Spring-IOC-默认元素解析源码分析

parseDefaultElement方法完成了默认标签Bean的解析。

parseDefaultElement主要完成了对import、alias、bean、beans标签的解析，本文仅以bean标签的解析为例。

beans.factory.xml.DefaultBeanDefinitionDocumentReader
    private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
        if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
            importBeanDefinitionResource(ele);
        }
        else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
            processAliasRegistration(ele);
        }
        else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
            processBeanDefinition(ele, delegate);
        }
        else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
            // recurse
            doRegisterBeanDefinitions(ele);
        }
    }

主方法processBeanDefinition流程

    /**
     * Process the given bean element, parsing the bean definition
     * and registering it with the registry.
     */
    protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
        BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
        if (bdHolder != null) {
            bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
            try {
                // Register the final decorated instance.
                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));
        }
    }

步骤说明：

1. 调用传递的BeanDefinitiondelegate类型实例参数delegate的parseBeanDefinitionElement方法进行元素解析，
   然后返回与此元素对应的BeanDefinitionHolder类型的实例bdHolder。
   说明：BeanDefinitionHolder定义比较简单，此类定义对Bean配置文件中配置的属性如：class、name、id、alias等的查询

2. 返回的bdHolder不为null，如果在默认标签节点下含有自定义属性，还需要对自定义标签进行解析。

3. 调用BeanDefinitionReaderUtils静态工具的registerBeanDefinition方法进行注册

4. 发出BeanDefinition注册完成的事件，通知相关的监听器

完成上述4个步骤BeanDefinition已经加载到BeanFactory中了。

其中上述第一步是最重要同时也是也是最复杂的，下面进行解析。

1- 获取信息并封装到bdHolder(BeanDefinitionHolder)

步骤：

1. 提取元素中的id以及name属性
2. 进一步解析其他所有属性并统一封装至GenericBeanDefinition类型的实例中
3. 如果检测到Bean没有指定beanName，那么使用默认规则为此Bean生成beanName
4. 将获取到的信息封装到BeanDefinitionHolder的实例中并返回这个实例

1.1- 提取元素中的id以及name/alias属性

beans.factory.xml.BeanDefinitionParserDelegate
    /**
     * Parses the supplied {@code <bean>} element. May return {@code null}
     * if there were errors during parse. Errors are reported to the
     * {@link org.springframework.beans.factory.parsing.ProblemReporter}.
     */
    public BeanDefinitionHolder parseBeanDefinitionElement(Element ele) {
        return parseBeanDefinitionElement(ele, null);
    }

    /**
     * Parses the supplied {@code <bean>} element. May return {@code null}
     * if there were errors during parse. Errors are reported to the
     * {@link org.springframework.beans.factory.parsing.ProblemReporter}.
     */
    public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, BeanDefinition containingBean) {
        String id = ele.getAttribute(ID_ATTRIBUTE);
        String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);

        List<String> aliases = new ArrayList<String>();
        if (StringUtils.hasLength(nameAttr)) {
            String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, MULTI_VALUE_ATTRIBUTE_DELIMITERS);
            aliases.addAll(Arrays.asList(nameArr));
        }

        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");
            }
        }

        if (containingBean == null) {
            checkNameUniqueness(beanName, aliases, ele);
        }

        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;
                }
            }
            String[] aliasesArray = StringUtils.toStringArray(aliases);
            return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);
        }

        return null;
    }

1.2- 解析其他默认属性

    /**
     * Parse the bean definition itself, without regard to name or aliases. May return
     * {@code null} if problems occurred during the parsing of the bean definition.
     */
    public AbstractBeanDefinition parseBeanDefinitionElement(
            Element ele, String beanName, BeanDefinition containingBean) {

        this.parseState.push(new BeanEntry(beanName));

        String className = null;
        if (ele.hasAttribute(CLASS_ATTRIBUTE)) {
            className = ele.getAttribute(CLASS_ATTRIBUTE).trim();
        }

        try {
            String parent = null;
            if (ele.hasAttribute(PARENT_ATTRIBUTE)) {
                parent = ele.getAttribute(PARENT_ATTRIBUTE);
            }
            AbstractBeanDefinition bd = createBeanDefinition(className, parent);

            parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);
            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;
    }

创建用于属性承载的BeanDefinition

BeanDefinition接口

BeanDefinition接口是配置文件<bean>元素标签在容器中的内部表现形式，<bean>元素含有class、scope
lazy-init等配置属性，BeanDefinition则提供了相应的beanclass、scope、lazyInit属性。综上BeanDefinition和<bean>标签中属性是一一对应的

BeanDefinition接口有三种实现，三种实现都继承了AbstractBeanDefinition

1. RootBeanDefinition：配置文件中可以定义父<bean>元素，父<bean>元素用RootBeanDefinition来表示
2. ChildBeanDefinition：子<bean>元素用ChildBeanDefinition来表示，而没有父<bean>的<bean>就使用RootBeanDefinition来表示
3. GenericBeanDefinition：2.5加入的bean配置文件定义类，提供了更多的拓展方法。

Spring通过将配置文件中的<bean>元素转换为BeanDefinition（这是Bean在容器中的内部表示），然后将这些BeanDefinition注册到BeanDefinitionRegistry中，BeanDefinitionRegistry就像是Spring配置信息的内存数据库，主要是以map的形式保存，后续操作直接从BeanDefinitionRegistry中读取配置信息，而不用重复加载Bean的配置文件。

1.2.1 创建用于Bean信息承载的BeanDefinition
    /**
     * Create a bean definition for the given class name and parent name.
     * @param className the name of the bean class
     * @param parentName the name of the bean's parent bean
     * @return the newly created bean definition
     * @throws ClassNotFoundException if bean class resolution was attempted but failed
     */
    protected AbstractBeanDefinition createBeanDefinition(String className, String parentName)
            throws ClassNotFoundException {

        return BeanDefinitionReaderUtils.createBeanDefinition(
                parentName, className, this.readerContext.getBeanClassLoader());
    }

beans.factory.support.BeanDefinitionReaderUtils 
    /**
     * Create a new GenericBeanDefinition for the given parent name and class name,
     * eagerly loading the bean class if a ClassLoader has been specified.
     * @param parentName the name of the parent bean, if any
     * @param className the name of the bean class, if any
     * @param classLoader the ClassLoader to use for loading bean classes
     * (can be {@code null} to just register bean classes by name)
     * @return the bean definition
     * @throws ClassNotFoundException if the bean class could not be loaded
     */
    public static AbstractBeanDefinition createBeanDefinition(
            String parentName, String className, ClassLoader classLoader) throws ClassNotFoundException {

        GenericBeanDefinition bd = new GenericBeanDefinition();
        bd.setParentName(parentName);
        if (className != null) {
            if (classLoader != null) {
                bd.setBeanClass(ClassUtils.forName(className, classLoader));
            }
            else {
                bd.setBeanClassName(className);
            }
        }
        return bd;
    }

解析属性

• meta子元素作用及使用
• lookup-method子元素的使用
• replaced-method子元素的使用
• constructor-arg子元素的使用
  ... 略

1.2.2 解析bean元素的属性
beans.factory.xml.BeanDefinitionParserDelegate
    /**
     * Apply the attributes of the given bean element to the given bean * definition.
     * @param ele bean declaration element
     * @param beanName bean name
     * @param containingBean containing bean definition
     * @return a bean definition initialized according to the bean element attributes
     */
    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());
        }

        if (ele.hasAttribute(ABSTRACT_ATTRIBUTE)) {
            bd.setAbstract(TRUE_VALUE.equals(ele.getAttribute(ABSTRACT_ATTRIBUTE)));
        }

        String lazyInit = ele.getAttribute(LAZY_INIT_ATTRIBUTE);
        if (DEFAULT_VALUE.equals(lazyInit)) {
            lazyInit = this.defaults.getLazyInit();
        }
        bd.setLazyInit(TRUE_VALUE.equals(lazyInit));

        String autowire = ele.getAttribute(AUTOWIRE_ATTRIBUTE);
        bd.setAutowireMode(getAutowireMode(autowire));

        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));
        }

        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));
        }

        if (ele.hasAttribute(PRIMARY_ATTRIBUTE)) {
            bd.setPrimary(TRUE_VALUE.equals(ele.getAttribute(PRIMARY_ATTRIBUTE)));
        }

        if (ele.hasAttribute(INIT_METHOD_ATTRIBUTE)) {
            String initMethodName = ele.getAttribute(INIT_METHOD_ATTRIBUTE);
            if (!"".equals(initMethodName)) {
                bd.setInitMethodName(initMethodName);
            }
        }
        else {
            if (this.defaults.getInitMethod() != null) {
                bd.setInitMethodName(this.defaults.getInitMethod());
                bd.setEnforceInitMethod(false);
            }
        }

        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);
            }
        }

        if (ele.hasAttribute(FACTORY_METHOD_ATTRIBUTE)) {
            bd.setFactoryMethodName(ele.getAttribute(FACTORY_METHOD_ATTRIBUTE));
        }
        if (ele.hasAttribute(FACTORY_BEAN_ATTRIBUTE)) {
            bd.setFactoryBeanName(ele.getAttribute(FACTORY_BEAN_ATTRIBUTE));
        }

        return bd;
    }

1.2.3 解析meta子元素
    public void parseMetaElements(Element ele, BeanMetadataAttributeAccessor attributeAccessor) {
        NodeList nl = ele.getChildNodes();
        for (int i = 0; i < nl.getLength(); i++) {
            Node node = nl.item(i);
            if (isCandidateElement(node) && nodeNameEquals(node, META_ELEMENT)) {
                Element metaElement = (Element) node;
                String key = metaElement.getAttribute(KEY_ATTRIBUTE);
                String value = metaElement.getAttribute(VALUE_ATTRIBUTE);
                BeanMetadataAttribute attribute = new BeanMetadataAttribute(key, value);
                attribute.setSource(extractSource(metaElement));
                attributeAccessor.addMetadataAttribute(attribute);
            }
        }
    }

1.2.4 解析lookup-method子元素
    /**
     * Parse lookup-override sub-elements of the given bean element.
     */
    public void parseLookupOverrideSubElements(Element beanEle, MethodOverrides overrides) {
        NodeList nl = beanEle.getChildNodes();
        for (int i = 0; i < nl.getLength(); i++) {
            Node node = nl.item(i);
            if (isCandidateElement(node) && nodeNameEquals(node, LOOKUP_METHOD_ELEMENT)) {
                Element ele = (Element) node;
                String methodName = ele.getAttribute(NAME_ATTRIBUTE);
                String beanRef = ele.getAttribute(BEAN_ELEMENT);
                LookupOverride override = new LookupOverride(methodName, beanRef);
                override.setSource(extractSource(ele));
                overrides.addOverride(override);
            }
        }
    }

1.2.5 解析replaced-method子元素
    /**
     * Parse replaced-method sub-elements of the given bean element.
     */
    public void parseReplacedMethodSubElements(Element beanEle, MethodOverrides overrides) {
        NodeList nl = beanEle.getChildNodes();
        for (int i = 0; i < nl.getLength(); i++) {
            Node node = nl.item(i);
            if (isCandidateElement(node) && nodeNameEquals(node, REPLACED_METHOD_ELEMENT)) {
                Element replacedMethodEle = (Element) node;
                String name = replacedMethodEle.getAttribute(NAME_ATTRIBUTE);
                String callback = replacedMethodEle.getAttribute(REPLACER_ATTRIBUTE);
                ReplaceOverride replaceOverride = new ReplaceOverride(name, callback);
                // Look for arg-type match elements.
                List<Element> argTypeEles = DomUtils.getChildElementsByTagName(replacedMethodEle, ARG_TYPE_ELEMENT);
                for (Element argTypeEle : argTypeEles) {
                    String match = argTypeEle.getAttribute(ARG_TYPE_MATCH_ATTRIBUTE);
                    match = (StringUtils.hasText(match) ? match : DomUtils.getTextValue(argTypeEle));
                    if (StringUtils.hasText(match)) {
                        replaceOverride.addTypeIdentifier(match);
                    }
                }
                replaceOverride.setSource(extractSource(replacedMethodEle));
                overrides.addOverride(replaceOverride);
            }
        }
    }