上一章我们介绍了Spring如何创建bean，<bean></bean>的命名空间是Spring默认的命名空间，那么对于<tx:advice></tx:advice>、<mvc:annotation-driven></mvc:annotation-driven>这种自定义的标签该如何解析呢？下面就以这几个标签为例进行说明，同时也说明下声明式Spring事物，还有我们在进行SpringMVC开发时配置的<mvc:annotation-driven/>到底起什么作用也会简单介绍。

现在我们回到org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader的parseBeanDefinitions方法中

//org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader  1 /** 2      * Parse the elements at the root level in the document: 3      * "import", "alias", "bean". 4      * @param root the DOM root element of the document 5      */ 6     protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) { 7         if (delegate.isDefaultNamespace(root)) { 8             NodeList nl = root.getChildNodes(); 9             for (int i = 0; i < nl.getLength(); i++) {10                 Node node = nl.item(i);11                 if (node instanceof Element) {12                     Element ele = (Element) node;13                     if (delegate.isDefaultNamespace(ele)) {14                         parseDefaultElement(ele, delegate);15                     }16                     else {17                         delegate.parseCustomElement(ele);18                     }19                 }20             }21         }22         else {23             delegate.parseCustomElement(root);24         }25     }

第17行，解析自定义的命名空间标签，跟踪方法

//org.springframework.beans.factory.xml.BeanDefinitionParserDelegate  1     public BeanDefinition parseCustomElement(Element ele) { 2         return parseCustomElement(ele, null); 3     } 4  5     public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) { 6         String namespaceUri = getNamespaceURI(ele); 7         NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri); 8         if (handler == null) { 9             error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele);10             return null;11         }12         return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));13     }

第6行，获取标签的命名空间，第7行，获取命名空间的处理器，第12行，调用命名空间处理器的parse方法解析标签。所有的自定义的命名空间的标签都是按照这种方式进行解析的，下面来看下具体都是怎么实现的

this.readerContext是通过org.springframework.beans.factory.xml.XmlBeanDefinitionReader的createReaderContext方法创建的

//org.springframework.beans.factory.xml.XmlBeanDefinitionReader  1     /** 2      * Create the {
    @link XmlReaderContext} to pass over to the document reader. 3      */ 4     public XmlReaderContext createReaderContext(Resource resource) { 5         return new XmlReaderContext(resource, this.problemReporter, this.eventListener, 6                 this.sourceExtractor, this, getNamespaceHandlerResolver()); 7     } 8  9     /**10      * Lazily create a default NamespaceHandlerResolver, if not set before.11      * @see #createDefaultNamespaceHandlerResolver()12      */13     public NamespaceHandlerResolver getNamespaceHandlerResolver() {14         if (this.namespaceHandlerResolver == null) {15             this.namespaceHandlerResolver = createDefaultNamespaceHandlerResolver();16         }17         return this.namespaceHandlerResolver;18     }19 20     /**21      * Create the default implementation of {
    @link NamespaceHandlerResolver} used if none is specified.22      * Default implementation returns an instance of {
    @link DefaultNamespaceHandlerResolver}.23      */24     protected NamespaceHandlerResolver createDefaultNamespaceHandlerResolver() {25         return new DefaultNamespaceHandlerResolver(getResourceLoader().getClassLoader());26     }

根据上边代码我们知道this.readerContext.getNamespaceHandlerResolver()方法返回的是DefaultNamespaceHandlerResolver实例，进入resolve方法

//org.springframework.beans.factory.xml.DefaultNamespaceHandlerResolver  1     /** 2      * Locate the {
    @link NamespaceHandler} for the supplied namespace URI 3      * from the configured mappings. 4      * @param namespaceUri the relevant namespace URI 5      * @return the located {
    @link NamespaceHandler}, or {
    @code null} if none found 6      */ 7     @Override 8     public NamespaceHandler resolve(String namespaceUri) { 9         Map
     
       handlerMappings = getHandlerMappings();10         Object handlerOrClassName = handlerMappings.get(namespaceUri);11         if (handlerOrClassName == null) {12             return null;13         }14         else if (handlerOrClassName instanceof NamespaceHandler) {15             return (NamespaceHandler) handlerOrClassName;16         }17         else {18             String className = (String) handlerOrClassName;19             try {20                 Class
       handlerClass = ClassUtils.forName(className, this.classLoader);21                 if (!NamespaceHandler.class.isAssignableFrom(handlerClass)) {22                     throw new FatalBeanException("Class [" + className + "] for namespace [" + namespaceUri +23                             "] does not implement the [" + NamespaceHandler.class.getName() + "] interface");24                 }25                 NamespaceHandler namespaceHandler = (NamespaceHandler) BeanUtils.instantiateClass(handlerClass);26                 namespaceHandler.init();27                 handlerMappings.put(namespaceUri, namespaceHandler);28                 return namespaceHandler;29             }30             catch (ClassNotFoundException ex) {31                 throw new FatalBeanException("NamespaceHandler class [" + className + "] for namespace [" +32                         namespaceUri + "] not found", ex);33             }34             catch (LinkageError err) {35                 throw new FatalBeanException("Invalid NamespaceHandler class [" + className + "] for namespace [" +36                         namespaceUri + "]: problem with handler class file or dependent class", err);37             }38         }39     }
     

关注第24--28行，根据Class对象实例化NamespaceHandler，调用NamespaceHandler实例的init()方法，把命名空间和NamespaceHandler实例映射关系放入handlerMappings，返回NamespaceHandler实例。那么handlerMappings是怎么来的呢，进入getHandlerMappings()方法

org.springframework.beans.factory.xml.DefaultNamespaceHandlerResolver  1     /** 2      * Load the specified NamespaceHandler mappings lazily. 3      */ 4     private Map
     
       getHandlerMappings() { 5         if (this.handlerMappings == null) { 6             synchronized (this) { 7                 if (this.handlerMappings == null) { 8                     try { 9                         Properties mappings =10                                 PropertiesLoaderUtils.loadAllProperties(this.handlerMappingsLocation, this.classLoader);11                         if (logger.isDebugEnabled()) {12                             logger.debug("Loaded NamespaceHandler mappings: " + mappings);13                         }14                         Map
      
        handlerMappings = new ConcurrentHashMap
       
        (mappings.size());15                         CollectionUtils.mergePropertiesIntoMap(mappings, handlerMappings);16                         this.handlerMappings = handlerMappings;17                     }18                     catch (IOException ex) {19                         throw new IllegalStateException(20                                 "Unable to load NamespaceHandler mappings from location [" + this.handlerMappingsLocation + "]", ex);21                     }22                 }23             }24         }25         return this.handlerMappings;26     }
       
      
     

第9、10行，加载所有Spring的jar文件中的"META-INF/spring.handlers"属性文件，第15行，把属性文件转换成map，第16行，转换后的map赋值到this.handlerMappings。

我们来看看spring-tx-4.0.2.RELEASE.jar和spring-webmvc-4.0.2.RELEASE.jar中spring.handlers文件

从上边两个图中我们可以清楚的看到，每个命名空间都对应命名空间处理器的类全路径。进入org.springframework.transaction.config.TxNamespaceHandler的init方法，看下这个方法都做了那些事情

//org.springframework.transaction.config.TxNamespaceHandler 1     @Override2     public void init() {3         registerBeanDefinitionParser("advice", new TxAdviceBeanDefinitionParser());4         registerBeanDefinitionParser("annotation-driven", new AnnotationDrivenBeanDefinitionParser());5         registerBeanDefinitionParser("jta-transaction-manager", new JtaTransactionManagerBeanDefinitionParser());6     }

//org.springframework.beans.factory.xml.NamespaceHandlerSupport 1     /**2      * Subclasses can call this to register the supplied {
    @link BeanDefinitionParser} to3      * handle the specified element. The element name is the local (non-namespace qualified)4      * name.5      */6     protected final void registerBeanDefinitionParser(String elementName, BeanDefinitionParser parser) {7         this.parsers.put(elementName, parser);8     }

注意registerBeanDefinitionParser方法是在父类org.springframework.beans.factory.xml.NamespaceHandlerSupport中。init()主要是注册了advice、annotation-driven、jta-transaction-manager标签对应的解析器。

回到org.springframework.beans.factory.xml.BeanDefinitionParserDelegate的parseCustomElement方法

//org.springframework.beans.factory.xml.BeanDefinitionParserDelegate   1     public BeanDefinition parseCustomElement(Element ele) { 2         return parseCustomElement(ele, null); 3 } 4 5 public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) { 6 String namespaceUri = getNamespaceURI(ele); 7 NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri); 8 if (handler == null) { 9 error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele); 10 return null; 11 } 12 return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd)); 13 }

 现在NamespaceHandler实例已经构建完成，同时也执行了init()方法，再来看下parse()方法（在父类org.springframework.beans.factory.xml.NamespaceHandlerSupport中），进入方法

1     /** 2      * Parses the supplied {
    @link Element} by delegating to the {
    @link BeanDefinitionParser} that is 3      * registered for that {
    @link Element}. 4      */ 5     @Override 6     public BeanDefinition parse(Element element, ParserContext parserContext) { 7         return findParserForElement(element, parserContext).parse(element, parserContext); 8     } 9 10     /**11      * Locates the {
    @link BeanDefinitionParser} from the register implementations using12      * the local name of the supplied {
    @link Element}.13      */14     private BeanDefinitionParser findParserForElement(Element element, ParserContext parserContext) {15         String localName = parserContext.getDelegate().getLocalName(element);16         BeanDefinitionParser parser = this.parsers.get(localName);17         if (parser == null) {18             parserContext.getReaderContext().fatal(19                     "Cannot locate BeanDefinitionParser for element [" + localName + "]", element);20         }21         return parser;22     }

第16行，得到的是在org.springframework.transaction.config.TxNamespaceHandler的init中注册的各个标签对应的解析器，<tx:advice/>对应的是TxAdviceBeanDefinitionParser实例，所以第7行调用的parse方法就是TxAdviceBeanDefinitionParser中的parse方法，其它的标签也是调用各自的解析器进行解析的。其它的命名空间也是同样的处理方式。

总结Spring解析自定义标签

1、在BeanDefinitionParserDelegate的parseCustomElement(）中调用DefaultNamespaceHandlerResolver的resolve方法

2、resolve方法根据属性文件创建了各个命名空间NameSpaceHandler实例，调用NameSpaceHandler的init()方法注册每个标签所对应的解析器

3、BeanDefinitionParserDelegate的parseCustomElement(）中调用DefaultNamespaceHandlerResolver的parse方法

4、parse方法查找第2步注册的解析器，调用其parse方法解析具体的标签