通俗理解spring源码（四）—— 获取Docment

通俗理解spring源码（四）—— 获取Docment

上节讲到了xmlBeanDefinitionReader.doLoadDocument(InputSource inputSource, Resource resource)方法：

    protected Document doLoadDocument(InputSource inputSource, Resource resource) throws Exception {
        return this.documentLoader.loadDocument(inputSource, getEntityResolver(), this.errorHandler,
                getValidationModeForResource(resource), isNamespaceAware());
    }

 getValidationModeForResource(resource)在这里，看看getEntityResolver():

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

　　这里有两种类型的entityResolver，对于xmlBeanDefinitionReader来说，会new一个ResourceEntityResolver，后面会解释什么是EntityResolver。

　　然后将EntityResolver返回作为loadDocument()的参数。

 protected Document doLoadDocument(InputSource inputSource, Resource resource) throws Exception {
        return this.documentLoader.loadDocument(inputSource, getEntityResolver(), this.errorHandler,
                getValidationModeForResource(resource), isNamespaceAware());
    }

　　调用DefaultDocumentLoader的loadDocument方法

    public Document loadDocument(InputSource inputSource, EntityResolver entityResolver,
            ErrorHandler errorHandler, int validationMode, boolean namespaceAware) throws Exception {

        DocumentBuilderFactory factory = createDocumentBuilderFactory(validationMode, namespaceAware);
        if (logger.isTraceEnabled()) {
            logger.trace("Using JAXP provider [" + factory.getClass().getName() + "]");
        }
        DocumentBuilder builder = createDocumentBuilder(factory, entityResolver, errorHandler);
        return builder.parse(inputSource);
    }
    protected DocumentBuilder createDocumentBuilder(DocumentBuilderFactory factory,
            @Nullable EntityResolver entityResolver, @Nullable ErrorHandler errorHandler)
            throws ParserConfigurationException {

        DocumentBuilder docBuilder = factory.newDocumentBuilder();
        if (entityResolver != null) {
            docBuilder.setEntityResolver(entityResolver);
        }
        if (errorHandler != null) {
            docBuilder.setErrorHandler(errorHandler);
        }
        return docBuilder;
    }

　　在loadDocument方法中，使用的DocumentBuilderFactory 、DocumentBuilder 等都是jdk中的类，加载xml资源也是调用的java中的方法，这里就不多说了，重点是docBuilder.setEntityResolver(entityResolver)方法，该方法将得到的entityResolver放到docBuilder中，也就是DocumentBuilder ，然后调用docBuilder.parse()。

　　那么EntityResolver在生成document过程中起到什么作用呢？

1、EntityResolver

　　EntityResolver不是属于spring定义的，是jdk中org.xml.sax下的一个接口。

　　官方是这样解释EntityResolver的：如果SAX应用程序实现自定义处理外部实体,则必须实现此接口，并使用setEntityResolver方法向SAX 驱动器注册一个实例。

　　也就是说,对于解析一个xml,sax首先会读取该xml文档上的声明,根据声明去寻找相应的dtd定义，以便对文档的进行验证,默认的寻找规则，(即:通过网络,实现上就是声明DTD的地址URI地址来下载DTD声明)，并进行认证，下载的过程是一个漫长的过程,而且当网络不可用时，就会找不到相应的dtd，就会报错。　　

　　EntityResolver 的作用就是项目本身就可以提供一个如何寻找DTD 的声明方法，即:由程序来实现寻找DTD声明的过程，比如我们将DTD放在项目的某处在实现时直接将此文档读取并返回个SAX即可，这样就避免了通过网络来寻找DTD的声明。

　　首先看看EntityResolver 接口声明的方法：

    public abstract InputSource resolveEntity (String publicId,
                                               String systemId)
        throws SAXException, IOException;

　　接收2个参数,publicId ,systemId ,并返回一个InputStream对象

　　如果我们在解析验证模式为xsd的配置文件,代码如下：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">
...
</beans>

　　读取得到以下参数
　　publicId : null
　　systemId : http://www.springframework.org/schema/beans/spring-beans-2.5.xsd

　　如果我们解析的是DTD的配置文件，代码如下

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE beans PUBLIC  "-//SPRING//DTD BEAN//EN"  "http://www.springframework.org/dtd/spring-beans.dtd">
<beans>
        ...
</beans>

　　上面说过，EntityResolver 会被set到DocumentBuilder 对象中，也就是说，在 DocumentBuilder .parse()过程中，EntityResolver 对象的resolveEntity会得到相应的参数publicId 和systemId ，然后调用其resolveEntity （）方法，返回的流InputSource 就是相应的DTD文件流或者XSD文件流，然后完成校验工作。

　　所以，现在重点就是，不同的EntityResolver实现是以何种方式返回DTD或XSD文件流的。来看具体实现。

2、DelegatingEntityResolver

    public InputSource resolveEntity(@Nullable String publicId, @Nullable String systemId)
            throws SAXException, IOException {

        if (systemId != null) {
            if (systemId.endsWith(DTD_SUFFIX)) {
                return this.dtdResolver.resolveEntity(publicId, systemId);
            }
            else if (systemId.endsWith(XSD_SUFFIX)) {
                return this.schemaResolver.resolveEntity(publicId, systemId);
            }
        }

        // Fall back to the parser's default behavior.
        return null;
    }

　　根据systemId后缀，委派给dtdResolver和schemaResolver完成，dtdResolver和schemaResolver也是EntityResolver的实现，在 DelegatingEntityResolver构造中初始化。

    public DelegatingEntityResolver(@Nullable ClassLoader classLoader) {
        this.dtdResolver = new BeansDtdResolver();
        this.schemaResolver = new PluggableSchemaResolver(classLoader);
    }

3、BeansDtdResolver

　　BeansDtdResolver负责找到classpath下spring-beans.dtd文件的位置，返回文件流。

    public InputSource resolveEntity(@Nullable String publicId, @Nullable String systemId) throws IOException {
        if (logger.isTraceEnabled()) {
            logger.trace("Trying to resolve XML entity with public ID [" + publicId +
                    "] and system ID [" + systemId + "]");
        }

        if (systemId != null && systemId.endsWith(DTD_EXTENSION)) {
            int lastPathSeparator = systemId.lastIndexOf('/');
            int dtdNameStart = systemId.indexOf(DTD_NAME, lastPathSeparator);
            if (dtdNameStart != -1) {
                String dtdFile = DTD_NAME + DTD_EXTENSION;
                //dtdFile="spring-beans.dtd"
                if (logger.isTraceEnabled()) {
                    logger.trace("Trying to locate [" + dtdFile + "] in Spring jar on classpath");
                }
                try {
                    //定位classpath资源文件
                    Resource resource = new ClassPathResource(dtdFile, getClass());
                    InputSource source = new InputSource(resource.getInputStream());
                    source.setPublicId(publicId);
                    source.setSystemId(systemId);
                    if (logger.isTraceEnabled()) {
                        logger.trace("Found beans DTD [" + systemId + "] in classpath: " + dtdFile);
                    }
                    return source;
                }
                catch (FileNotFoundException ex) {
                    if (logger.isDebugEnabled()) {
                        logger.debug("Could not resolve beans DTD [" + systemId + "]: not found in classpath", ex);
                    }
                }
            }
        }

        // Fall back to the parser's default behavior.
        return null;
    }

spring-beans.dtd文件在如下位置：

4、PluggableSchemaResolver

    public static final String DEFAULT_SCHEMA_MAPPINGS_LOCATION = "META-INF/spring.schemas";
    private volatile Map<String, String> schemaMappings;
    public PluggableSchemaResolver(@Nullable ClassLoader classLoader) {
        this.classLoader = classLoader;
        this.schemaMappingsLocation = DEFAULT_SCHEMA_MAPPINGS_LOCATION;
    }

　　PluggableSchemaResolver是重点，负责找到相对应的XSD文件，一般我们都是使用XSD作为检验文件。 　

　　DEFAULT_SCHEMA_MAPPINGS_LOCATION ，指明资源文件路径。

 　　可以看到，spring很贴心的将所有版本的xsd文件都准备了，即使没有指定版本号，也会有默认的xsd文件。

　　schemaMappings负责保存资源路径与校验文件url的映射。即systemId为key，resourceLocation为value。

    public InputSource resolveEntity(@Nullable String publicId, @Nullable String systemId) throws IOException {
        if (logger.isTraceEnabled()) {
            logger.trace("Trying to resolve XML entity with public id [" + publicId +
                    "] and system id [" + systemId + "]");
        }

        if (systemId != null) {
            //根据systemId，也就是验证文件的url，找到本地文件路径
            String resourceLocation = getSchemaMappings().get(systemId);
            //如果没有找到，并且url是https开头，就转换为http开头的url
            if (resourceLocation == null && systemId.startsWith("https:")) {
                // Retrieve canonical http schema mapping even for https declaration
                resourceLocation = getSchemaMappings().get("http:" + systemId.substring(6));
            }
            //定位本地资源文件
            if (resourceLocation != null) {
                Resource resource = new ClassPathResource(resourceLocation, this.classLoader);
                try {
                    InputSource source = new InputSource(resource.getInputStream());
                    source.setPublicId(publicId);
                    source.setSystemId(systemId);
                    if (logger.isTraceEnabled()) {
                        logger.trace("Found XML schema [" + systemId + "] in classpath: " + resourceLocation);
                    }
                    return source;
                }
                catch (FileNotFoundException ex) {
                    if (logger.isDebugEnabled()) {
                        logger.debug("Could not find XML schema [" + systemId + "]: " + resource, ex);
                    }
                }
            }
        }

        // Fall back to the parser's default behavior.
        return null;
    }

　　其中getSchemaMappings获取或初始化map容器，这里用到了懒加载

    private Map<String, String> getSchemaMappings() {
        Map<String, String> schemaMappings = this.schemaMappings;
        if (schemaMappings == null) {
            synchronized (this) {
                schemaMappings = this.schemaMappings;
                if (schemaMappings == null) {
                    if (logger.isTraceEnabled()) {
                        logger.trace("Loading schema mappings from [" + this.schemaMappingsLocation + "]");
                    }
                    try {
                        //这里负责初始化map
                        Properties mappings =
                                PropertiesLoaderUtils.loadAllProperties(this.schemaMappingsLocation, this.classLoader);
                        if (logger.isTraceEnabled()) {
                            logger.trace("Loaded schema mappings: " + mappings);
                        }
                        schemaMappings = new ConcurrentHashMap<>(mappings.size());
                        CollectionUtils.mergePropertiesIntoMap(mappings, schemaMappings);
                        this.schemaMappings = schemaMappings;
                    }
                    catch (IOException ex) {
                        throw new IllegalStateException(
                                "Unable to load schema mappings from location [" + this.schemaMappingsLocation + "]", ex);
                    }
                }
            }
        }
        return schemaMappings;
    }

　　 通过调试，可以看到map中的中的内容

 4、ResourceEntityResolver

　　在xmlBeanDefinitionReader中，默认会用这种EntityResolver。

public class ResourceEntityResolver extends DelegatingEntityResolver {
public InputSource resolveEntity(@Nullable String publicId, @Nullable String systemId)
            throws SAXException, IOException {

        InputSource source = super.resolveEntity(publicId, systemId);
        //调用父类DelegatingEntityResolver的方法，如果没有，就从网络获取
        if (source == null && systemId != null) {
            String resourcePath = null;
            try {
                String decodedSystemId = URLDecoder.decode(systemId, "UTF-8");
                String givenUrl = new URL(decodedSystemId).toString();
                String systemRootUrl = new File("").toURI().toURL().toString();
                // Try relative to resource base if currently in system root.
                if (givenUrl.startsWith(systemRootUrl)) {
                    resourcePath = givenUrl.substring(systemRootUrl.length());
                }
            }
            catch (Exception ex) {
                // Typically a MalformedURLException or AccessControlException.
                if (logger.isDebugEnabled()) {
                    logger.debug("Could not resolve XML entity [" + systemId + "] against system root URL", ex);
                }
                // No URL (or no resolvable URL) -> try relative to resource base.
                resourcePath = systemId;
            }
            if (resourcePath != null) {
                if (logger.isTraceEnabled()) {
                    logger.trace("Trying to locate XML entity [" + systemId + "] as resource [" + resourcePath + "]");
                }
                Resource resource = this.resourceLoader.getResource(resourcePath);
                source = new InputSource(resource.getInputStream());
                source.setPublicId(publicId);
                source.setSystemId(systemId);
                if (logger.isDebugEnabled()) {
                    logger.debug("Found XML entity [" + systemId + "]: " + resource);
                }
            }
            else if (systemId.endsWith(DTD_SUFFIX) || systemId.endsWith(XSD_SUFFIX)) {
                // External dtd/xsd lookup via https even for canonical http declaration
                String url = systemId;
                if (url.startsWith("http:")) {
                    url = "https:" + url.substring(5);
                }
                try {
                    source = new InputSource(new URL(url).openStream());
                    source.setPublicId(publicId);
                    source.setSystemId(systemId);
                }
                catch (IOException ex) {
                    if (logger.isDebugEnabled()) {
                        logger.debug("Could not resolve XML entity [" + systemId + "] through URL [" + url + "]", ex);
                    }
                    // Fall back to the parser's default behavior.
                    source = null;
                }
            }
        }

        return source;
    }

}

　　该类首先会调用父类DelegatingEntityResolver的方法获取资源文件，如果没有就会从网络获取。不过一般都能从本地获取到。 

　　走的太远，不要忘记为什么出发！

　　再来看看这个方法，是不是清楚多了？

    protected Document doLoadDocument(InputSource inputSource, Resource resource) throws Exception {
        return this.documentLoader.loadDocument(inputSource, getEntityResolver(), this.errorHandler,
                getValidationModeForResource(resource), isNamespaceAware());
    }

 　　至此，document对象获取完成。

　　参考：spring源码深度解析。