BeanDefinition的载入

在IoC容器初始化过程（1）-Resource定位中知道，调用AbstractBeanDefinitionReader中的loadBeanDefinitions方法时不管传入的参数是String类型还是Resource类型，最终都是由XmlBeanDefinitionReader中的loadBeanDefinitions方法实现的，那么就从XmlBeanDefinitionReader开始，看一下载入BeanDefinition的过程。

1.XmlBeanDefinitionReader

public class XmlBeanDefinitionReader extends AbstractBeanDefinitionReader {
    
    @Override
    public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
        //载入XML形式的资源文件，并使用Resource对象创建EncodedResource对象  
        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());
        }
        //获取资源对象集合
        Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
        if (currentResources == null) {
            currentResources = new HashSet<>(4);
            this.resourcesCurrentlyBeingLoaded.set(currentResources);
        }
        //将当前的资源对象添加到EncodedResource
        if (!currentResources.add(encodedResource)) {
            throw new BeanDefinitionStoreException(
                    "Detected cyclic loading of " + encodedResource + " - check your import definitions!");
        }
        try {
             //从EncodedResource获取资源对象的输入流，得到XML文件  
            InputStream inputStream = encodedResource.getResource().getInputStream();
            try {
                 //创建InputSource准备进行读取  
                InputSource inputSource = new InputSource(inputStream);
                if (encodedResource.getEncoding() != null) {
                    //设置编码 
                    inputSource.setEncoding(encodedResource.getEncoding());
                }
                //进行BeanDefinition的载入
                return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
            }
            finally {
                inputStream.close();
            }
        }
        catch (IOException ex) {
            throw new BeanDefinitionStoreException(
                    "IOException parsing XML document from " + encodedResource.getResource(), ex);
        }
        finally {
            //移除已经读取过的资源对象  
            currentResources.remove(encodedResource);
            if (currentResources.isEmpty()) {
                this.resourcesCurrentlyBeingLoaded.remove();
            }
        }
    }

    public int loadBeanDefinitions(InputSource inputSource) throws BeanDefinitionStoreException {
        return loadBeanDefinitions(inputSource, "resource loaded through SAX InputSource");
    }

    public int loadBeanDefinitions(InputSource inputSource, @Nullable String resourceDescription)
            throws BeanDefinitionStoreException {

        return doLoadBeanDefinitions(inputSource, new DescriptiveResource(resourceDescription));
    }
    
    /**
     * 从XML文件具体载入BeanDefinition的方法
     * @param inputSource the SAX InputSource to read from
     * @param resource the resource descriptor for the XML file
     */
    protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
            throws BeanDefinitionStoreException {
        try {
            //取得XML文件的Document对象  
            Document doc = doLoadDocument(inputSource, resource);
            //注册BeanDefinition
            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);
        }
    }

    /**
     * 向容器中注册BeanDefinition
     * 按照Spring的Bean语义要求进行解析资源文件并转换为内部的数据结构BeanDefinition 
     */
    public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
        //创建BeanDefinitionDocumentReader用来解析Document，也就是解析XML配置文件
        BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
        //具体的解析过程在registerBeanDefinitions方法中完成,该方法在DefaultBeanDefinitionDocumentReader中有实现 
        int countBefore = getRegistry().getBeanDefinitionCount();
        //注册BeanDefinition
        documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
        //对载入Bean的数量进行统计  
        return getRegistry().getBeanDefinitionCount() - countBefore;
    }
    
    /**
     * 创建BeanDefinitionDocumentReader
     * 作用是从XML文件中读取bean信息转为BeanDefinition
     */
    protected BeanDefinitionDocumentReader createBeanDefinitionDocumentReader() {
        return BeanDefinitionDocumentReader.class.cast(BeanUtils.instantiateClass(this.documentReaderClass));
    }
    
    protected Document doLoadDocument(InputSource inputSource, Resource resource) throws Exception {
        //资源文件转为Document对象
        return this.documentLoader.loadDocument(inputSource, getEntityResolver(), this.errorHandler,
                getValidationModeForResource(resource), isNamespaceAware());
    }

}

• BeanDefinition载入时首先通过DocumentLoader将资源文件转为Document对象，然后使用BeanDefinitionDocumentReader进行解析。
• BeanDefinitionDocumentReader是一个接口，具体由DefaultBeanDefinitionDocumentReader实现BeanDefinition的注册。

2.DefaultBeanDefinitionDocumentReader

public class DefaultBeanDefinitionDocumentReader implements BeanDefinitionDocumentReader {

    /**
     * 注册BeanDefinition
     */
    @Override
    public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
        this.readerContext = readerContext;
        logger.debug("Loading bean definitions");
        //获取根节点
        Element root = doc.getDocumentElement();
        //注册每一个BeanDefinition  
        doRegisterBeanDefinitions(root);
    }
    
    /**
     * 根据根节点注册每一个BeanDefinition
     */
    protected void doRegisterBeanDefinitions(Element root) {
        // 用于解析BeanDefinition的委托类
        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)) {
                    if (logger.isInfoEnabled()) {
                        logger.info("Skipped XML bean definition file due to specified profiles [" + profileSpec +
                                "] not matching: " + getReaderContext().getResource());
                    }
                    return;
                }
            }
        }
        //在开始处理bean定义之前，允许XML通过处理任何自定义元素类型进行扩展
        preProcessXml(root);
        //从根元素开始解析  
        parseBeanDefinitions(root, this.delegate);
        postProcessXml(root);

        this.delegate = parent;
    }
    /**
     * 从根元素开始解析BeanDefinition 
     */
    protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
        if (delegate.isDefaultNamespace(root)) {
            //获取根节点的所有子节点  
            NodeList nl = root.getChildNodes();
            //遍历根节点的子节点集合  
            for (int i = 0; i < nl.getLength(); i++) {
                Node node = nl.item(i);
                if (node instanceof Element) {
                    Element ele = (Element) node;
                    //如果是默认的元素  
                    if (delegate.isDefaultNamespace(ele)) {
                        //解析默认元素  
                        parseDefaultElement(ele, delegate);
                    }
                    else {
                         //解析自定义元素  
                        delegate.parseCustomElement(ele);
                    }
                }
            }
        }
        else {
            delegate.parseCustomElement(root);
        }
    }
    
    /** 
     * 解析默认的元素（import、alias、bean、beans） 
     */
    private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
        //解析import
        if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
            importBeanDefinitionResource(ele);
        }
        else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {//alias
            processAliasRegistration(ele);
        }
        else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {//bean
            processBeanDefinition(ele, delegate);
        }
        else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
            // recurse
            doRegisterBeanDefinitions(ele);
        }
    }
    
    /**
     * 处理给定的Element元素，解析BeanDefinition并注册 
     */
    protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
        BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
        if (bdHolder != null) {
            bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
            try {
                // 向IoC容器注册解析得到的BeanDefnition
                BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
            }
            catch (BeanDefinitionStoreException ex) {
                getReaderContext().error("Failed to register bean definition with name '" +
                        bdHolder.getBeanName() + "'", ele, ex);
            }
            // 注册完成后，发送消息 
            getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
        }
    }
}

• processBeanDefinition()方法中，使用BeanDefinitionParserDelegate对Element进行解析，返回的是BeanDefinitionHolder对象，用它来完成向IoC容器的注册 。BeanDefinitionHolder是BeanDefinition的封装类，除了BeanDefinition还持有其他与BeanDefinition的使用相关的信息，比如Bean的名字、别名集合等。
• BeanDefinition生成后，调用了BeanDefinitionReaderUtils的registerBeanDefinition()完成BeanDefinition的注册（注册BeanDefinition的过程会从该方法入手）。

BeanDefinitionHolder

public class BeanDefinitionHolder implements BeanMetadataElement {

    private final BeanDefinition beanDefinition;

    private final String beanName;

    @Nullable
    private final String[] aliases;
}

3.BeanDefinitionParserDelegate

BeanDefinitionParserDelegate是具体解析XML中的元素与生成BeanDefinition的地方。

public class BeanDefinitionParserDelegate {
    /**
     * 解析XML元素 
     */
    @Nullable
    public BeanDefinitionHolder parseBeanDefinitionElement(Element ele) {
        return parseBeanDefinitionElement(ele, null);
    }

    /**
     * 解析元素的详细过程，并返回BeanDefinitionHolder对象 
     */
    @Nullable
    public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, @Nullable BeanDefinition containingBean) {
        //获取<bean>元素中定义的属性值  
        String id = ele.getAttribute(ID_ATTRIBUTE);//获取ID属性
        String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);//获取name属性
        //别名
        List<String> aliases = new ArrayList<>();
        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);
        }
        //对Bean元素的详细解析，生成BeanDefinition
        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);
            //对BeanDefinition封装返回BeanDefinitionHolder
            return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);
        }

        return null;
    }
    
    /**
     * 解析元素生成BeanDefinition
     */
    @Nullable
    public AbstractBeanDefinition parseBeanDefinitionElement(
            Element ele, String beanName, @Nullable BeanDefinition containingBean) {

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

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

        try {
            // 创建BeanDefinition
            AbstractBeanDefinition bd = createBeanDefinition(className, parent);

            parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);//解析attributes元素
            bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));

            parseMetaElements(ele, bd);//解析meta元素
            parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
            parseReplacedMethodSubElements(ele, bd.getMethodOverrides());

            parseConstructorArgElements(ele, bd);
            parsePropertyElements(ele, bd);//解析Property元素
            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;
    }
}

在parseBeanDefinitionElement(Element ele, String beanName, @Nullable BeanDefinition containingBean)中生成了BeanDefinition对象。然后进行了一系列的解析XML元素的方法，设置到BeanDefinition中，最后将创建的BeanDefinition返回，完成BeanDefinition的载入。

BeanDefinition的注册

BeanDefinition的载入过程中，在DefaultBeanDefinitionDocumentReader中创建BeanDefinition后调用了BeanDefinitionReaderUtils对BeanDefiniton进行注册。

1.BeanDefinitionReaderUtils

public class BeanDefinitionReaderUtils {
    /**
     * 注册BeanDefinition
     */
    public static void registerBeanDefinition(
            BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
            throws BeanDefinitionStoreException {

        // 获取bean的name
        String beanName = definitionHolder.getBeanName();
        //注册BeanDefinition
        registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());

        // 注册别名
        String[] aliases = definitionHolder.getAliases();
        if (aliases != null) {
            for (String alias : aliases) {
                registry.registerAlias(beanName, alias);
            }
        }
    }
}

BeanDefinitionRegistry是一个接口，registerBeanDefinition方法由它的子类DefaultListableBeanFactory实现。

2.DefaultListableBeanFactory

在DefaultListableBeanFactory可以看到有一个beanDefinitionMap成员变量，每一个BeanDefinition都会注册到该集合中，IoC容器通过这个HashMap来持有这些BeanDefinition数据。

public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory
        implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {
    
    /** 存放BeanDefinition对象的Map集合，key为bean的name */
    private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);
        
    //---------------------------------------------------------------------
    //  注册BeanDefinition
    //---------------------------------------------------------------------

    @Override
    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);
            }
        }

        BeanDefinition oldBeanDefinition;
        //检查是不是有相同名字的BeanDefinition已经在IoC容器中注册了，如果有相同的BeanDefinition，但又不允许覆盖，会抛出异常  
        oldBeanDefinition = this.beanDefinitionMap.get(beanName);
        if (oldBeanDefinition != null) {
            if (!isAllowBeanDefinitionOverriding()) {
                throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                        "Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
                        "': There is already [" + oldBeanDefinition + "] bound.");
            }
            else if (oldBeanDefinition.getRole() < beanDefinition.getRole()) {
                // e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
                if (this.logger.isWarnEnabled()) {
                    this.logger.warn("Overriding user-defined bean definition for bean '" + beanName +
                            "' with a framework-generated bean definition: replacing [" +
                            oldBeanDefinition + "] with [" + beanDefinition + "]");
                }
            }
            else if (!beanDefinition.equals(oldBeanDefinition)) {
                if (this.logger.isInfoEnabled()) {
                    this.logger.info("Overriding bean definition for bean '" + beanName +
                            "' with a different definition: replacing [" + oldBeanDefinition +
                            "] with [" + beanDefinition + "]");
                }
            }
            else {
                if (this.logger.isDebugEnabled()) {
                    this.logger.debug("Overriding bean definition for bean '" + beanName +
                            "' with an equivalent definition: replacing [" + oldBeanDefinition +
                            "] with [" + beanDefinition + "]");
                }
            }
            this.beanDefinitionMap.put(beanName, beanDefinition);
        }
        else {
            if (hasBeanCreationStarted()) {//是否已经启动注册
                // 使用synchronized保持数据的一致性  
                synchronized (this.beanDefinitionMap) {
                    //注册BeanDefinition
                    this.beanDefinitionMap.put(beanName, beanDefinition);
                    List<String> updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
                    updatedDefinitions.addAll(this.beanDefinitionNames);
                    updatedDefinitions.add(beanName);
                    this.beanDefinitionNames = updatedDefinitions;
                    if (this.manualSingletonNames.contains(beanName)) {
                        Set<String> updatedSingletons = new LinkedHashSet<>(this.manualSingletonNames);
                        updatedSingletons.remove(beanName);
                        this.manualSingletonNames = updatedSingletons;
                    }
                }
            }
            else {
                //注册BeanDefinition，将解析后BeanDefinition放入map，key为bean name，value为BeanDefinition对象 
                this.beanDefinitionMap.put(beanName, beanDefinition);
                this.beanDefinitionNames.add(beanName);
                this.manualSingletonNames.remove(beanName);
            }
            this.frozenBeanDefinitionNames = null;
        }

        if (oldBeanDefinition != null || containsSingleton(beanName)) {
            resetBeanDefinition(beanName);
        }
    }
}

参考:

spring技术内幕:深入解析spring架构与设计原理

IoC容器初始化过程（二）：BeanDefinition载入和注册 | SHAN (shan-ml.github.io)