Spring Ioc 源码分析(四)--parseBeanDefinitions()与BeanDefinitionParserDelegate

上一篇博客说到,BeanDefinition的解析,已经走到了DefaultBeanDefinitionDocumentR
eader里,这时候配置文件已经被加载,并解析成w3c的Document对象。这篇博客就接着介绍,DefaultBeanDefinitionDocumentReader和BeanDefinitionParserDelegate类,是怎么协同完成bean的解析和注册的。

        BeanDefinitionParserDelegate parent = this.delegate;
        this.delegate = createHelper(readerContext, root, parent);
        preProcessXml(root);
        parseBeanDefinitions(root, this.delegate);
        postProcessXml(root);
        this.delegate = parent;

这段代码,创建了一个BeanDefinitionParserDelegate组件,然后就是preProcessXml()、parseBeanDefinitions()、postProcessXml()方法
其中preProcessXml()和postProcessXml()默认是空方法,接下来就看下parseBeanDefinitions()方法

protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
        if (delegate.isDefaultNamespace(root)) {
            NodeList nl = root.getChildNodes();
            for (int i = 0; i < nl.getLength(); i++) {
                Node node = nl.item(i);
                if (node instanceof Element) {
                    Element ele = (Element) node;
                    if (delegate.isDefaultNamespace(ele)) {
                        parseDefaultElement(ele, delegate);
                    }
                    else {
                        delegate.parseCustomElement(ele);
                    }
                }
            }
        }
        else {
            delegate.parseCustomElement(root);
        }
    }

从这个方法开始,BeanDefinitionParserDelegate就开始发挥作用了,判断当前解析元素是否属于默认的命名空间,如果是的话,就调用parseDefaultElement()方法,否则调用delegate上parseCustomElement()方法

public boolean isDefaultNamespace(String namespaceUri) {
        return (!StringUtils.hasLength(namespaceUri) || BEANS_NAMESPACE_URI.equals(namespaceUri));
    }
    public boolean isDefaultNamespace(Node node) {
        return isDefaultNamespace(getNamespaceURI(node));
    }

只有http://www.springframework.org/schema/beans,会被认为是默认的命名空间。也就是说,beans、bean这些元素,会认为属于默认的命名空间,而像task:scheduled这些,就认为不属于默认命名空间。
根节点beans的一个子节点bean,是属于默认命名空间的,所以会进入parseDefaultElement()方法

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);
        }
    }

这里可能会有4种情况,import、alias、bean、beans,分别有一个方法与之对应,这里解析的是bean元素,所以会进入processBeanDefinition()方法

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));
        }
    }

这里主要有3个步骤,先是委托delegate对bean进行解析,然后委托delegate对bean进行装饰,最后由一个工具类来完成BeanDefinition的注册
可以看出来,DefaultBeanDefinitionDocumentReader不负责任何具体的bean解析,它面向的是xml Document对象,根据其元素的命名空间和名称,起一个类似路由的作用(不过,命名空间的判断,也是委托给delegate来做的)。所以这个类的命名,是比较贴切的,突出了其面向Document的特性。具体的工作,是由BeanDefinitionParserDelegate来完成的
下面就看下parseBeanDefinitionElement()方法

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;
    }

这个方法很长,可以分成三段来看

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);
        }

这一段,主要是处理一些跟alias,id等标识相关的东西

AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);

这一行是核心,进行实际的解析

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);
        }

这段是后置处理,对beanName进行处理
前置处理和后置处理,不是核心,就不细看了,重点看下核心的那一行调用

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;
    }

这个方法也挺长的,拆开看看

this.parseState.push(new BeanEntry(beanName));
        String className = null;
        if (ele.hasAttribute(CLASS_ATTRIBUTE)) {
            className = ele.getAttribute(CLASS_ATTRIBUTE).trim();
        }

这段是从配置中抽取出类名。接下来的长长一段,把异常处理先抛开,看看实际的业务

            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;

这里每个方法的命名,就说明了是要干什么,可以一个个跟进去看,本文就不细说了。总之,经过这里的解析,就得到了一个完整的BeanDefinitionHolder。只是说明一下,如果在配置文件里,没有对一些属性进行设置,比如autowire-candidate等,那么这个解析生成的BeanDefinition,都会得到一个默认值
然后,对这个Bean做一些必要的装饰

public BeanDefinitionHolder decorateBeanDefinitionIfRequired(
            Element ele, BeanDefinitionHolder definitionHolder, BeanDefinition containingBd) {
        BeanDefinitionHolder finalDefinition = definitionHolder;
        // Decorate based on custom attributes first.
        NamedNodeMap attributes = ele.getAttributes();
        for (int i = 0; i < attributes.getLength(); i++) {
            Node node = attributes.item(i);
            finalDefinition = decorateIfRequired(node, finalDefinition, containingBd);
        }
        // Decorate based on custom nested elements.
        NodeList children = ele.getChildNodes();
        for (int i = 0; i < children.getLength(); i++) {
            Node node = children.item(i);
            if (node.getNodeType() == Node.ELEMENT_NODE) {
                finalDefinition = decorateIfRequired(node, finalDefinition, containingBd);
            }
        }
        return finalDefinition;
    }

持续单步调试,代码继续运行到DefaultBeanDefinitionDocumentReader中的processBeanDefinition中的registerBeanDefinition()

BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, 
getReaderContext().getRegistry());

单步进入代码发现BeanDefinitionReaderUtils静态方法registerBeanDefinition()

public static void registerBeanDefinition(
            BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
            throws BeanDefinitionStoreException {
        // Register bean definition under primary name.
        String beanName = definitionHolder.getBeanName();
        // 其实调用的是DefaultListableBeanFactory中的registerBeanDefinition方法
        registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
        // Register aliases for bean name, if any.
        String[] aliases = definitionHolder.getAliases();
        if (aliases != null) {
            for (String aliase : aliases) {
                registry.registerAlias(beanName, aliase);
            }
        }
    }

解释一下其实调用的是DefaultListableBeanFactory中的registerBeanDefinition方法这句话,因为DefaultListableBeanFactory实现BeanDefinitionRegistry接口,BeanDefinitionRegistry接口中定义了registerBeanDefinition()方法
看下DefaultListableBeanFactory中registerBeanDefinition()实例方法的具体实现:

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 {
                ((AbstractBeanDefinition) beanDefinition).validate();
            }
            catch (BeanDefinitionValidationException ex) {
                throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                        "Validation of bean definition failed", ex);
            }
        }
        synchronized (this.beanDefinitionMap) {
            Object oldBeanDefinition = this.beanDefinitionMap.get(beanName);
            if (oldBeanDefinition != null) {
                if (!this.allowBeanDefinitionOverriding) {
                    throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                            "Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
                            "': There is already [" + oldBeanDefinition + "] bound.");
                }
                else {
                    if (this.logger.isInfoEnabled()) {
                        this.logger.info("Overriding bean definition for bean '" + beanName +
                                "': replacing [" + oldBeanDefinition + "] with [" + beanDefinition + "]");
                    }
                }
            }
            else {
                this.beanDefinitionNames.add(beanName);
                this.frozenBeanDefinitionNames = null;
            }
            this.beanDefinitionMap.put(beanName, beanDefinition);
            resetBeanDefinition(beanName);
        }
    }

代码追溯之后发现这个方法里,最关键的是以下2行:

this.beanDefinitionNames.add(beanName);
this.beanDefinitionMap.put(beanName, beanDefinition);

前者是把beanName放到队列里,后者是把BeanDefinition放到map中,到此注册就完成了。在后面实例化的时候,就是把beanDefinitionMap中的BeanDefinition取出来,逐一实例化
BeanFactory准备完毕之后,代码又回到了ClassPathXmlApplicationContext里

public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            prepareRefresh();
            // Tell the subclass to refresh the internal bean factory.
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
            // Prepare the bean factory for use in this context.
            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) {
                // Destroy already created singletons to avoid dangling resources.
                destroyBeans();
                // Reset 'active' flag.
                cancelRefresh(ex);
                // Propagate exception to caller.
                throw ex;
            }
        }
    }

也就是obtainFreshBeanFactory()方法执行之后,再进行下面的步骤。
总结来说,ApplicationContext将解析配置文件的工作委托给BeanDefinitionReader,然后BeanDefinitionReader将配置文件读取为xml的Document文档之后,又委托给BeanDefinitionDocumentReader
BeanDefinitionDocumentReader这个组件是根据xml元素的命名空间和元素名,起到一个路由的作用,实际的解析工作,是委托给BeanDefinitionParserDelegate来完成的
BeanDefinitionParserDelegate的解析工作完成以后,会返回BeanDefinitionHolder给BeanDefinitionDocumentReader,在这里,会委托给DefaultListableBeanFactory完成bean的注册
XmlBeanDefinitionReader(计数、解析XML文档),BeanDefinitionDocumentReader(依赖xml文档,进行解析和注册),BeanDefinitionParserDelegate(实际的解析工作)。可以看出,在解析bean的过程中,这3个组件的分工是比较清晰的,各司其职,这种设计思想值得学习
到此为止,bean的解析、注册、spring ioc 容器的实例化过程就基本分析结束了。

博客搬家:大坤的个人博客
欢迎评论哦~

    原文作者:Spring MVC
    原文地址: https://blog.csdn.net/u012780655/article/details/83353230
    本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系博主进行删除。
点赞