Spring IOC容器的初始化-（二）BeanDefinition的载入和解析

前言

1.在讲BeanDefinition的载入和解析之前，我们先来看看什么是BeanDefinition。

Bean对象在Spring中是以BeanDefinition来描述的，也就是说在Spring中，BeanDefinition用来表示bean对象。

2. 对于Spring IOC容器，BeanDefinition的载入过程，相当于把Spring的配置文件转换成Spring的内部数据结构。

在上一篇中容器初始化各个父类方法调用图，如下图所示：

1.BeanDefinition的载入和解析

我们接着上一篇的代码，在AbstractBeanDefinitionReader的loadBeanDefinitions()中，我们定位到了Resource[]，看下代码：

AbstractBeanDefinitionReader的loadBeanDefinitions

public int loadBeanDefinitions(String location, Set<Resource> actualResources) throws BeanDefinitionStoreException {
        ResourceLoader resourceLoader = getResourceLoader();
        if (resourceLoader == null) {
            throw new BeanDefinitionStoreException(
                    "Cannot import bean definitions from location [" + location + "]: no ResourceLoader available");
        }

        if (resourceLoader instanceof ResourcePatternResolver) {
            // Resource pattern matching available.
            try {
                Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
                int loadCount = loadBeanDefinitions(resources);
                if (actualResources != null) {
                    for (Resource resource : resources) {
                        actualResources.add(resource);
                    }
                }
                if (logger.isDebugEnabled()) {
                    logger.debug("Loaded " + loadCount + " bean definitions from location pattern [" + location + "]");
                }
                return loadCount;
            }
            catch (IOException ex) {
                throw new BeanDefinitionStoreException(
                        "Could not resolve bean definition resource pattern [" + location + "]", ex);
            }
        }
        else {
            // Can only load single resources by absolute URL.
            Resource resource = resourceLoader.getResource(location);
            int loadCount = loadBeanDefinitions(resource);
            if (actualResources != null) {
                actualResources.add(resource);
            }
            if (logger.isDebugEnabled()) {
                logger.debug("Loaded " + loadCount + " bean definitions from location [" + location + "]");
            }
            return loadCount;
        }
    }

1.1. 将xml文件转换成DOM对象

通过getResouce()方法得到Resouce之后，那么就开始了真正加载Bean资源。因为Spring可以对应不同形式的BeanDefition，我们这里讲的是xml的形式，所以就需要到xmlBeanDefinitonReader的实现中去看源码。

XmlBeanDefinitionReader的loadBeanDefinitions()及doLoadBeanDefinitions()

public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
        return loadBeanDefinitions(new EncodedResource(resource));
    }

public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
        Assert.notNull(encodedResource, "EncodedResource must not be null");
        if (logger.isInfoEnabled()) {
            logger.info("Loading XML bean definitions from " + encodedResource.getResource());
        }

        Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
        if (currentResources == null) {
            currentResources = new HashSet<EncodedResource>(4);
            this.resourcesCurrentlyBeingLoaded.set(currentResources);
        }
        if (!currentResources.add(encodedResource)) {
            throw new BeanDefinitionStoreException(
                    "Detected cyclic loading of " + encodedResource + " - check your import definitions!");
        }
        try {
            //将资源文件转换为IO输入流
            InputStream inputStream = encodedResource.getResource().getInputStream();
            try {
                InputSource inputSource = new InputSource(inputStream);
                if (encodedResource.getEncoding() != null) {
                    inputSource.setEncoding(encodedResource.getEncoding());
                }
                return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
            }
            finally {
                inputStream.close();
            }
        }
        catch (IOException ex) {
            throw new BeanDefinitionStoreException(
                    "IOException parsing XML document from " + encodedResource.getResource(), ex);
        }
        finally {
            currentResources.remove(encodedResource);
            if (currentResources.isEmpty()) {
                this.resourcesCurrentlyBeingLoaded.remove();
            }
        }
    }

//该方法-从特定XML文件中实际载入Bean定义资源
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
            throws BeanDefinitionStoreException {
        try {
             //将XML文件转换为DOM对象
             Document doc = doLoadDocument(inputSource, resource);
             //这里是启动对Bean定义解析的详细过程，该解析过程会用到Spring的Bean配置规则[下文讲解]
            return registerBeanDefinitions(doc, resource);
        }
        catch (BeanDefinitionStoreException ex) {
            throw ex;
        }
        catch (SAXParseException ex) {
            throw new XmlBeanDefinitionStoreException(resource.getDescription(),
                    "Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
        }
        catch (SAXException ex) {
            throw new XmlBeanDefinitionStoreException(resource.getDescription(),
                    "XML document from " + resource + " is invalid", ex);
        }
        catch (ParserConfigurationException ex) {
            throw new BeanDefinitionStoreException(resource.getDescription(),
                    "Parser configuration exception parsing XML from " + resource, ex);
        }
        catch (IOException ex) {
            throw new BeanDefinitionStoreException(resource.getDescription(),
                    "IOException parsing XML document from " + resource, ex);
        }
        catch (Throwable ex) {
            throw new BeanDefinitionStoreException(resource.getDescription(),
                    "Unexpected exception parsing XML document from " + resource, ex);
        }
    }

上面的过程完成了两个动作：（1）资源文件转换成IO输入流；（2）将xml文件转换成DOM对象。至于解析xml文件的方法，里面是通过创建解析工厂，设置xml解析校验，通过工厂创建解析器，然后parse()，所以这个过程我们就不看了

1.2 按照Spring的Bean规则对Document对象进行解析

走主线：得到dom对象后，开始启动对bean定义的详细解析，来看registerBeanDefinition()方法

XmlBeanDefinitionReader的loadBeanDefinitions()方法

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
        
        BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
        int countBefore = getRegistry().getBeanDefinitionCount();
        //解析过程入口，BeanDefinitionDocumentReader只是个接口，具体的解析实现过程有实现类
         //DefaultBeanDefinitionDocumentReader完成
        documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
        return getRegistry().getBeanDefinitionCount() - countBefore;
    }

我们总结下上面那么多绕人的代码。其实就2个思想：

（1）首先，通过调用XML解析器将Bean定义资源文件转换成DOM对象，但这些dom对象并没有按照Spring的bean规则进行解析。这一步是载入的过程。

（2）其次，在完成通用的XML解析之后，按照Spring的bean规则对document对象进行解析。

按照Spring的Bean规则对Document对象解析的过程是在接口BeanDefinitionDocumentReader的实现类DefaultBeanDefinitionDocumentReader中实现的。OK,重点来了。（友情提示，会绕好几个方法。）

DefaultBeanDefinitionDocumentReader的registerBeanDefinitions()及doRegisterBeanDefinitions(）方法

    @Override
    public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
        this.readerContext = readerContext;
        logger.debug("Loading bean definitions");
        Element root = doc.getDocumentElement();
        doRegisterBeanDefinitions(root);
    }

/**
     * Register each bean definition within the given root {@code <beans/>} element.
     */
    protected void doRegisterBeanDefinitions(Element root) {
        // Any nested <beans> elements will cause recursion in this method. In
        // order to propagate and preserve <beans> default-* attributes correctly,
        // keep track of the current (parent) delegate, which may be null. Create
        // the new (child) delegate with a reference to the parent for fallback purposes,
        // then ultimately reset this.delegate back to its original (parent) reference.
        // this behavior emulates a stack of delegates without actually necessitating one.
        BeanDefinitionParserDelegate parent = this.delegate;
        this.delegate = createDelegate(getReaderContext(), root, parent);

        if (this.delegate.isDefaultNamespace(root)) {
            String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
            if (StringUtils.hasText(profileSpec)) {
                String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
                        profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
                if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
                    return;
                }
            }
        }

        preProcessXml(root);
        parseBeanDefinitions(root, this.delegate);
        postProcessXml(root);

        this.delegate = parent;
    }

/**
     * Parse the elements at the root level in the document:
     * "import", "alias", "bean".
     * @param root the DOM root element of the document
     */
    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);
        }
    }

上面这段代码又墨迹了一会，这里主要是看节点元素是否是Spring的规范，因为它允许我们自定义解析规范，那么正常我们是利用Spring的规则，所以我们来看parseDefaultElement(ele, delegate)方法

private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
        //如果元素节点是<import>导入元素，进行导入解析
        if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
            importBeanDefinitionResource(ele);
        }
      //如果元素节点时<Alias>别名元素，进行别名解析
        else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
            processAliasRegistration(ele);
        }
      //如果是<Bean>元素，按照Spring的Bean规则解析元素
         else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
            processBeanDefinition(ele, delegate);
        }
        else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
            // recurse
            doRegisterBeanDefinitions(ele);
        }
    }

终于看到自己熟悉的一点东西了

1.3  Bean资源的解析

//解析bean资源
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
        //BeanDefinitionHolder是对BeanDefinition的封装，包括BeanDefinition,beanName,aliases
        BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
        if (bdHolder != null) {
            bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
            try {
                //向SpringIOC容器注册解析得到的bean定义
                //这是bean定义向IOC容器注册的入口，实际上是放到一个map里面
                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));
        }
    }

注意：在解析<Bean>元素过程中没有创建和实例化Bean对象，只是创建了Bean对象的定义类型BeanDefinition，将<Bean>元素中的配置信息设置到BeanDefiniton作记录，当依赖注入时才使用这些记录信息创建和实例化具体的Bean对象。