一、BeanDefinition介绍

BeanDefinition包含了spring应用中各对象属性以及对象间的相互依赖关系，Bean对象的创建与依赖注入都是依照BeanDefinition来进行初始化、注入等，就好比Bean创建的配方。

通过上面的属性可以看到，基本和我们xml里的配置一一对应的，它就是我们xml里bean配置的载体。

二、BeanDefinition的装载

主链路

主入口DefaultBeanDefinitionDocumentReader.doRegisterBeanDefinitions

protected void doRegisterBeanDefinitions(Element root) {
    //...............................................................
    BeanDefinitionParserDelegate parent = this.delegate;
    this.delegate = createDelegate(this.readerContext, root, parent);

    preProcessXml(root);
    //开始解析document
    parseBeanDefinitions(root, this.delegate);
    postProcessXml(root);

    this.delegate = parent;
}

DefaultBeanDefinitionDocumentReader.parseBeanDefinitions

protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
    //判断是否是spring 默认的 namespace
    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);
    }
}

判断是否是spring默认的配置文件，具体逻辑见代码截图

BeanDefinitionParserDelegate.isDefaultNamespace

2.1 spring默认的解析策略

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

DefaultBeanDefinitionDocumentReader.processBeanDefinition

protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
    //配置文件解析完毕后放到 BeanDefinitionHolder
    BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
    if (bdHolder != null) {
        bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
        try {
            //将BeanDefinition注册到容器中，也就是我们所说的bean的创建与依赖注入
            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));
    }
}

具体的解析逻辑见
BeanDefinitionParserDelegate.parseBeanDefinitionElement

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

    //parseState 是一个解析栈，在异常时候方便记录
    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);
        }
        //创建 BeanDefinition,将name和class属性注入， class是使用XmlReaderContext的classloader,根据配置加载
        AbstractBeanDefinition bd = createBeanDefinition(className, parent);

        //解析bean 属性配置 如 initMethod 这些
        parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);
        bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));

        parseMetaElements(ele, bd);
        parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
        parseReplacedMethodSubElements(ele, bd.getMethodOverrides());

        //解析构造方法注入配置
        parseConstructorArgElements(ele, bd);
        //解析 bean property配置
        parsePropertyElements(ele, bd);
        //解析 bean qualifier配置
        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);
        //省略若干代码
    finally {
        //成功解析后，移除该 BeanEntry
        this.parseState.pop();
    }

    return null;
}

创建beandefinition实现代码BeanDefinitionReaderUtils.createBeanDefinition

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) {
            //设置class对象
            bd.setBeanClass(ClassUtils.forName(className, classLoader));
        }
        else {
            //如果classloader为空则设置className
            bd.setBeanClassName(className);
        }
    }
    return bd;
}

解析 bean property配置parsePropertyElements具体执行单元parsePropertyElement，当然如果bean配置没有property,自然也不会走到parsePropertyElement逻辑

public void parsePropertyElement(Element ele, BeanDefinition bd) {
        //..................

        //解析property value
        Object val = parsePropertyValue(ele, bd, propertyName);
        PropertyValue pv = new PropertyValue(propertyName, val);
        parseMetaElements(ele, pv);
        pv.setSource(extractSource(ele));
        bd.getPropertyValues().addPropertyValue(pv);
    }
    finally {
        this.parseState.pop();
    }
}

public Object parsePropertyValue(Element ele, BeanDefinition bd, String propertyName) {
    //...................................

    //属性是否是 ref
    boolean hasRefAttribute = ele.hasAttribute(REF_ATTRIBUTE);
    //属性是否是 value
    boolean hasValueAttribute = ele.hasAttribute(VALUE_ATTRIBUTE);
    //...................................

    if (hasRefAttribute) {
        String refName = ele.getAttribute(REF_ATTRIBUTE);
        if (!StringUtils.hasText(refName)) {
            error(elementName + " contains empty 'ref' attribute", ele);
        }
        //RuntimeBeanReference ref的载体
        RuntimeBeanReference ref = new RuntimeBeanReference(refName);
        ref.setSource(extractSource(ele));
        return ref;
    }
    else if (hasValueAttribute) {
        //TypedStringValue 普通value的载体
        TypedStringValue valueHolder = new TypedStringValue(ele.getAttribute(VALUE_ATTRIBUTE));
        valueHolder.setSource(extractSource(ele));
        return valueHolder;
    }
    //是否还有有子节点 比如 list,map这些就有子节点
    else if (subElement != null) {
        return parsePropertySubElement(subElement, bd);
    }
    else {
        // Neither child element nor "ref" or "value" attribute found.
        error(elementName + " must specify a ref or value", ele);
        return null;
    }
}

public Object parsePropertySubElement(Element ele, BeanDefinition bd, String defaultValueType) {
    //如果不是spring默认解析继续走自定义handler解析
    if (!isDefaultNamespace(ele)) {
        return parseNestedCustomElement(ele, bd);
    }
    //如果节点是 bean
    else if (nodeNameEquals(ele, BEAN_ELEMENT)) {
        BeanDefinitionHolder nestedBd = parseBeanDefinitionElement(ele, bd);
        if (nestedBd != null) {
            nestedBd = decorateBeanDefinitionIfRequired(ele, nestedBd, bd);
        }
        return nestedBd;
    }
    //如果节点是 ref
    else if (nodeNameEquals(ele, REF_ELEMENT)) {
        // ......................
        RuntimeBeanReference ref = new RuntimeBeanReference(refName, toParent);
        ref.setSource(extractSource(ele));
        return ref;
    }
    else if (nodeNameEquals(ele, IDREF_ELEMENT)) {
        return parseIdRefElement(ele);
    }
    //如果节点是 value
    else if (nodeNameEquals(ele, VALUE_ELEMENT)) {
        return parseValueElement(ele, defaultValueType);
    }
    //如果节点是 null
    else if (nodeNameEquals(ele, NULL_ELEMENT)) {
        // ......................
        TypedStringValue nullHolder = new TypedStringValue(null);
        nullHolder.setSource(extractSource(ele));
        return nullHolder;
    }
    //如果节点是 array
    else if (nodeNameEquals(ele, ARRAY_ELEMENT)) {
        return parseArrayElement(ele, bd);
    }
    else if (nodeNameEquals(ele, LIST_ELEMENT)) {
        return parseListElement(ele, bd);
    }
    else if (nodeNameEquals(ele, SET_ELEMENT)) {
        return parseSetElement(ele, bd);
    }
    else if (nodeNameEquals(ele, MAP_ELEMENT)) {
        return parseMapElement(ele, bd);
    }
    else if (nodeNameEquals(ele, PROPS_ELEMENT)) {
        return parsePropsElement(ele);
    }
    else {
        error("Unknown property sub-element: [" + ele.getNodeName() + "]", ele);
        return null;
    }
}

parsePropertySubElement，解析代码就不贴了，篇幅有限，我们可以看里面list节点的解析链路
parsePropertySubElement() -> parseListElement() -> parseCollectionElements() -> parsePropertySubElement()，对没有看错，最后又回到了 parsePropertySubElement（），不过逻辑分支自然是不同的

public List parseListElement(Element collectionEle, BeanDefinition bd) {
    String defaultElementType = collectionEle.getAttribute(VALUE_TYPE_ATTRIBUTE);
    NodeList nl = collectionEle.getChildNodes();
    //整体被包装成ManagedList类型
    ManagedList<Object> target = new ManagedList<Object>(nl.getLength());
    target.setSource(extractSource(collectionEle));
    target.setElementTypeName(defaultElementType);
    target.setMergeEnabled(parseMergeAttribute(collectionEle));
    //解析list 子节点值， 集合类的都用该方法进行解析
    parseCollectionElements(nl, target, bd, defaultElementType);
    return target;
}

其它类型的节点解析就不一一贴代码了，只要调用链路清晰后，顺着代码也基本就看明白了

2.2 自定义的解析

如果不是spring默认解析的则有自定的handler进行解析

//BeanDefinitionParserDelegate
public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) {
    String namespaceUri = getNamespaceURI(ele);
    NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);
    if (handler == null) {
        error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele);
        return null;
    }
    return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));
}

this.readerContext.getNamespaceHandlerResolver()也就是XmlReaderContext.namespaceHandlerResolver，这是什么东西呢？我们回到 XmlReaderContext创建的代码

//XmlBeanDefinitionReader.registerBeanDefinitions
public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
    BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
    documentReader.setEnvironment(getEnvironment());
    int countBefore = getRegistry().getBeanDefinitionCount();
    //createReaderContext(resource) 创建XmlReaderContext
    documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
    return getRegistry().getBeanDefinitionCount() - countBefore;
}

protected XmlReaderContext createReaderContext(Resource resource) {
    if (this.namespaceHandlerResolver == null) {
        //这里创建了默认的 NamespaceHandlerResolver
        this.namespaceHandlerResolver = createDefaultNamespaceHandlerResolver();
    }
    return new XmlReaderContext(resource, this.problemReporter, this.eventListener,
            this.sourceExtractor, this, this.namespaceHandlerResolver);
}

如何根据xml的namespace获取对应的handler?

//DefaultNamespaceHandlerResolver
public NamespaceHandler resolve(String namespaceUri) {
    Map<String, Object> handlerMappings = getHandlerMappings();
    //根据namespace获取对应的handler
    Object handlerOrClassName = handlerMappings.get(namespaceUri);
    if (handlerOrClassName == null) {
        return null;
    }
    //判断handler是否实现接口NamespaceHandler
    else if (handlerOrClassName instanceof NamespaceHandler) {
        return (NamespaceHandler) handlerOrClassName;
    }
    else {
        //如果handler首次加载，那么此时value只是个className
        String className = (String) handlerOrClassName;
        try {
            Class<?> handlerClass = ClassUtils.forName(className, this.classLoader);
            //.........?????
            if (!NamespaceHandler.class.isAssignableFrom(handlerClass)) {
                throw new FatalBeanException("Class [" + className + "] for namespace [" + namespaceUri +
                        "] does not implement the [" + NamespaceHandler.class.getName() + "] interface");
            }
            NamespaceHandler namespaceHandler = (NamespaceHandler) BeanUtils.instantiateClass(handlerClass);
            namespaceHandler.init();
            handlerMappings.put(namespaceUri, namespaceHandler);
            return namespaceHandler;
        }
        //..............................
    }
}

DefaultNamespaceHandlerResolver.getHandlerMappings

Properties mappings =
PropertiesLoaderUtils.loadAllProperties(this.handlerMappingsLocation, this.classLoader);

这行代码会将classLoader classpath下的所有META-INF/spring.handlers给加载并存放在 handlerMappings， nameSpaceUri作为key，handler class作为value

以dubbo的spring.handlers为例

http\://code.alibabatech.com/schema/dubbo=com.alibaba.dubbo.config.spring.schema.DubboNamespaceHandler

自定义解析，通过nameSpace找到对应的handler，然后由对应的handler来解析对应的xml，这么一个解析的方式。

三 BeanDefinition 注册到容器

DefaultBeanDefinitionDocumentReader.parseBeanDefinitions
解析

protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
    //解析后的bean，BeanDefinitionHolder
    BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
    if (bdHolder != null) {
        bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
        try {
            // 将解析后的bean，BeanDefinitionHolder注册到容器
            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));
    }
}

执行具体动作
BeanDefinitionReaderUtils.registerBeanDefinition -> DefaultListableBeanFactory.registerBeanDefinition

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;

    //注册到beanDefinitionMap ，启用synchronized保证整个操作是原子性的，达到数据一致性
    synchronized (this.beanDefinitionMap) {

        //检查是否已经有同名的注册进来了，如果有则判断是否允许覆盖，若不允许则抛异常
        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);
    }

    //覆盖掉beanName对应的原BeanDefinition，当然不仅仅是简单的覆盖，还需要将已经实例化的bean对象销毁、父类同名的覆盖等
    if (oldBeanDefinition != null || containsSingleton(beanName)) {
        resetBeanDefinition(beanName);
    }
}

整个过程，所谓的容器注册，就是以 beanName为key， BeanDefinition为value，存放到beanDefinitionMap

private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<String, BeanDefinition>(64);

PS. ConcurrentHashMap已经是线程安全了为什么还加synchronized？
注意观察synchronized，锁的块，spring要保证读写一致。ConcurrentHashMap只能保证单个操作的线程安全。

四 结语

至此，beanDefinition信息已经注册到spring容器中，接下来就可以被容器使用，都在beanDefinitionMap进行检索，这些信息是容器建议依赖反转的基础。

源码版本：3.2.18.RELEASE

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spring源码探索(2)-IOC容器-BeanDefinition加载与注册
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