上一篇博客中我们讲到了registerBeanDefinitions方法，这一篇我们继续，先让我们看看源码是怎么实现的

//根据给定的DOM文件注册Bean定义
public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
        //得到BeanDefinitionDocumentReader来对xml格式的BeanDefinition解析
        BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
        //获得容器中已经注册的Bean数量
        int countBefore = getRegistry().getBeanDefinitionCount();
        //核心代码，接下来详细分析
        //createReaderContext方法用来创建一个XmlReaderContext实例
        documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
        //统计出本次注册bean的数量
        return getRegistry().getBeanDefinitionCount() - countBefore;
    }

BeanDefinitionDocumentReader是一个接口，我们先来看看他的默认实现类DefaultBeanDefinitionDocumentReader

DefaultBeanDefinitionDocumentReader

1.成员变量

    public static final String BEAN_ELEMENT = BeanDefinitionParserDelegate.BEAN_ELEMENT;

    public static final String NESTED_BEANS_ELEMENT = "beans";

    public static final String ALIAS_ELEMENT = "alias";

    public static final String NAME_ATTRIBUTE = "name";

    public static final String ALIAS_ATTRIBUTE = "alias";

    public static final String IMPORT_ELEMENT = "import";

    public static final String RESOURCE_ATTRIBUTE = "resource";

    public static final String PROFILE_ATTRIBUTE = "profile";


    protected final Log logger = LogFactory.getLog(getClass());

    private XmlReaderContext readerContext;

    private BeanDefinitionParserDelegate delegate;

**2.BeanDefinitionParserDelegate **

public class BeanDefinitionParserDelegate {

    public static final String BEANS_NAMESPACE_URI = "http://www.springframework.org/schema/beans";

    public static final String MULTI_VALUE_ATTRIBUTE_DELIMITERS = ",; ";

    /**
     * Value of a T/F attribute that represents true.
     * Anything else represents false. Case seNsItive.
     */
    public static final String TRUE_VALUE = "true";

    public static final String FALSE_VALUE = "false";

    public static final String DEFAULT_VALUE = "default";

    public static final String DESCRIPTION_ELEMENT = "description";

    public static final String AUTOWIRE_NO_VALUE = "no";

    public static final String AUTOWIRE_BY_NAME_VALUE = "byName";

    public static final String AUTOWIRE_BY_TYPE_VALUE = "byType";

    public static final String AUTOWIRE_CONSTRUCTOR_VALUE = "constructor";

    public static final String AUTOWIRE_AUTODETECT_VALUE = "autodetect";

    public static final String DEPENDENCY_CHECK_ALL_ATTRIBUTE_VALUE = "all";

    public static final String DEPENDENCY_CHECK_SIMPLE_ATTRIBUTE_VALUE = "simple";

    public static final String DEPENDENCY_CHECK_OBJECTS_ATTRIBUTE_VALUE = "objects";

    public static final String NAME_ATTRIBUTE = "name";

    public static final String BEAN_ELEMENT = "bean";

    public static final String META_ELEMENT = "meta";

    public static final String ID_ATTRIBUTE = "id";

    public static final String PARENT_ATTRIBUTE = "parent";

    public static final String CLASS_ATTRIBUTE = "class";

    public static final String ABSTRACT_ATTRIBUTE = "abstract";

    public static final String SCOPE_ATTRIBUTE = "scope";

    private static final String SINGLETON_ATTRIBUTE = "singleton";

    public static final String LAZY_INIT_ATTRIBUTE = "lazy-init";

    public static final String AUTOWIRE_ATTRIBUTE = "autowire";

    public static final String AUTOWIRE_CANDIDATE_ATTRIBUTE = "autowire-candidate";

    public static final String PRIMARY_ATTRIBUTE = "primary";

    public static final String DEPENDENCY_CHECK_ATTRIBUTE = "dependency-check";

    public static final String DEPENDS_ON_ATTRIBUTE = "depends-on";

    public static final String INIT_METHOD_ATTRIBUTE = "init-method";

    public static final String DESTROY_METHOD_ATTRIBUTE = "destroy-method";

    public static final String FACTORY_METHOD_ATTRIBUTE = "factory-method";

    public static final String FACTORY_BEAN_ATTRIBUTE = "factory-bean";

    public static final String CONSTRUCTOR_ARG_ELEMENT = "constructor-arg";

    public static final String INDEX_ATTRIBUTE = "index";

    public static final String TYPE_ATTRIBUTE = "type";

    public static final String VALUE_TYPE_ATTRIBUTE = "value-type";

    public static final String KEY_TYPE_ATTRIBUTE = "key-type";

    public static final String PROPERTY_ELEMENT = "property";

    public static final String REF_ATTRIBUTE = "ref";

    public static final String VALUE_ATTRIBUTE = "value";

    public static final String LOOKUP_METHOD_ELEMENT = "lookup-method";

    public static final String REPLACED_METHOD_ELEMENT = "replaced-method";

    public static final String REPLACER_ATTRIBUTE = "replacer";

    public static final String ARG_TYPE_ELEMENT = "arg-type";

    public static final String ARG_TYPE_MATCH_ATTRIBUTE = "match";

    public static final String REF_ELEMENT = "ref";

    public static final String IDREF_ELEMENT = "idref";

    public static final String BEAN_REF_ATTRIBUTE = "bean";

    public static final String LOCAL_REF_ATTRIBUTE = "local";

    public static final String PARENT_REF_ATTRIBUTE = "parent";

    public static final String VALUE_ELEMENT = "value";

    public static final String NULL_ELEMENT = "null";

    public static final String ARRAY_ELEMENT = "array";

    public static final String LIST_ELEMENT = "list";

    public static final String SET_ELEMENT = "set";

    public static final String MAP_ELEMENT = "map";

    public static final String ENTRY_ELEMENT = "entry";

    public static final String KEY_ELEMENT = "key";

    public static final String KEY_ATTRIBUTE = "key";

    public static final String KEY_REF_ATTRIBUTE = "key-ref";

    public static final String VALUE_REF_ATTRIBUTE = "value-ref";

    public static final String PROPS_ELEMENT = "props";

    public static final String PROP_ELEMENT = "prop";

    public static final String MERGE_ATTRIBUTE = "merge";

    public static final String QUALIFIER_ELEMENT = "qualifier";

    public static final String QUALIFIER_ATTRIBUTE_ELEMENT = "attribute";

    public static final String DEFAULT_LAZY_INIT_ATTRIBUTE = "default-lazy-init";

    public static final String DEFAULT_MERGE_ATTRIBUTE = "default-merge";

    public static final String DEFAULT_AUTOWIRE_ATTRIBUTE = "default-autowire";

    public static final String DEFAULT_DEPENDENCY_CHECK_ATTRIBUTE = "default-dependency-check";

    public static final String DEFAULT_AUTOWIRE_CANDIDATES_ATTRIBUTE = "default-autowire-candidates";

    public static final String DEFAULT_INIT_METHOD_ATTRIBUTE = "default-init-method";

    public static final String DEFAULT_DESTROY_METHOD_ATTRIBUTE = "default-destroy-method";


    protected final Log logger = LogFactory.getLog(getClass());

    private final XmlReaderContext readerContext;

    private final DocumentDefaultsDefinition defaults = new DocumentDefaultsDefinition();

    private final ParseState parseState = new ParseState();

    /**
     * Stores all used bean names so we can enforce uniqueness on a per
     * beans-element basis. Duplicate bean ids/names may not exist within the
     * same level of beans element nesting, but may be duplicated across levels.
     */
    private final Set<String> usedNames = new HashSet<String>();
    ...
    }

3.registerBeanDefinitions方法

    @Override
    public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
        //获得XML描述符(拥有XmlBeanDefinitionReader，NamespaceHandlerResolver)2个成员变量
        this.readerContext = readerContext;
        logger.debug("Loading bean definitions");
        //获得Document的根元素  （<beans>）
        Element root = doc.getDocumentElement();
        doRegisterBeanDefinitions(root);
    }

    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.
        //从上述的注释中可以看出里面有一个隐式的递归
        BeanDefinitionParserDelegate parent = this.delegate;
        //创建BeanDefinitionParserDelegate，用于完成真正的解析过程  
        this.delegate = createDelegate(getReaderContext(), root, parent);
        //对beans中的profile属性做处理，不是很重要，这里不做解释了
        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;
                }
            }
        }
        //这里用到了模板模式
        //在解析Bean定义之前，进行自定义的解析，增强解析过程的可扩展性
        preProcessXml(root);
        //解析bean定义，下章详解
        parseBeanDefinitions(root, this.delegate);
        //在解析Bean定义之后，进行自定义的解析，增强解析过程的可扩展性
        postProcessXml(root);

        this.delegate = parent;
    }

    protected BeanDefinitionParserDelegate createDelegate(
            XmlReaderContext readerContext, Element root, BeanDefinitionParserDelegate parentDelegate) {
        //构造BeanDefinitionParserDelegate
        BeanDefinitionParserDelegate delegate = new BeanDefinitionParserDelegate(readerContext);
        delegate.initDefaults(root, parentDelegate);
        return delegate;
    }

   public void initDefaults(Element root, BeanDefinitionParserDelegate parent) {
        //对dom的根文件(<beans>)默认初始化
        populateDefaults(this.defaults, (parent != null ? parent.defaults : null), root);
        //注册事件监听
        this.readerContext.fireDefaultsRegistered(this.defaults);
    }

   protected void populateDefaults(DocumentDefaultsDefinition defaults, DocumentDefaultsDefinition parentDefaults, Element root) {
        String lazyInit = root.getAttribute(DEFAULT_LAZY_INIT_ATTRIBUTE);
        if (DEFAULT_VALUE.equals(lazyInit)) {
            // Potentially inherited from outer <beans> sections, otherwise falling back to false.
            lazyInit = (parentDefaults != null ? parentDefaults.getLazyInit() : FALSE_VALUE);
        }
        defaults.setLazyInit(lazyInit);

        String merge = root.getAttribute(DEFAULT_MERGE_ATTRIBUTE);
        if (DEFAULT_VALUE.equals(merge)) {
            // Potentially inherited from outer <beans> sections, otherwise falling back to false.
            merge = (parentDefaults != null ? parentDefaults.getMerge() : FALSE_VALUE);
        }
        defaults.setMerge(merge);

        String autowire = root.getAttribute(DEFAULT_AUTOWIRE_ATTRIBUTE);
        if (DEFAULT_VALUE.equals(autowire)) {
            // Potentially inherited from outer <beans> sections, otherwise falling back to 'no'.
            autowire = (parentDefaults != null ? parentDefaults.getAutowire() : AUTOWIRE_NO_VALUE);
        }
        defaults.setAutowire(autowire);

        // Don't fall back to parentDefaults for dependency-check as it's no longer supported in
        // <beans> as of 3.0. Therefore, no nested <beans> would ever need to fall back to it.
        defaults.setDependencyCheck(root.getAttribute(DEFAULT_DEPENDENCY_CHECK_ATTRIBUTE));

        if (root.hasAttribute(DEFAULT_AUTOWIRE_CANDIDATES_ATTRIBUTE)) {
            defaults.setAutowireCandidates(root.getAttribute(DEFAULT_AUTOWIRE_CANDIDATES_ATTRIBUTE));
        }
        else if (parentDefaults != null) {
            defaults.setAutowireCandidates(parentDefaults.getAutowireCandidates());
        }

        if (root.hasAttribute(DEFAULT_INIT_METHOD_ATTRIBUTE)) {
            defaults.setInitMethod(root.getAttribute(DEFAULT_INIT_METHOD_ATTRIBUTE));
        }
        else if (parentDefaults != null) {
            defaults.setInitMethod(parentDefaults.getInitMethod());
        }

        if (root.hasAttribute(DEFAULT_DESTROY_METHOD_ATTRIBUTE)) {
            defaults.setDestroyMethod(root.getAttribute(DEFAULT_DESTROY_METHOD_ATTRIBUTE));
        }
        else if (parentDefaults != null) {
            defaults.setDestroyMethod(parentDefaults.getDestroyMethod());
        }

        defaults.setSource(this.readerContext.extractSource(root));
    }