spring自定义xml标签

spring自定义xml标签

要实现自定义的xml配置，需要有两个默认spring配置文件来支持。一个是spring.schemas,一个是spring.handlers，前者是为了验证你自定义的xml配置文件是否符合你的格式要求，后者是告诉spring该如何来解析你自定义的配置文件。

自定义标签涉及的核心接口为：

• NamespaceHandler
• BeanDefinitionParser

实际使用的时候，一般分别继承类：

• NamespaceHandlerSupport：init
• AbstractSingleBeanDefinitionParser：parse

以spring事务标签为例（前提：要了解bean注册过程）：

1. 配置文件：

• spring.handlers
  

• spring.schemas
  

2. 调用逻辑

org.springframework.context.support.AbstractApplicationContext#refresh
org.springframework.context.support.AbstractApplicationContext#obtainFreshBeanFactory
org.springframework.context.support.AbstractApplicationContext#refreshBeanFactory
org.springframework.context.support.AbstractRefreshableApplicationContext#refreshBeanFactory
org.springframework.context.support.AbstractRefreshableApplicationContext#loadBeanDefinitions
org.springframework.context.support.AbstractXmlApplicationContext#loadBeanDefinitions(org.springframework.beans.factory.support.DefaultListableBeanFactory)

• 在第3步中 registerBeanDefinitions 调用 createReaderContext

    //class XmlBeanDefinitionReader

    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;
    }

• 第4步 createReaderContext 中调用getNamespaceHandlerResolver方法获取： NamespaceHandlerResolver

    //class XmlBeanDefinitionReader

    public XmlReaderContext createReaderContext(Resource resource) {
        return new XmlReaderContext(resource, this.problemReporter, this.eventListener,
                this.sourceExtractor, this, getNamespaceHandlerResolver());
    }

    public NamespaceHandlerResolver getNamespaceHandlerResolver() {
        if (this.namespaceHandlerResolver == null) {
            this.namespaceHandlerResolver = createDefaultNamespaceHandlerResolver();
        }
        return this.namespaceHandlerResolver;
    }

new 一个 DefaultNamespaceHandlerResolver

    //class XmlBeanDefinitionReader
    protected NamespaceHandlerResolver createDefaultNamespaceHandlerResolver() {
        return new DefaultNamespaceHandlerResolver(getResourceLoader().getClassLoader());
    }

DEFAULT_HANDLER_MAPPINGS_LOCATION: META-INF/spring.handlers

    //DefaultNamespaceHandlerResolver

    public DefaultNamespaceHandlerResolver(ClassLoader classLoader) {
        this(classLoader, DEFAULT_HANDLER_MAPPINGS_LOCATION);
    }

    public DefaultNamespaceHandlerResolver(ClassLoader classLoader, String handlerMappingsLocation) {
        Assert.notNull(handlerMappingsLocation, "Handler mappings location must not be null");
        this.classLoader = (classLoader != null ? classLoader : ClassUtils.getDefaultClassLoader());
        this.handlerMappingsLocation = handlerMappingsLocation;
    }

DefaultNamespaceHandlerResolver的属性：private volatile Map handlerMappings保存的是
namespace URI 与 NamespaceHandler的映射

• 第5步调用DefaultBeanDefinitionDocumentReader（父类BeanDefinitionDocumentReader）的 registerBeanDefinitions 方法

    //BeanDefinitionDocumentReader

    public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
        this.readerContext = readerContext;
        logger.debug("Loading bean definitions");
        Element root = doc.getDocumentElement();
        doRegisterBeanDefinitions(root);
    }

    protected void doRegisterBeanDefinitions(Element root) {
        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;
    }

delegate.parseCustomElement(ele); 这一行代码是解析自定义标签

    //class DefaultBeanDefinitionDocumentReader

    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);
        }
    }

1. 根据元素标签获取：NamespaceHandler，并调用其 init 方法
2. 调用 NamespaceHandler 的 parse 方法，在 parse 方法里解析标签，然后返回BeanDefinition，后续会被spring注册

    //class BeanDefinitionParserDelegate

    public BeanDefinition parseCustomElement(Element ele) {
        return parseCustomElement(ele, null);
    }

    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));
    }

3. 定制点

• 定制 init 方法：一般在init方法中注册BeanDefinitionParser, 存储在其父类NamespaceHandlerSupport的属性：parsers 中

• 定制 parse 方法：一般在parse方法中返回BeanDefinition