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nullpublic 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() */@Overridepublic 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.handlersProperties 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userString 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BeanDefinitionHolderBeanDefinitionHolder 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 */@Overrideprotected 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();}