spring源码（四）自定义标签

自定义标签也叫自定义命名空间

用过dubbo的同学应该都知道怎么在xml中配置服务的发布与引用，而且配置dubbo的配置文件是被spring容器加载的，原生的spring可是没有< dubbo:xxx >这样的标签的,spring是如何解析和加载这种标签的对象呢

自己实现一套自定义标签

如何构建其实网上都有很多实现，主要分为以下几个步骤：

1. 在资源文件夹MATE-INF下创建spring.schemas和spring.handlers；
2. 创建标签元素定义的xsd文件，并在spring.schemas中指定xsd位置；
3. 创建命名空间处理器NamespaceHandler，并在spring.handlers中指定处理器位置；
4. 创建解析对应标签元素的解析器BeanDefinitionParser；
5. 在配置文件中使用我们的自定义标签；

具体实现

spring.schemas的内容提示自定义标签xsd文件的位置

//这里的key和value，如果value对于的位置没有找到xsd文件，就会去key的路径网上下载
http\://www.lzl.com/schema/lzl/lzl.xsd=META-INF/lzl.xsd

spring.handlers的内容指定了标签的命名空间处理器所在位置

http\://www.lzl.com/schema/lzl=com.lzl.springscope.selfNameSpace.LZLNameSpaceHandler

xsd文件是我们最需要考虑如何设计的

配置文件引入自定义标签

实现自定义的命名空间处理器

public class LZLNameSpaceHandler extends NamespaceHandlerSupport {
    @Override
    public void init() {
        registerBeanDefinitionParser("person",new LZLBeanDefinitionParser());
    }
}

实现自定义的解析器

public class LZLBeanDefinitionParser extends AbstractSingleBeanDefinitionParser {
    public void doParse(Element element, BeanDefinitionBuilder builder){
        String name = element.getAttribute("name");
        if(null!= name && name.length()>0){
            builder.addPropertyValue("name",name);
        }
        String sex = element.getAttribute("sex");
        if(null!= name && name.length()>0){
            builder.addPropertyValue("sex",sex);
        }
        String age = element.getAttribute("age");
        if(null!= name && name.length()>0){
            builder.addPropertyValue("age",Integer.valueOf(age));
        }
        builder.getRawBeanDefinition().setBeanClass(Person.class);
    }

//    protected Class getBeanClass(Element element) {
//        String clazz = element.getAttribute("class");
//        if(clazz!=null&&clazz.length()>0){
//            try {
//                return Class.forName(clazz);
//            } catch (ClassNotFoundException e) {
//                return Person.class;
//            }
//        }
//        return Person.class;
//    }
}

这里解析器的原理是首先spring把xml文件解析成dom元素，然后调用对应的命名空间处理器来对指定的元素进行解析，解析的主要逻辑是根据dom元素的属性来构建一个beanDefinition，这里需要注意的一点生成的beanDefinition要指定beanClass，才能在之后构建成你想要的bean，最后把构建好的beanDefinition注册到容器中，放到beanDefinitionMap中。

源码解析

首先看spring是如何加载配置文件的

ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("classpath:application.xml");

在构造函数中，spring把配置文件的路径存到成员变量configLocations中，然后调用主流程refresh()方法

public ClassPathXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)
			throws BeansException {

		super(parent);
		setConfigLocations(configLocations);
		if (refresh) {
			refresh();
		}
	}

在refresh方法的主流程中

public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// Prepare this context for refreshing.
			prepareRefresh();

			// Tell the subclass to refresh the internal bean factory.
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// Prepare the bean factory for use in this context.
			prepareBeanFactory(beanFactory);

			try {
				// Allows post-processing of the bean factory in context subclasses.
				postProcessBeanFactory(beanFactory);

				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);

				// Register bean processors that intercept bean creation.
				registerBeanPostProcessors(beanFactory);

				// Initialize message source for this context.
				initMessageSource();

				// Initialize event multicaster for this context.
				initApplicationEventMulticaster();

				// Initialize other special beans in specific context subclasses.
				onRefresh();

				// Check for listener beans and register them.
				registerListeners();

				// Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory);

				// Last step: publish corresponding event.
				finishRefresh();
			}

			catch (BeansException ex) {
				if (logger.isWarnEnabled()) {
					logger.warn("Exception encountered during context initialization - " +
							"cancelling refresh attempt: " + ex);
				}

				// Destroy already created singletons to avoid dangling resources.
				destroyBeans();

				// Reset 'active' flag.
				cancelRefresh(ex);

				// Propagate exception to caller.
				throw ex;
			}

			finally {
				// Reset common introspection caches in Spring's core, since we
				// might not ever need metadata for singleton beans anymore...
				resetCommonCaches();
			}
		}
	}

我们需要跟到刷新beanFactory的方法中refreshBeanFactory()

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
		refreshBeanFactory();
		ConfigurableListableBeanFactory beanFactory = getBeanFactory();
		if (logger.isDebugEnabled()) {
			logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
		}
		return beanFactory;
	}

在AbstractRefreshableApplicationContext类中，我们可以看到这里开始了加载beanDefinition

protected final void refreshBeanFactory() throws BeansException {
		if (hasBeanFactory()) {
			destroyBeans();
			closeBeanFactory();
		}
		try {
			DefaultListableBeanFactory beanFactory = createBeanFactory();
			beanFactory.setSerializationId(getId());
			customizeBeanFactory(beanFactory);
			loadBeanDefinitions(beanFactory);
			synchronized (this.beanFactoryMonitor) {
				this.beanFactory = beanFactory;
			}
		}
		catch (IOException ex) {
			throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
		}
	}

在AbstractXmlApplicationContext类中，创建了XmlBeanDefinitionReader来加载beanDefinition，并且取的资源就是我们传进来的配置文件。

protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
		// Create a new XmlBeanDefinitionReader for the given BeanFactory.
		XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

		// Configure the bean definition reader with this context's
		// resource loading environment.
		beanDefinitionReader.setEnvironment(this.getEnvironment());
		beanDefinitionReader.setResourceLoader(this);
		beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

		// Allow a subclass to provide custom initialization of the reader,
		// then proceed with actually loading the bean definitions.
		initBeanDefinitionReader(beanDefinitionReader);
		loadBeanDefinitions(beanDefinitionReader);
	}


protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
		Resource[] configResources = getConfigResources();
		if (configResources != null) {
			reader.loadBeanDefinitions(configResources);
		}
		String[] configLocations = getConfigLocations();
		if (configLocations != null) {
			reader.loadBeanDefinitions(configLocations);
		}
	}

在XmlBeanDefinitionReader类中实现了如何加载资源文件，先将资源文件传成数据流的形式，然后根据dom解析成dom元素，并创建BeanDefinitionDocumentReader 来实现解析dom元素。

public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
		Assert.notNull(encodedResource, "EncodedResource must not be null");
		if (logger.isInfoEnabled()) {
			logger.info("Loading XML bean definitions from " + encodedResource.getResource());
		}

		Set currentResources = this.resourcesCurrentlyBeingLoaded.get();
		if (currentResources == null) {
			currentResources = new HashSet(4);
			this.resourcesCurrentlyBeingLoaded.set(currentResources);
		}
		if (!currentResources.add(encodedResource)) {
			throw new BeanDefinitionStoreException(
					"Detected cyclic loading of " + encodedResource + " - check your import definitions!");
		}
		try {
			InputStream inputStream = encodedResource.getResource().getInputStream();
			try {
				InputSource inputSource = new InputSource(inputStream);
				if (encodedResource.getEncoding() != null) {
					inputSource.setEncoding(encodedResource.getEncoding());
				}
				return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
			}
			finally {
				inputStream.close();
			}
		}
		catch (IOException ex) {
			throw new BeanDefinitionStoreException(
					"IOException parsing XML document from " + encodedResource.getResource(), ex);
		}
		finally {
			currentResources.remove(encodedResource);
			if (currentResources.isEmpty()) {
				this.resourcesCurrentlyBeingLoaded.remove();
			}
		}
	}

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
			throws BeanDefinitionStoreException {
		try {
			Document doc = doLoadDocument(inputSource, resource);
			return registerBeanDefinitions(doc, resource);
		}
		catch (BeanDefinitionStoreException ex) {
			throw ex;
		}
		catch (SAXParseException ex) {
			throw new XmlBeanDefinitionStoreException(resource.getDescription(),
					"Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
		}
		catch (SAXException ex) {
			throw new XmlBeanDefinitionStoreException(resource.getDescription(),
					"XML document from " + resource + " is invalid", ex);
		}
		catch (ParserConfigurationException ex) {
			throw new BeanDefinitionStoreException(resource.getDescription(),
					"Parser configuration exception parsing XML from " + resource, ex);
		}
		catch (IOException ex) {
			throw new BeanDefinitionStoreException(resource.getDescription(),
					"IOException parsing XML document from " + resource, ex);
		}
		catch (Throwable ex) {
			throw new BeanDefinitionStoreException(resource.getDescription(),
					"Unexpected exception parsing XML document from " + resource, ex);
		}
	}

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
		BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
		int countBefore = getRegistry().getBeanDefinitionCount();
		documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
		return getRegistry().getBeanDefinitionCount() - countBefore;
	}

在DefaultBeanDefinitionDocumentReader类中解析dom元素，主要有两种模式，一种是含有bean、alias、import标签的元素使用默认的解析逻辑，其他的采用委托的方式，使用用户自定义的解析器去解析。

protected void doRegisterBeanDefinitions(Element root) {
		// Any nested  elements will cause recursion in this method. In
		// order to propagate and preserve  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.
		BeanDefinitionParserDelegate parent = this.delegate;
		this.delegate = createDelegate(getReaderContext(), root, parent);

		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)) {
					if (logger.isInfoEnabled()) {
						logger.info("Skipped XML bean definition file due to specified profiles [" + profileSpec +
								"] not matching: " + getReaderContext().getResource());
					}
					return;
				}
			}
		}

		preProcessXml(root);
		parseBeanDefinitions(root, this.delegate);
		postProcessXml(root);

		this.delegate = parent;
	}

protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
		if (delegate.isDefaultNamespace(root)) {
			NodeList nl = root.getChildNodes();
			for (int i = 0; i < nl.getLength(); i++) {
				Node node = nl.item(i);
				if (node instanceof Element) {
					Element ele = (Element) node;
					if (delegate.isDefaultNamespace(ele)) {
						parseDefaultElement(ele, delegate);
					}
					else {
						delegate.parseCustomElement(ele);
					}
				}
			}
		}
		else {
			delegate.parseCustomElement(root);
		}
	}

在BeanDefinitionParserDelegate类中首先找到元素对应的命名空间处理器，然后调用注册在容器中对应元素的解析器beanDefinitionParser

public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) {
		String namespaceUri = getNamespaceURI(ele);
		NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);
		if (handler == null) {
			error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele);
			return null;
		}
		return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));
	}

在NamespaceHandlerSupport类中调用自定义解析器解析元素，并注册beanDefinition

public BeanDefinition parse(Element element, ParserContext parserContext) {
		return findParserForElement(element, parserContext).parse(element, parserContext);
	}