Spring中Bean解析:从DOM对象到Bean的定义对象

1. Bean的定义对象

Spring中Bean的定义对象，由以下接口BeanDefinition来定义，其代码如下：

http://www.easysb.cn/2019/04/275.html

public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {

    /**
     * Scope identifier for the standard singleton scope: "singleton".
     * <p>Note that extended bean factories might support further scopes.
     * @see #setScope
     */
    String SCOPE_SINGLETON = ConfigurableBeanFactory.SCOPE_SINGLETON;

    /**
     * Scope identifier for the standard prototype scope: "prototype".
     * <p>Note that extended bean factories might support further scopes.
     * @see #setScope
     */
    String SCOPE_PROTOTYPE = ConfigurableBeanFactory.SCOPE_PROTOTYPE;


    /**
     * Role hint indicating that a {@code BeanDefinition} is a major part
     * of the application. Typically corresponds to a user-defined bean.
     */
    int ROLE_APPLICATION = 0;

    /**
     * Role hint indicating that a {@code BeanDefinition} is a supporting
     * part of some larger configuration, typically an outer
     * {@link org.springframework.beans.factory.parsing.ComponentDefinition}.
     * {@code SUPPORT} beans are considered important enough to be aware
     * of when looking more closely at a particular
     * {@link org.springframework.beans.factory.parsing.ComponentDefinition},
     * but not when looking at the overall configuration of an application.
     */
    int ROLE_SUPPORT = 1;

    /**
     * Role hint indicating that a {@code BeanDefinition} is providing an
     * entirely background role and has no relevance to the end-user. This hint is
     * used when registering beans that are completely part of the internal workings
     * of a {@link org.springframework.beans.factory.parsing.ComponentDefinition}.
     */
    int ROLE_INFRASTRUCTURE = 2;


    /**
     * Return the name of the parent definition of this bean definition, if any.
     */
    String getParentName();

    /**
     * Set the name of the parent definition of this bean definition, if any.
     */
    void setParentName(String parentName);

    /**
     * Return the current bean class name of this bean definition.
     * <p>Note that this does not have to be the actual class name used at runtime, in
     * case of a child definition overriding/inheriting the class name from its parent.
     * Hence, do <i>not</i> consider this to be the definitive bean type at runtime but
     * rather only use it for parsing purposes at the individual bean definition level.
     */
    String getBeanClassName();

    /**
     * Override the bean class name of this bean definition.
     * <p>The class name can be modified during bean factory post-processing,
     * typically replacing the original class name with a parsed variant of it.
     */
    void setBeanClassName(String beanClassName);

    /**
     * Return the factory bean name, if any.
     */
    String getFactoryBeanName();

    /**
     * Specify the factory bean to use, if any.
     */
    void setFactoryBeanName(String factoryBeanName);

    /**
     * Return a factory method, if any.
     */
    String getFactoryMethodName();

    /**
     * Specify a factory method, if any. This method will be invoked with
     * constructor arguments, or with no arguments if none are specified.
     * The method will be invoked on the specified factory bean, if any,
     * or otherwise as a static method on the local bean class.
     * @param factoryMethodName static factory method name,
     * or {@code null} if normal constructor creation should be used
     * @see #getBeanClassName()
     */
    void setFactoryMethodName(String factoryMethodName);

    /**
     * Return the name of the current target scope for this bean,
     * or {@code null} if not known yet.
     */
    String getScope();

    /**
     * Override the target scope of this bean, specifying a new scope name.
     * @see #SCOPE_SINGLETON
     * @see #SCOPE_PROTOTYPE
     */
    void setScope(String scope);

    /**
     * Return whether this bean should be lazily initialized, i.e. not
     * eagerly instantiated on startup. Only applicable to a singleton bean.
     */
    boolean isLazyInit();

    /**
     * Set whether this bean should be lazily initialized.
     * <p>If {@code false}, the bean will get instantiated on startup by bean
     * factories that perform eager initialization of singletons.
     */
    void setLazyInit(boolean lazyInit);

    /**
     * Return the bean names that this bean depends on.
     */
    String[] getDependsOn();

    /**
     * Set the names of the beans that this bean depends on being initialized.
     * The bean factory will guarantee that these beans get initialized first.
     */
    void setDependsOn(String... dependsOn);

    /**
     * Return whether this bean is a candidate for getting autowired into some other bean.
     */
    boolean isAutowireCandidate();

    /**
     * Set whether this bean is a candidate for getting autowired into some other bean.
     */
    void setAutowireCandidate(boolean autowireCandidate);

    /**
     * Return whether this bean is a primary autowire candidate.
     * If this value is true for exactly one bean among multiple
     * matching candidates, it will serve as a tie-breaker.
     */
    boolean isPrimary();

    /**
     * Set whether this bean is a primary autowire candidate.
     * <p>If this value is true for exactly one bean among multiple
     * matching candidates, it will serve as a tie-breaker.
     */
    void setPrimary(boolean primary);


    /**
     * Return the constructor argument values for this bean.
     * <p>The returned instance can be modified during bean factory post-processing.
     * @return the ConstructorArgumentValues object (never {@code null})
     */
    ConstructorArgumentValues getConstructorArgumentValues();

    /**
     * Return the property values to be applied to a new instance of the bean.
     * <p>The returned instance can be modified during bean factory post-processing.
     * @return the MutablePropertyValues object (never {@code null})
     */
    MutablePropertyValues getPropertyValues();


    /**
     * Return whether this a <b>Singleton</b>, with a single, shared instance
     * returned on all calls.
     * @see #SCOPE_SINGLETON
     */
    boolean isSingleton();

    /**
     * Return whether this a <b>Prototype</b>, with an independent instance
     * returned for each call.
     * @see #SCOPE_PROTOTYPE
     */
    boolean isPrototype();

    /**
     * Return whether this bean is "abstract", that is, not meant to be instantiated.
     */
    boolean isAbstract();

    /**
     * Get the role hint for this {@code BeanDefinition}. The role hint
     * provides the frameworks as well as tools with an indication of
     * the role and importance of a particular {@code BeanDefinition}.
     * @see #ROLE_APPLICATION
     * @see #ROLE_SUPPORT
     * @see #ROLE_INFRASTRUCTURE
     */
    int getRole();

    /**
     * Return a human-readable description of this bean definition.
     */
    String getDescription();

    /**
     * Return a description of the resource that this bean definition
     * came from (for the purpose of showing context in case of errors).
     */
    String getResourceDescription();

    /**
     * Return the originating BeanDefinition, or {@code null} if none.
     * Allows for retrieving the decorated bean definition, if any.
     * <p>Note that this method returns the immediate originator. Iterate through the
     * originator chain to find the original BeanDefinition as defined by the user.
     */
    BeanDefinition getOriginatingBeanDefinition();

}

2. 元素解析成Bean定义对象

Bean的XML配置等多种形式，都会由 BeanDefinitionParserDelegate.parseBeanDefinitionElement 进行解析，其流程如下：

public BeanDefinitionHolder parseBeanDefinitionElement(Element ele) {
        return parseBeanDefinitionElement(ele, null);
    }


public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, BeanDefinition containingBean) {
        // 获取id属性
        String id = ele.getAttribute(ID_ATTRIBUTE);
        // 获取name属性
        String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);

        List<String> aliases = new ArrayList<String>();
        if (StringUtils.hasLength(nameAttr)) {
            // 对所有的别名进行解析，以逗号，分号，空格隔开的别名列表
            String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, MULTI_VALUE_ATTRIBUTE_DELIMITERS);
            aliases.addAll(Arrays.asList(nameArr));
        }

        String beanName = id;
        if (!StringUtils.hasText(beanName) && !aliases.isEmpty()) {
            // 如果没有id属性，则以第一个别名作为id本命
            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) {
            如果没有子bean直接检测本名、别名是否有重复
            checkNameUniqueness(beanName, aliases, ele);
        }
        // 对Bean的更多属性进行解析
        AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);
        if (beanDefinition != null) {
            // 如果bean没有定义id，name
            if (!StringUtils.hasText(beanName)) {
                try {
                    if (containingBean != null) {
                        // 假如包含子bean，那么会生成一个唯一name
                        beanName = BeanDefinitionReaderUtils.generateBeanName(
                                beanDefinition, this.readerContext.getRegistry(), true);
                    }
                    else {
                        // 如果不包含子bean，则由context来生成唯一name
                        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)) {
                            // 向上兼容，增加对应的class名字别名
                            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);
            // 构建一个包含bean定义的holder对象
            return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);
        }

        return null;
    }

上面的检测本名、别名的唯一性的函数 checkNameUniqueness 实现如下，

protected void checkNameUniqueness(String beanName, List<String> aliases, Element beanElement) {
        String foundName = null;

        // 检查本名是否冲突
        if (StringUtils.hasText(beanName) && this.usedNames.contains(beanName)) {
            foundName = beanName;
        }
        // 检查别名是否冲突
        if (foundName == null) {
            foundName = CollectionUtils.findFirstMatch(this.usedNames, aliases);
        }
        if (foundName != null) {
            error("Bean name '" + foundName + "' is already used in this <beans> element", beanElement);
        }
        // 如果没有冲突则直接将现有的加入到集合中
        this.usedNames.add(beanName);
        this.usedNames.addAll(aliases);
    }

接下来再详细了解下 parseBeanDefinitionElement 是如何将DOM元素解析成Bean的定义对象的:

public AbstractBeanDefinition parseBeanDefinitionElement(
            Element ele, String beanName, BeanDefinition containingBean) {

        // 记录解析的对象本名
        this.parseState.push(new BeanEntry(beanName));

        String className = null;
        // 解析是否含有class属性，记录类名
        if (ele.hasAttribute(CLASS_ATTRIBUTE)) {
            className = ele.getAttribute(CLASS_ATTRIBUTE).trim();
        }

        try {
            String parent = null;
            // 解析是否含有parent属性，记录下来
            if (ele.hasAttribute(PARENT_ATTRIBUTE)) {
                parent = ele.getAttribute(PARENT_ATTRIBUTE);
            }
            // 根据类名和父bean构建一个bean定义结构对象
            AbstractBeanDefinition bd = createBeanDefinition(className, parent);

            // 解析Bean的其他定义的属性
            parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);
            // 获取bean的description描述信息
            bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));
            // 对meta属性进行解析
            parseMetaElements(ele, bd);
            // 对lookup-method属性解析
            parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
            // 对replace-method属性解析
            parseReplacedMethodSubElements(ele, bd.getMethodOverrides());
            // 解析构造函数的参数
            parseConstructorArgElements(ele, bd);
            // 解析成员属性
            parsePropertyElements(ele, bd);
            // 解析qulifier属性
            parseQualifierElements(ele, bd);

            // 设置当前bean定义结构的生成的数据来源信息
            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;
    }

parseBeanDefinitionAttributes 是解析Bean定义的各种属性的，请实现如下：

/**
     * 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;
    }

parseMetaElements 是用来解析meta属性，解析出所有的key->value属性值对，其实现如下：

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;
                // 获取key属性和value属性值
                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);
            }
        }
    }

parseLookupOverrideSubElements 解析所有的lookup-method属性，其实现如下：

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);
            }
        }
    }

parseReplacedMethodSubElements 解析所有的replaced-method属性，其实现如下：

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);
            }
        }
    }

parseConstructorArgElements 是解析constructor-arg构造函数的参数信息，其实现如下：

public void parseConstructorArgElements(Element beanEle, BeanDefinition bd) {
        NodeList nl = beanEle.getChildNodes();
        for (int i = 0; i < nl.getLength(); i++) {
            Node node = nl.item(i);
            if (isCandidateElement(node) && nodeNameEquals(node, CONSTRUCTOR_ARG_ELEMENT)) {
                parseConstructorArgElement((Element) node, bd);
            }
        }
    }

parsePropertyElements 是解析property属性，其实现如下：

public void parsePropertyElements(Element beanEle, BeanDefinition bd) {
        NodeList nl = beanEle.getChildNodes();
        for (int i = 0; i < nl.getLength(); i++) {
            Node node = nl.item(i);
            if (isCandidateElement(node) && nodeNameEquals(node, PROPERTY_ELEMENT)) {
                parsePropertyElement((Element) node, bd);
            }
        }
    }

parseQualifierElements 是解析qualifier属性，其实现如下：

public void parseQualifierElements(Element beanEle, AbstractBeanDefinition bd) {
        NodeList nl = beanEle.getChildNodes();
        for (int i = 0; i < nl.getLength(); i++) {
            Node node = nl.item(i);
            if (isCandidateElement(node) && nodeNameEquals(node, QUALIFIER_ELEMENT)) {
                parseQualifierElement((Element) node, bd);
            }
        }
    }

至此，我们可以差不多了解都配置文件由相应的Reader读取后解析成DOM对象后，会由 BeanDefinitionParserDelegate 解析成Bean定义结构对象，为后续的Bean的生成提供了数据源参考。