spring源码剖析（四）自定义标签解析流程

解析总流程

自定义标签解析入口

 让我们先看BeanDefinitionParserDelegate类的parseCustomElement方法:

	public BeanDefinition parseCustomElement(Element ele) {
		return parseCustomElement(ele, null);
	}
	//containingBd为父类bean,对顶层元素的解析应设置为null
	public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) {
		String namespaceUri = getNamespaceURI(ele);
		//根据命名空间找到对应的NamespaceHandler进行解析
		NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);
		if (handler == null) {
			error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele);
			return null;
		}
		//调用自定义的NamespaceHandler进行解析
		return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));
	}

提取自定义标签处理器

跟踪this.readerContext.getNamespaceHandlerResolver(). resolve(namespaceUri);

进入DefaultNamespaceHandlerResolver中的resolve方法

	/**
	 * Locate the {@link NamespaceHandler} for the supplied namespace URI
	 * from the configured mappings.
	 * @param namespaceUri the relevant namespace URI
	 * @return the located {@link NamespaceHandler}, or {@code null} if none found
	 */
	public NamespaceHandler resolve(String namespaceUri) {
		//获取所有已经配置的handler映射
		Map<String, Object> handlerMappings = getHandlerMappings();
		//根据命名空间找打对应信息
		Object handlerOrClassName = handlerMappings.get(namespaceUri);
		if (handlerOrClassName == null) {
			return null;
		}
		else if (handlerOrClassName instanceof NamespaceHandler) {
			//已做过解析的情况,直接从缓存读取
			return (NamespaceHandler) handlerOrClassName;
		}
		else {
			//没有做过解析,则返回的是类路径
			String className = (String) handlerOrClassName;
			try {
				//使用反射将类路径转换成类
				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");
				}
				//初始化类
				NamespaceHandler namespaceHandler = (NamespaceHandler) BeanUtils.instantiateClass(handlerClass);
				//调用自定义的NamespaceHandler的初始化方法
				namespaceHandler.init();
				//记录在缓存
				handlerMappings.put(namespaceUri, namespaceHandler);
				return namespaceHandler;
			}
			catch (ClassNotFoundException ex) {
				throw new FatalBeanException("NamespaceHandler class [" + className + "] for namespace [" +
						namespaceUri + "] not found", ex);
			}
			catch (LinkageError err) {
				throw new FatalBeanException("Invalid NamespaceHandler class [" + className + "] for namespace [" +
						namespaceUri + "]: problem with handler class file or dependent class", err);
			}
		}
	}

回忆上一篇的自定义标签使用中的MyNamespaceHandler便可以发现,其实上面的方法就是初始化了MyNamespaceHandler,然后调用了他的init()方法

/**
 * 
 * @author Administrator
 */
public class MyNamespaceHandler extends NamespaceHandlerSupport {

	/* (non-Javadoc)
	 * @see org.springframework.beans.factory.xml.NamespaceHandler#init()
	 */
	@Override
	public void init() {
		registerBeanDefinitionParser("user", new UserBeanDefinitionParser());
	}

}

让我们在看看spring是如何获取handler映射的,查看getHandlerMappings()方法

/**
	 * Load the specified NamespaceHandler mappings lazily.
	 */
	private Map<String, Object> getHandlerMappings() {
		//如果没有被缓存,则开始进入缓存
		if (this.handlerMappings == null) {
			synchronized (this) {
				if (this.handlerMappings == null) {
					try {
						//this.handlerMappings在构造函数已被初始化为META-INF/Spring.handlers
						Properties mappings =
								PropertiesLoaderUtils.loadAllProperties(this.handlerMappingsLocation, this.classLoader);
						if (logger.isDebugEnabled()) {
							logger.debug("Loaded NamespaceHandler mappings: " + mappings);
						}
						Map<String, Object> handlerMappings = new ConcurrentHashMap<String, Object>(mappings.size());
						//将properties文件合并到Map格式的handlerMappings中
						CollectionUtils.mergePropertiesIntoMap(mappings, handlerMappings);
						this.handlerMappings = handlerMappings;
					}
					catch (IOException ex) {
						throw new IllegalStateException(
								"Unable to load NamespaceHandler mappings from location [" + this.handlerMappingsLocation + "]", ex);
					}
				}
			}
		}
		return this.handlerMappings;
	}

标签解析

得到了解析器和分析的元素侯,Spring就可以将解析工作委托给自定义解析器去解析了

让我们回到BeanDefinitionParserDelegate的handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));中

进入NamespaceHandlerSupport中的parse方法

	/**
	 * Parses the supplied {@link Element} by delegating to the {@link BeanDefinitionParser} that is
	 * registered for that {@link Element}.
	 */
	public BeanDefinition parse(Element element, ParserContext parserContext) {
		//寻找解析器,并进行解析操作
		return findParserForElement(element, parserContext).parse(element, parserContext);
	}

	/**
	 * Locates the {@link BeanDefinitionParser} from the register implementations using
	 * the local name of the supplied {@link Element}.
	 */
	private BeanDefinitionParser findParserForElement(Element element, ParserContext parserContext) {
		//获取元素名称,也就是<myname:user 中的user,此时localname为user
		String localName = parserContext.getDelegate().getLocalName(element);
		//根据user找到对应的解析器也就是在
		//registerBeanDefinitionParser("user", new UserBeanDefinitionParser());注册的解析器
		BeanDefinitionParser parser = this.parsers.get(localName);
		if (parser == null) {
			parserContext.getReaderContext().fatal(
					"Cannot locate BeanDefinitionParser for element [" + localName + "]", element);
		}
		return parser;
	}

让我们再跟踪到AbstractBeanDefinitionParser 的parse()方法

public final BeanDefinition parse(Element element, ParserContext parserContext) {
		AbstractBeanDefinition definition = parseInternal(element, parserContext);
		if (definition != null && !parserContext.isNested()) {
			try {
				String id = resolveId(element, definition, parserContext);
				if (!StringUtils.hasText(id)) {
					parserContext.getReaderContext().error(
							"Id is required for element '" + parserContext.getDelegate().getLocalName(element)
									+ "' when used as a top-level tag", element);
				}
				String[] aliases = new String[0];
				String name = element.getAttribute(NAME_ATTRIBUTE);
				if (StringUtils.hasLength(name)) {
					aliases = StringUtils.trimArrayElements(StringUtils.commaDelimitedListToStringArray(name));
				}
				//将AbstractBeanDefinition转换成BeanDefinitionHolder
				BeanDefinitionHolder holder = new BeanDefinitionHolder(definition, id, aliases);
				registerBeanDefinition(holder, parserContext.getRegistry());
				if (shouldFireEvents()) {
					//需要通知监听器进行处理
					BeanComponentDefinition componentDefinition = new BeanComponentDefinition(holder);
					postProcessComponentDefinition(componentDefinition);
					parserContext.registerComponent(componentDefinition);
				}
			}
			catch (BeanDefinitionStoreException ex) {
				parserContext.getReaderContext().error(ex.getMessage(), element);
				return null;
			}
		}
		return definition;
	}

虽然说是对自定义配置文件的解析,但是我们可以看到,在这个函数中大部分的大马士用来处理将解析后的AbstractBeanDefinition转换为BeanDefinitionHolder并注册的功能,而真正去做解析的事情委托了给parseInternal,真是这句代码调用了我们的自定义解析函数。在parseInternal中，并不是直接调用自定义的doParse函数，而是惊醒了一些列的数据准备，包括对beanClass，scope，lazyInit等属性的准备。

接下来，让我们看看parseInternal方法，先跟踪到AbstractSingleBeanDefinitionParser

	/**
	 * Creates a {@link BeanDefinitionBuilder} instance for the
	 * {@link #getBeanClass bean Class} and passes it to the
	 * {@link #doParse} strategy method.
	 * @param element the element that is to be parsed into a single BeanDefinition
	 * @param parserContext the object encapsulating the current state of the parsing process
	 * @return the BeanDefinition resulting from the parsing of the supplied {@link Element}
	 * @throws IllegalStateException if the bean {@link Class} returned from
	 * {@link #getBeanClass(org.w3c.dom.Element)} is {@code null}
	 * @see #doParse
	 */
	@Override
	protected final AbstractBeanDefinition parseInternal(Element element, ParserContext parserContext) {
		BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition();
		String parentName = getParentName(element);
		if (parentName != null) {
			builder.getRawBeanDefinition().setParentName(parentName);
		}
		//获取自定义标签中的class,此时会调用自定义解析器,如UserBeanDefinitionParser中的getBeanClass方法
		Class<?> beanClass = getBeanClass(element);
		if (beanClass != null) {
			builder.getRawBeanDefinition().setBeanClass(beanClass);
		}
		else {
			//若子类没有重写getBeanClass方法则尝试检查子类是否重写getBeanClassName方法
			String beanClassName = getBeanClassName(element);
			if (beanClassName != null) {
				builder.getRawBeanDefinition().setBeanClassName(beanClassName);
			}
		}
		builder.getRawBeanDefinition().setSource(parserContext.extractSource(element));
		if (parserContext.isNested()) {
			// Inner bean definition must receive same scope as containing bean.
			builder.setScope(parserContext.getContainingBeanDefinition().getScope());
		}
		if (parserContext.isDefaultLazyInit()) {
			// Default-lazy-init applies to custom bean definitions as well.
			builder.setLazyInit(true);
		}
		doParse(element, parserContext, builder);
		return builder.getBeanDefinition();
	}