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

準備工作

Spring的XmlBeanDefinitionReader通過ResourceLoader建立了Resource對象後，又如何處理Resource對象呢？XmlBeanDefinitionReader拿到Resource對象後，會調用它的loadBeanDefinitions(Resource resource)方法，下面我們就根據這個方法為入口來探讨這個問題，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對象後，首先把它封裝成EncodedResource 對象來調用它的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>();
            this.resourcesCurrentlyBeingLoaded.set(currentResources);
        }
        if (!currentResources.add(encodedResource)) {
            throw new BeanDefinitionStoreException(
                    "Detected cyclic loading of " + encodedResource + " - check your import definitions!");
        }
        try {
            // 讀取資源檔案輸入流
            InputStream inputStream = encodedResource.getResource().getInputStream();
            try {
                InputSource inputSource = new InputSource(inputStream);
                if (encodedResource.getEncoding() != null) {
                    inputSource.setEncoding(encodedResource.getEncoding());
                }
                // 根據指定的XML檔案加載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檔案所需的InputSource對象，這個對象被SAX解析器用來決定怎麼讀取xml檔案中的内容。我們繼續看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.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);
        }
    }

進入doLoadBeanDefinitions方法，也就離我們探讨的主題不遠了，doLoadDocument方法傳回的正是我們要探讨的目标Document對象，下面是XmlBeanDefinitionReader的doLoadDocument方法源碼。

/**
    * 擷取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，但這裡在使用DocumentLoader對象之前還需要做以下5個準備工作

a. 擷取DocumentLoader對象。

b. 擷取EntityResolver對象。

c. 擷取ErrorHandler對象。

d. 擷取xml驗證模式。

e. 設定xml命名空間是否敏感

其中DocumentLoader對象預設為DefaultDocumentLoader；ErrorHandler對象預設為SimpleSaxErrorHandler；namespaceAware預設為false，即xml命名空間不敏感。這三個預設對象都可以通過XmlBeanDefinitionReader所提供的setter方法更改。下面來看看EntityResolver對象和xml驗證模式的擷取。

（1）擷取EntityResolver對象

XmlBeanDefinitionReader通過它的getEntityResolver方法擷取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，則直接傳回。

如果XmlBeanDefinitionReader持有的ResourceLoader對象不為空，則傳回ResourceEntityResolver對象，否則傳回DelegatingEntityResolver對象。

（2）擷取xml驗證模式

protected int getValidationModeForResource(Resource resource) {
        // 擷取XmlBeanDefinitionReader設定的驗證模式
        int validationModeToUse = getValidationMode();
        if (validationModeToUse != VALIDATION_AUTO) {
            return validationModeToUse;
        }
        // 沒有明确的驗證模式，從Resource對象中檢測驗證模式
        int detectedMode = detectValidationMode(resource);
        if (detectedMode != VALIDATION_AUTO) {
            return detectedMode;
        }
        // 傳回預設的驗證模式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中預設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驗證模式标志
            boolean isDtdValidated = false;
            String content;
            while ((content = reader.readLine()) != null) {
                content = consumeCommentTokens(content);
                if (this.inComment || !StringUtils.hasText(content)) {
                    continue;
                }
                // 判斷目前行是否包含DOCTYPE，有則是dtd模式
                if (hasDoctype(content)) {
                    isDtdValidated = true;
                    break;
                }
                // 判斷目前行是否以‘<’+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檔案流的前幾行來判斷是否含有DOCTYPE字元串，如果有則是dtd驗證模式，否則是xsd驗證模式。

使用DocumentLoader對象建立Document對象

前面我們已經探讨了spring使用DocumentLoader對象前需要做的準備工作，包括擷取解析xml檔案中的實體的解析器EntityResolver對象、擷取xml檔案的驗證模式、擷取解析xml檔案需要的InputSource對象以及擷取處理xml檔案解析錯誤的ErrorHandler對象。現在我們開始探讨DocumentLoader的執行流程。

Spring提供DocumentLoader接口來加載Document對象。并提供了DocumentLoader的預設實作類DefaultDocumentLoader。下面是DefaultDocumentLoader實作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方法首先建立DocumentBuilderFactory 對象，預設使用com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderFactoryImpl。然後使用DocumentBuilderFactory 來建立DocumentBuilder 對象。最後使用DocumentBuilder 對象來解析持有xml輸入流的InputSource對象并傳回建立的Document對象。下面我們來看看這三步的執行過程。

（1）建立DocumentBuilderFactory 對象

loadDocument方法調用DefaultDocumentLoader的createDocumentBuilderFactory方法來建立DocumentBuilderFactory 對象，此方法的源碼如下。

protected DocumentBuilderFactory createDocumentBuilderFactory(int validationMode, boolean namespaceAware)
            throws ParserConfigurationException {

        // 執行個體化DocumentBuilderFactory 
        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();

        // 下面配置factory

        factory.setNamespaceAware(namespaceAware);
        if (validationMode != XmlValidationModeDetector.VALIDATION_NONE) {
            // 設定使用驗證模式
            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方法通過調用抽象類DocumentBuilderFactory的靜态方法newInstance()來建立DocumentBuilderFactory對象，預設使用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，這裡就不深入探讨了。

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

（2）建立DocumentBuilder 對象

loadDocument方法調用DefaultDocumentLoader的createDocumentBuilder方法來傳回一個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 對象建立DocumentBuilder 對象，然後把EntityResolver 和ErrorHandler 對象設定給DocumentBuilder 對象。其中我們來看看預設的DocumentBuilderFactory 對象的newDocumentBuilder方法傳回的是一個怎麼樣的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. */
        // 檢查是否已經指定了Schema對象。
        if (grammar != null && attributes != null) {
            // 是否已經設定了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 {
            // 建立一個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方法傳回一個com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderImpl對象，DocumentBuilderImpl的上述構造方法代碼如下。

DocumentBuilderImpl(DocumentBuilderFactoryImpl dbf, Hashtable dbfAttrs, Hashtable features, boolean secureProcessing)
        throws SAXNotRecognizedException, SAXNotSupportedException
    {
        domParser = new DOMParser();

        // 設定ErrorHandler對象
        if (dbf.isValidating()) {
            fInitErrorHandler = new DefaultValidationErrorHandler(domParser.getXMLParserConfiguration().getLocale());
            setErrorHandler(fInitErrorHandler);
        }
        else {
            fInitErrorHandler = domParser.getErrorHandler();
        }

        domParser.setFeature(VALIDATION_FEATURE, dbf.isValidating());

        // 設定命名空間是否敏感
        domParser.setFeature(NAMESPACES_FEATURE, dbf.isNamespaceAware());

        // 通過DocumentBuilderFactory設定各種變量
        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());

        // 設定是否支撐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，使用内置的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子產品. **/
                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）建立Document對象

loadDocument方法調用DocumentBuilder的parse方法來傳回一個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檔案
        domParser.parse(is);
        // 擷取Document對象
        Document doc = domParser.getDocument();
        // 删除與Document建立有關的引用
        domParser.dropDocumentReferences();
        return doc;
    }

關于這裡parse方法，就不過多的探讨它。我們隻需知道通過它，可以擷取到Document對象就行了。

總結

（1）Spring預設的DocumentLoader是DefaultDocumentLoader。

（2）DefaultDocumentLoader通過建立DocumentBuilderFactory工廠對象來建立文檔建構器DocumentBuilder對象，最後使用DocumentBuilder對象來擷取Document對象。

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

準備工作

使用DocumentLoader對象建立Document對象

總結

繼續閱讀

MyBatis-Plus 之AR模式

馴服爛代碼_馴服Tomcat：Tomcat 5的過濾技巧

基于Springboot+VUE+Mysql的大學生校園疫情防控管理系統

Using MessageConverter

馴服爛代碼_C＃中的馴服配置

Spring Cloud整合Sleuth，當請求完成後，Zipkin沒有鍊路資訊

JAVA自學之路——馬士兵

Rabbitmq指令行手動建立隊列

整合 Spring 開發架構及服務端技術

SSM整合02---Spring-SpringMVC層一、添加web架構支援二、web.xml三、spring-mvc.xml

SSM架構（二）------------表現層的SpringMVC

Intellij idea基于Maven 的SSM 架構整合

Spring資料和Redis

SpringMVC的啟動過程源碼分析1 SpringMVC 配置XML的方式啟動2 SpringMVC純注解模式啟動

Java Spring 架構

SpringCloud微服務（四）之ribbon筆記ribbon是什麼？入門案例自定義均衡負載算法