Spring4.3.x 淺析xml配置的解析過程（3）——使用DocumentLoader創(chuàng)建Document對象

2019-11-10 20:13:09

字體：大中小

供稿：網(wǎng)友

準備工作

SPRing的xmlBeanDefinitionReader通過ResourceLoader創(chuàng)建了Resource對象后，又如何處理Resource對象呢？XmlBeanDefinitionReader拿到Resource對象后，會調(diào)用它的loadBeanDefinitions(Resource resource)方法，下面我們就根據(jù)這個方法為入口來探討這個問題，見下面的代碼。

public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException { return loadBeanDefinitions(new EncodedResource(resource)); }

通過上面的代碼我們看到XmlBeanDefinitionReader拿到Resource對象后，首先把它封裝成EncodedResource 對象來調(diào)用它的loadBeanDefinitions(EncodedResource encodedResource)方法，下面是此方法的源碼。

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<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get(); if (currentResources == null) { currentResources = new HashSet<EncodedResource>(4); this.resourcesCurrentlyBeingLoaded.set(currentResources); } if (!currentResources.add(encodedResource)) { throw new BeanDefinitionStoreException( "Detected cyclic loading of " + encodedResource + " - check your import definitions!"); } try { // 讀取資源文件輸入流 InputStream inputStream = encodedResource.getResource().getInputStream(); try { InputSource inputSource = new InputSource(inputStream); if (encodedResource.getEncoding() != null) { inputSource.setEncoding(encodedResource.getEncoding()); } // 根據(jù)指定的XML文件加載BeanDefinition 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(); } } }

上面的代碼把Resource對象持有的xml輸入流對象封裝成解析XML文件所需的InputSource對象，這個對象被SAX解析器用來決定怎么讀取xml文件中的內(nèi)容。我們繼續(xù)看doLoadBeanDefinitions方法在XmlBeanDefinitionReader類中的源碼。

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource) throws BeanDefinitionStoreException { try { // 加載Document對象 Document doc = doLoadDocument(inputSource, resource); // 注冊BeanDefinition return registerBeanDefinitions(doc, resource); } catch (BeanDefinitionStoreException ex) { throw ex; } catch (SAXParseException ex) { throw new XmlBeanDefinitionStoreException(resource.getDescr進入doLoadBeanDefinitions方法，也就離我們探討的主題不遠了，doLoadDocument方法返回的正是我們要探討的目標Document對象，下面是XmlBeanDefinitionReader的doLoadDocument方法源碼。

/** * 獲取Document對象 **/ protected Document doLoadDocument(InputSource inputSource, Resource resource) throws Exception { // 使用DocumentLoader來加載Document對象 return this.documentLoader.loadDocument(inputSource, getEntityResolver(), this.errorHandler, getValidationModeForResource(resource), isNamespaceAware()); }

doLoadDocument通過使用DocumentLoader對象來加載Document對象，但這里在使用DocumentLoader對象之前還需要做以下5個準備工作 a. 獲取DocumentLoader對象。 b. 獲取EntityResolver對象。 c. 獲取ErrorHandler對象。 d. 獲取xml驗證模式。 e. 設置xml命名空間是否敏感其中DocumentLoader對象默認為DefaultDocumentLoader；ErrorHandler對象默認為SimpleSaxErrorHandler；namespaceAware默認為false，即xml命名空間不敏感。這三個默認對象都可以通過XmlBeanDefinitionReader所提供的setter方法更改。下面來看看EntityResolver對象和xml驗證模式的獲取。（1）獲取EntityResolver對象 XmlBeanDefinitionReader通過它的getEntityResolver方法獲取EntityResolver對象，getEntityResolver的代碼如下。

protected EntityResolver getEntityResolver() { if (this.entityResolver == null) { ResourceLoader resourceLoader = getResourceLoader(); if (resourceLoader != null) { this.entityResolver = new ResourceEntityResolver(resourceLoader); } else { this.entityResolver = new DelegatingEntityResolver(getBeanClassLoader()); } } return this.entityResolver; }

如果XmlBeanDefinitionReader持有的EntityResolver對象不為空，則直接返回。如果XmlBeanDefinitionReader持有的ResourceLoader對象不為空，則返回ResourceEntityResolver對象，否則返回DelegatingEntityResolver對象。

（2）獲取xml驗證模式

protected int getValidationModeForResource(Resource resource) { // 獲取XmlBeanDefinitionReader設置的驗證模式 int validationModeToUse = getValidationMode(); if (validationModeToUse != VALIDATION_AUTO) { return validationModeToUse; } // 沒有明確的驗證模式，從Resource對象中檢測驗證模式 int detectedMode = detectValidationMode(resource); if (detectedMode != VALIDATION_AUTO) { return detectedMode; } // 返回默認的驗證模式xsd return VALIDATION_XSD; }

上面的代碼中我們來看看從Resource對象中檢測驗證模式的detectValidationMode方法的代碼，如下。

protected int detectValidationMode(Resource resource) { if (resource.isOpen()) { throw new BeanDefinitionStoreException( "Passed-in Resource [" + resource + "] contains an open stream: " + "cannot determine validation mode automatically. Either pass in a Resource " + "that is able to create fresh streams, or explicitly specify the validationMode " + "on your XmlBeanDefinitionReader instance."); } InputStream inputStream; try { inputStream = resource.getInputStream(); } catch (IOException ex) { throw new BeanDefinitionStoreException( "Unable to determine validation mode for [" + resource + "]: cannot open InputStream. " + "Did you attempt to load directly from a SAX InputSource without specifying the " + "validationMode on your XmlBeanDefinitionReader instance?", ex); } try { return this.validationModeDetector.detectValidationMode(inputStream); } catch (IOException ex) { throw new BeanDefinitionStoreException("Unable to determine validation mode for [" + resource + "]: an error occurred whilst reading from the InputStream.", ex); } }

detectValidationMode方法使用驗證模式檢測器來從xml輸入流中檢測，XmlBeanDefinitionReader中默認的驗證模式檢測器為XmlValidationModeDetector。我們來看看XmlValidationModeDetector的detectValidationMode方法的代碼，如下。

public int detectValidationMode(InputStream inputStream) throws IOException { // Peek into the file to look for DOCTYPE. BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream)); try { // dtd驗證模式標志 boolean isDtdValidated = false; String content; while ((content = reader.readLine()) != null) { content = consumeCommentTokens(content); if (this.inComment || !StringUtils.hasText(content)) { continue; } // 判斷當前行是否包含DOCTYPE，有則是dtd模式 if (hasDoctype(content)) { isDtdValidated = true; break; } // 判斷當前行是否以‘<’+字母開始 if (hasOpeningTag(content)) { break; } } // 聲明：public static final int VALIDATION_XSD = 3; // 聲明：public static final int VALIDATION_DTD = 2; return (isDtdValidated ? VALIDATION_DTD : VALIDATION_XSD); } catch (CharConversionException ex) { return VALIDATION_AUTO; } finally { reader.close(); } }

這段代碼主要是讀取xml文件流的前幾行來判斷是否含有DOCTYPE字符串，如果有則是dtd驗證模式，否則是xsd驗證模式。

使用DocumentLoader對象創(chuàng)建Document對象

前面我們已經(jīng)探討了spring使用DocumentLoader對象前需要做的準備工作，包括獲取解析xml文件中的實體的解析器EntityResolver對象、獲取xml文件的驗證模式、獲取解析xml文件需要的InputSource對象以及獲取處理xml文件解析錯誤的ErrorHandler對象。現(xiàn)在我們開始探討DocumentLoader的執(zhí)行流程。

Spring提供DocumentLoader接口來加載Document對象。并提供了DocumentLoader的默認實現(xiàn)類DefaultDocumentLoader。下面是DefaultDocumentLoader實現(xiàn)loadDocument方法的源代碼。

@Override public Document loadDocument(InputSource inputSource, EntityResolver entityResolver, ErrorHandler errorHandler, int validationMode, boolean namespaceAware) throws Exception { DocumentBuilderFactory factory = createDocumentBuilderFactory(validationMode, namespaceAware); if (logger.isDebugEnabled()) { logger.debug("Using JAXP provider [" + factory.getClass().getName() + "]"); } DocumentBuilder builder = createDocumentBuilder(factory, entityResolver, errorHandler); return builder.parse(inputSource); }

loadDocument方法首先創(chuàng)建DocumentBuilderFactory 對象，默認使用com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderFactoryImpl。然后使用DocumentBuilderFactory 來創(chuàng)建DocumentBuilder 對象。最后使用DocumentBuilder 對象來解析持有xml輸入流的InputSource對象并返回創(chuàng)建的Document對象。下面我們來看看這三步的執(zhí)行過程。

（1）創(chuàng)建DocumentBuilderFactory 對象 loadDocument方法調(diào)用DefaultDocumentLoader的createDocumentBuilderFactory方法來創(chuàng)建DocumentBuilderFactory 對象，此方法的源碼如下。

protected DocumentBuilderFactory createDocumentBuilderFactory(int validationMode, boolean namespaceAware) throws ParserConfigurationException { // 實例化DocumentBuilderFactory DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance(); // 下面配置factory factory.setNamespaceAware(namespaceAware); if (validationMode != XmlValidationModeDetector.VALIDATION_NONE) { // 設置使用驗證模式 factory.setValidating(true); if (validationMode == XmlValidationModeDetector.VALIDATION_XSD) { // xsd強制命名空間敏感 factory.setNamespaceAware(true); try { // 聲明：private static final String SCHEMA_LANGUAGE_ATTRIBUTE = "http://java.sun.com/xml/jaxp/properties/schemaLanguage"; // 聲明：private static final String XSD_SCHEMA_LANGUAGE = "http://www.w3.org/2001/XMLSchema"; factory.setAttribute(SCHEMA_LANGUAGE_ATTRIBUTE, XSD_SCHEMA_LANGUAGE); } catch (IllegalArgumentException ex) { ParserConfigurationException pcex = new ParserConfigurationException( "Unable to validate using XSD: Your JAXP provider [" + factory + "] does not support XML Schema. Are you running on Java 1.4 with Apache Crimson? " + "Upgrade to Apache Xerces (or Java 1.5) for full XSD support."); pcex.initCause(ex); throw pcex; } } } return factory; }

createDocumentBuilderFactory方法通過調(diào)用抽象類DocumentBuilderFactory的靜態(tài)方法newInstance()來創(chuàng)建DocumentBuilderFactory對象，默認使用com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderFactoryImpl。當然jdk還提供了4中方式指定自己定義的DocumentBuilderFactory，這里就不深入探討了。

獲取到DocumentBuilderFactory對象后，createDocumentBuilderFactory方法它做了一些定制設置。比如，xsd驗證模式強制命名空間敏感。

（2）創(chuàng)建DocumentBuilder 對象 loadDocument方法調(diào)用DefaultDocumentLoader的createDocumentBuilder方法來返回一個DocumentBuilder 對象，這個方法的源代碼如下。

protected DocumentBuilder createDocumentBuilder( DocumentBuilderFactory factory, EntityResolver entityResolver, ErrorHandler errorHandler) throws ParserConfigurationException { DocumentBuilder docBuilder = factory.newDocumentBuilder(); if (entityResolver != null) { docBuilder.setEntityResolver(entityResolver); } if (errorHandler != null) { docBuilder.setErrorHandler(errorHandler); } return docBuilder; }

createDocumentBuilder方法首先使用DocumentBuilderFactory 對象創(chuàng)建DocumentBuilder 對象，然后把EntityResolver 和ErrorHandler 對象設置給DocumentBuilder 對象。其中我們來看看默認的DocumentBuilderFactory 對象的newDocumentBuilder方法返回的是一個怎么樣的DocumentBuilder 對象，源代碼如下。

public DocumentBuilder newDocumentBuilder() throws ParserConfigurationException { /** Check that if a Schema has been specified that neither of the schema properties have been set. */ // 檢查是否已經(jīng)指定了Schema對象。 if (grammar != null && attributes != null) { // 是否已經(jīng)設置了schema的屬性 if (attributes.containsKey(JAXPConstants.JAXP_SCHEMA_LANGUAGE)) { throw new ParserConfigurationException( SAXMessageFormatter.formatMessage(null, "schema-already-specified", new Object[] {JAXPConstants.JAXP_SCHEMA_LANGUAGE})); } else if (attributes.containsKey(JAXPConstants.JAXP_SCHEMA_SOURCE)) { throw new ParserConfigurationException( SAXMessageFormatter.formatMessage(null, "schema-already-specified", new Object[] {JAXPConstants.JAXP_SCHEMA_SOURCE})); } } try { // 創(chuàng)建一個com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderImpl對象。 return new DocumentBuilderImpl(this, attributes, features, fSecureProcess); } catch (SAXException se) { throw new ParserConfigurationException(se.getMessage()); } }

newDocumentBuilder方法返回一個com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderImpl對象，DocumentBuilderImpl的上述構(gòu)造方法代碼如下。

DocumentBuilderImpl(DocumentBuilderFactoryImpl dbf, Hashtable dbfAttrs, Hashtable features, boolean secureProcessing) throws SAXNotRecognizedException, SAXNotSupportedException { domParser = new DOMParser(); // 設置ErrorHandler對象 if (dbf.isValidating()) { fInitErrorHandler = new DefaultValidationErrorHandler(domParser.getXMLParserConfiguration().getLocale()); setErrorHandler(fInitErrorHandler); } else { fInitErrorHandler = domParser.getErrorHandler(); } domParser.setFeature(VALIDATION_FEATURE, dbf.isValidating()); // 設置命名空間是否敏感 domParser.setFeature(NAMESPACES_FEATURE, dbf.isNamespaceAware()); // 通過DocumentBuilderFactory設置各種變量 domParser.setFeature(INCLUDE_IGNORABLE_WHITESPACE, !dbf.isIgnoringElementContentWhitespace()); domParser.setFeature(CREATE_ENTITY_REF_NODES_FEATURE, !dbf.isExpandEntityReferences()); domParser.setFeature(INCLUDE_COMMENTS_FEATURE, !dbf.isIgnoringComments()); domParser.setFeature(CREATE_CDATA_NODES_FEATURE, !dbf.isCoalescing()); // 設置是否支撐XInclude. if (dbf.isXIncludeAware()) { domParser.setFeature(XINCLUDE_FEATURE, true); } fSecurityPropertyMgr = new XMLSecurityPropertyManager(); domParser.setProperty(XML_SECURITY_PROPERTY_MANAGER, fSecurityPropertyMgr); fSecurityManager = new XMLSecurityManager(secureProcessing); domParser.setProperty(SECURITY_MANAGER, fSecurityManager); if (secureProcessing) { if (features != null) { Object temp = features.get(XMLConstants.FEATURE_SECURE_PROCESSING); if (temp != null) { boolean value = ((Boolean) temp).booleanValue(); if (value && Constants.IS_JDK8_OR_ABOVE) { fSecurityPropertyMgr.setValue(Property.access_EXTERNAL_DTD, State.FSP, Constants.EXTERNAL_ACCESS_DEFAULT_FSP); fSecurityPropertyMgr.setValue(Property.ACCESS_EXTERNAL_SCHEMA, State.FSP, Constants.EXTERNAL_ACCESS_DEFAULT_FSP); } } } } this.grammar = dbf.getSchema(); if (grammar != null) { XMLParserConfiguration config = domParser.getXMLParserConfiguration(); XMLComponent validatorComponent = null; // 對于Xerces grammars，使用內(nèi)置的schema驗證器 if (grammar instanceof XSGrammarPoolContainer) { validatorComponent = new XMLSchemaValidator(); fSchemaValidationManager = new ValidationManager(); fUnparsedEntityHandler = new UnparsedEntityHandler(fSchemaValidationManager); config.setDTDHandler(fUnparsedEntityHandler); fUnparsedEntityHandler.setDTDHandler(domParser); domParser.setDTDSource(fUnparsedEntityHandler); fSchemaValidatorComponentManager = new SchemaValidatorConfiguration(config, (XSGrammarPoolContainer) grammar, fSchemaValidationManager); } else { /** 對于第三方grammars, 使用JAXP validator模塊. **/ validatorComponent = new JAXPValidatorComponent(grammar.newValidatorHandler()); fSchemaValidationManager = null; fUnparsedEntityHandler = null; fSchemaValidatorComponentManager = config; } config.addRecognizedFeatures(validatorComponent.getRecognizedFeatures()); config.addRecognizedProperties(validatorComponent.getRecognizedProperties()); setFeatures(features); config.setDocumentHandler((XMLDocumentHandler) validatorComponent); ((XMLDocumentSource)validatorComponent).setDocumentHandler(domParser); domParser.setDocumentSource((XMLDocumentSource) validatorComponent); fSchemaValidator = validatorComponent; } else { fSchemaValidationManager = null; fUnparsedEntityHandler = null; fSchemaValidatorComponentManager = null; fSchemaValidator = null; setFeatures(features); } setDocumentBuilderFactoryAttributes(dbfAttrs); // 初始化EntityResolver fInitEntityResolver = domParser.getEntityResolver(); }

（3）創(chuàng)建Document對象 loadDocument方法調(diào)用DocumentBuilder的parse方法來返回一個Document對象，我們來看看com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderImpl的parse方法源碼。

public Document parse(InputSource is) throws SAXException, IOException { if (is == null) { throw new IllegalArgumentException( DOMMessageFormatter.formatMessage(DOMMessageFormatter.DOM_DOMAIN, "jaxp-null-input-source", null)); } // 重置schema驗證器 if (fSchemaValidator != null) { if (fSchemaValidationManager != null) { fSchemaValidationManager.reset(); fUnparsedEntityHandler.reset(); } resetSchemaValidator(); } // 使用DomParser對象解析xml文件 domParser.parse(is); // 獲取Document對象 Document doc = domParser.getDocument(); // 刪除與Document創(chuàng)建有關(guān)的引用 domParser.dropDocumentReferences(); return doc; }

關(guān)于這里parse方法，就不過多的探討它。我們只需知道通過它，可以獲取到Document對象就行了。