd.setDestroyMethodName(destroyMethodName);
		}
		else {
			if (this.defaults.getDestroyMethod() != null) {
				bd.setDestroyMethodName(this.defaults.getDestroyMethod());
				bd.setEnforceDestroyMethod(false);
			}
		}
		//设置factoryMethodName； spring通过工厂方法配置Bean
		//Https://blog.csdn.net/mrwuyi/article/details/51578654
		if (ele.hasAttribute(FACTORY_METHOD_ATTRIBUTE)) {
			bd.setFactoryMethodName(ele.getAttribute(FACTORY_METHOD_ATTRIBUTE));
		}
		//设置factoryBeanName   spring通过工厂方法配置Bean
		//https://blog.csdn.net/mrwuyi/article/details/51578654
		if (ele.hasAttribute(FACTORY_BEAN_ATTRIBUTE)) {
			bd.setFactoryBeanName(ele.getAttribute(FACTORY_BEAN_ATTRIBUTE));
		}

		return bd;
	}

	
	public void parseConstructorArgElement(Element ele, BeanDefinition bd) {
		String indexAttr = ele.getAttribute(INDEX_ATTRIBUTE);
		String typeAttr = ele.getAttribute(TYPE_ATTRIBUTE);
		String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);
		//如果xml配置了index
		if (StringUtils.hasLength(indexAttr)) {
			try {
				int index = Integer.parseInt(indexAttr);
				if (index < 0) {
					error("'index' cannot be lower than 0", ele);
				}
				else {
					try {
						
						this.parseState.push(new ConstructorArgumentEntry(index));
							
						Object value = parsePropertyValue(ele, bd, null);
						//新建一个ValueHolder实例，这个专门存放构造参数的一些 属性值的
						ConstructorArgumentValues.ValueHolder valueHolder = new ConstructorArgumentValues.ValueHolder(value);
						//设置类型
						if (StringUtils.hasLength(typeAttr)) {
							valueHolder.setType(typeAttr);
						}
						//设置name
						if (StringUtils.hasLength(nameAttr)) {
							valueHolder.setName(nameAttr);
						}
						valueHolder.setSource(extractSource(ele));
						//验证index是否设置正确
						if (bd.getConstructorArgumentValues().hasIndexedArgumentValue(index)) {
							error("Ambiguous constructor-arg entries for index " + index, ele);
						}
						else {
						//ConstructorArgumentValues有个 Map<Integer, ValueHolder> indexedArgumentValues = new LinkedHashMap<Integer, ValueHolder>(0)来hold参数
							bd.getConstructorArgumentValues().addIndexedArgumentValue(index, valueHolder);
						}
					}
					finally {
						//出栈；简单的栈结构来跟踪逻辑位置中解析过程，主要是记录异常信息的吧
						this.parseState.pop();
					}
				}
			}
			catch (NumberFORMatException ex) {
				error("Attribute 'index' of tag 'constructor-arg' must be an integer", ele);
			}
		}
		else {
			try {
				//与上无二
				this.parseState.push(new ConstructorArgumentEntry());
				Object value = parsePropertyValue(ele, bd, null);
				ConstructorArgumentValues.ValueHolder valueHolder = new ConstructorArgumentValues.ValueHolder(value);
				if (StringUtils.hasLength(typeAttr)) {
					valueHolder.setType(typeAttr);
				}
				if (StringUtils.hasLength(nameAttr)) {
					valueHolder.setName(nameAttr);
				}
				valueHolder.setSource(extractSource(ele));
				bd.getConstructorArgumentValues().addGenericArgumentValue(valueHolder);
			}
			finally {
				this.parseState.pop();
			}
		}
	}

    <bean class="src.HelloWorld">
    <property name="beanA" ref="beanA" />
    <property name="name" value="姓名" />
    <constructor-arg name="a" value="1" />
    <constructor-arg name="list">
      <list>
        <value>1</value>
        <value>2</value>
        <value>3</value>
      </list>
    </constructor-arg>
    <meta key="meta1" value="meta1value"/>
  </bean>

public Object parsePropertyValue(Element ele, BeanDefinition bd, String propertyName) {
		String elementName = (propertyName != null) ?
						"<property> element for property '" + propertyName + "'" :
						"<constructor-arg> element";

		// 最多只能有其中一个元素：ref，value,子元素(ref，value,list,set array 等等形式的元素) 等等
		
		nodeList nl = ele.getChildNodes();
		Element subElement = null;
		
		for (int i = 0; i < nl.getLength(); i++) {
			Node node = nl.item(i);
			if (node instanceof Element && !nodeNameEquals(node, DESCRIPTION_ELEMENT) &&
					!nodeNameEquals(node, META_ELEMENT)) {
				// Child element is what we're looking for.
				if (subElement != null) {
					error(elementName + " must not contain more than one sub-element", ele);
				}
				else {
					subElement = (Element) node;
				}
			}
		}
		
		boolean hasRefAttribute = ele.hasAttribute(REF_ATTRIBUTE);
		boolean hasValueAttribute = ele.hasAttribute(VALUE_ATTRIBUTE);
		//1.如果 ref 和 value都存在  抛异常
		//2.如果ref或者value存在 一个并且 subElement也存在  抛异常
		if ((hasRefAttribute && hasValueAttribute) ||
				((hasRefAttribute || hasValueAttribute) && subElement != null)) {
			error(elementName +
					" is only allowed to contain either 'ref' attribute OR 'value' attribute OR sub-element", ele);
		}

//如果是ref；返回RuntimeBeanReference implements BeanReference；有beanName，toParent，source等属性
		if (hasRefAttribute) {
			String refName = ele.getAttribute(REF_ATTRIBUTE);
			if (!StringUtils.hasText(refName)) {
				error(elementName + " contains empty 'ref' attribute", ele);
			}
			RuntimeBeanReference ref = new RuntimeBeanReference(refName);
			ref.setSource(extractSource(ele));
			return ref;
		}
		//如果是value；返回TypedStringValue；设置属性 value和source 
		else if (hasValueAttribute) {
			TypedStringValue valueHolder = new TypedStringValue(ele.getAttribute(VALUE_ATTRIBUTE));
			valueHolder.setSource(extractSource(ele));
			return valueHolder;
		}
		//如果是list；解析一下子元素
		else if (subElement != null) {
			return parsePropertySubElement(subElement, bd);
		}
		else {
			// Neither child element nor "ref" or "value" attribute found.
			error(elementName + " must specify a ref or value", ele);
			return null;
		}
	}
//解析子元素
public Object parsePropertySubElement(Element ele, BeanDefinition bd, String defaultValueType) {
		//如果不是默认的命名空间，则最终调用public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) 来解析；
		//第一篇文章分析过 根据元素选择不同的解析器来解析；
		if (!isDefaultNamespace(ele)) {
			return parseNestedCustomElement(ele, bd);
		}
		//解析<bean>元素；这里就是我们本文正在分析的方法，所以不概述了
		else if (nodeNameEquals(ele, BEAN_ELEMENT)) {
			BeanDefinitionHolder nestedBd = parseBeanDefinitionElement(ele, bd);
			if (nestedBd != null) {
				nestedBd = decorateBeanDefinitionIfRequired(ele, nestedBd, bd);
			}
			return nestedBd;
		}
		//如果是ref 元素 RuntimeBeanReference
		else if (nodeNameEquals(ele, REF_ELEMENT)) {
			// A generic reference to any name of any bean.
			String refName = ele.getAttribute(BEAN_REF_ATTRIBUTE);
			boolean toParent = false;
			if (!StringUtils.hasLength(refName)) {
				// A reference to the id of another bean in the same XML file.
				refName = ele.getAttribute(LOCAL_REF_ATTRIBUTE);
				if (!StringUtils.hasLength(refName)) {
					// A reference to the id of another bean in a parent context.
					refName = ele.getAttribute(PARENT_REF_ATTRIBUTE);
					toParent = true;
					if (!StringUtils.hasLength(refName)) {
						error("'bean', 'local' or 'parent' is required for <ref> element", ele);
						return null;
					}
				}
			}
			if (!StringUtils.hasText(refName)) {
				error("<ref> element contains empty target attribute", ele);
				return null;
			}
			RuntimeBeanReference ref = new RuntimeBeanReference(refName, toParent);
			ref.setSource(extractSource(ele));
			return ref;
		}
		//解析idref
		else if (nodeNameEquals(ele, IDREF_ELEMENT)) {
			return parseIdRefElement(ele);
		}
		//解析value
		else if (nodeNameEquals(ele, VALUE_ELEMENT)) {
			
			return parseValueElement(ele, defaultValueType);
		}
		//解析null
		else if (nodeNameEquals(ele, NULL_ELEMENT)) {
			// It's a distinguished null value. Let's wrap it in a TypedStringValue
			// object in order to preserve the source location.
			TypedStringValue nullHolder = new TypedStringValue(null);
			nullHolder.setSource(extractSource(ele));
			return nullHolder;
		}
		//解析array
		else if (nodeNameEquals(ele, ARRAY_ELEMENT)) {
			return parseArrayElement(ele, bd);
		}
		//list
		else if (nodeNameEquals(ele, LIST_ELEMENT)) {
			return parseListElement(ele, bd);
		}
		//set
		else if (nodeNameEquals(ele, SET_ELEMENT)) {
			return parseSetElement(ele, bd);
		}
		//map
		else if (nodeNameEquals(ele, MAP_ELEMENT)) {
			return parseMapElement(ele, bd);
		}
		//props
		else if (nodeNameEquals(ele, PROPS_ELEMENT)) {
			return parsePropsElement(ele);
		}
		else {
			error("Unknown property sub-element: [" + ele.getNodeName() + "]", ele);
			return null;
		}
	}