Spring5源码分析(012)——IoC篇之解析bean标签：解析 BeanDefinition

/**
 * Parses the supplied {@code } element. May return {@code null}
 * if there were errors during parse. Errors are reported to the
 * {@link org.springframework.beans.factory.parsing.ProblemReporter}.
 * 
解析提供的  标签元素。如果在解析过程中出现错误，可以返回 null 。
 * 错误将会报告给 {@link org.springframework.beans.factory.parsing.ProblemReporter} 进行处理。
 */
@Nullable
public BeanDefinitionHolder parseBeanDefinitionElement(Element ele) {
    return parseBeanDefinitionElement(ele, null);
}

/**
 * Parses the supplied {@code } element. May return {@code null}
 * if there were errors during parse. Errors are reported to the
 * {@link org.springframework.beans.factory.parsing.ProblemReporter}.
 */
@Nullable
public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, @Nullable BeanDefinition containingBean) {
    // 1.1、解析 id 、 name 属性
    String id = ele.getAttribute(ID_ATTRIBUTE);
    String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);

    // 1.2、分割 name 属性，作为别名 alias 集合
    List aliases = new ArrayList();
    if (StringUtils.hasLength(nameAttr)) {
        String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, MULTI_VALUE_ATTRIBUTE_DELIMITERS);
        aliases.addAll(Arrays.asList(nameArr));
    }
    // 1.3、beanName ： 先默认使用 id
    String beanName = id;
    if (!StringUtils.hasText(beanName) && !aliases.isEmpty()) {
        // 若 id 没有设置，则使用别名 alias 的第一个，同时移除这个别名
        beanName = aliases.remove(0);
        if (logger.isTraceEnabled()) {
            logger.trace("No XML ‘id‘ specified - using ‘" + beanName +
                    "‘ as bean name and " + aliases + " as aliases");
        }
    }
    // 1.4、检查 beanName 和 alias 的唯一性
    if (containingBean == null) {
        checkNameUniqueness(beanName, aliases, ele);
    }
    // 2、解析属性，构造 AbstractBeanDefinition 对象
    AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);
    if (beanDefinition != null) {
        // 再次检查 beanName
        if (!StringUtils.hasText(beanName)) {
            try {
                if (containingBean != null) {
                    // 3.1、beanName 还是不存在，则根据 Spring 提供的命名规则生成唯一的 beanName ，
                    // 这个是内部 bean ，比较常见的格式就是： 类全限定名#十六进制字符串（对象的标识哈希码）
                    beanName = BeanDefinitionReaderUtils.generateBeanName(
                            beanDefinition, this.readerContext.getRegistry(), true);
                }
                else {
                    // 3.2、生成唯一的 beanName ，比较常见的格式就是： 类全限定名#counter(从0开始)
                    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.
                    // 增加别名 beanClassName ，兼容 1.2/2.0
                    String beanClassName = beanDefinition.getBeanClassName();
                    if (beanClassName != null &&
                            beanName.startsWith(beanClassName) && beanName.length() > beanClassName.length() &&
                            !this.readerContext.getRegistry().isBeanNameInUse(beanClassName)) {
                        aliases.add(beanClassName);
                    }
                }
                if (logger.isTraceEnabled()) {
                    logger.trace("Neither XML ‘id‘ nor ‘name‘ specified - " +
                            "using generated bean name [" + beanName + "]");
                }
            }
            catch (Exception ex) {
                error(ex.getMessage(), ele);
                return null;
            }
        }
        // 4、创建 BeanDefinitionHolder 对象
        String[] aliasesArray = StringUtils.toStringArray(aliases);
        return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);
    }

    return null;
}

/**
 * Validate that the specified bean name and aliases have not been used already
 * within the current level of beans element nesting.
 */
protected void checkNameUniqueness(String beanName, List aliases, Element beanElement) {
    // 检查 beanName 和 alias 是否已经被使用
    String foundName = null;

    if (StringUtils.hasText(beanName) && this.usedNames.contains(beanName)) {
        foundName = beanName;
    }
    if (foundName == null) {
        foundName = CollectionUtils.findFirstMatch(this.usedNames, aliases);
    }
    // 如果 beanName 已存在，则使用 problemReporter 提示错误
    if (foundName != null) {
        error("Bean name ‘" + foundName + "‘ is already used in this  element", beanElement);
    }
    // 将 beanName 和 alias 添加到 usedNames 集合中
    this.usedNames.add(beanName);
    this.usedNames.addAll(aliases);
}

private final BeanDefinition beanDefinition;

private final String beanName;

@Nullable
private final String[] aliases;

/**
 * Parse the bean definition itself, without regard to name or aliases. May return
 * {@code null} if problems occurred during the parsing of the bean definition.
 */
@Nullable
public AbstractBeanDefinition parseBeanDefinitionElement(
        Element ele, String beanName, @Nullable BeanDefinition containingBean) {

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

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

    try {
        // 创建用于承载属性的 AbstractBeanDefinition 类型的 GenericBeanDefinition 实例对象
        AbstractBeanDefinition bd = createBeanDefinition(className, parent);
        // 解析默认 bean 标签的各种属性
        parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);
        // 提取 description
        bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));

        /// 下面这些是解析 bean 标签的子元素

        // 解析元数据 
        parseMetaElements(ele, bd);
        // 解析 lookup-method 子元素 
        parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
        // 解析 replaced-method 子元素 
        parseReplacedMethodSubElements(ele, bd.getMethodOverrides());

        // 解析构造函数参数 
        parseConstructorArgElements(ele, bd);
        // 解析 property 子元素 
        parsePropertyElements(ele, bd);
        // 解析 qualifier 子元素 
        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;
}

/**
 * Create a bean definition for the given class name and parent name.
 * @param className the name of the bean class
 * @param parentName the name of the bean‘s parent bean
 * @return the newly created bean definition
 * @throws ClassNotFoundException if bean class resolution was attempted but failed
 */
protected AbstractBeanDefinition createBeanDefinition(@Nullable String className, @Nullable String parentName)
        throws ClassNotFoundException {

    return BeanDefinitionReaderUtils.createBeanDefinition(
            parentName, className, this.readerContext.getBeanClassLoader());
}

/// org.springframework.beans.factory.support.BeanDefinitionReaderUtils

/**
 * Create a new GenericBeanDefinition for the given parent name and class name,
 * eagerly loading the bean class if a ClassLoader has been specified.
 * @param parentName the name of the parent bean, if any
 * @param className the name of the bean class, if any
 * @param classLoader the ClassLoader to use for loading bean classes
 * (can be {@code null} to just register bean classes by name)
 * @return the bean definition
 * @throws ClassNotFoundException if the bean class could not be loaded
 */
public static AbstractBeanDefinition createBeanDefinition(
        @Nullable String parentName, @Nullable String className, @Nullable ClassLoader classLoader) throws ClassNotFoundException {

    GenericBeanDefinition bd = new GenericBeanDefinition();
    bd.setParentName(parentName);
    if (className != null) {
        if (classLoader != null) {
            bd.setBeanClass(ClassUtils.forName(className, classLoader));
        }
        else {
            bd.setBeanClassName(className);
        }
    }
    return bd;
}