Java中，不同来源的资源抽象成URL，通过注册不同的handler（URLStreamHandler)来处理不同来源的资源的读取逻辑。一般handler的类型使用不同的前缀（协议，protocal）来识别，如：“file：”、“http：“、”jar：”等。
对于Spring，URL没有定义相应的，如“classpath：“的handler，定义也相对麻烦，Spring对配置文件的读取做了相应的封装，通过Resource接口来抽象底层资源。如下：
/**
* Interface for a resource descriptor that abstracts from the actual
* type of underlying resource, such as a file or class path resource.
*
* <p>An InputStream can be opened for every resource if it exists in
* physical form, but a URL or File handle can just be returned for
* certain resources. The actual behavior is implementation-specific.
*
* @author Juergen Hoeller
* @since 28.12.2003
* @see #getInputStream()
* @see #getURL()
* @see #getURI()
* @see #getFile()
* @see WritableResource
* @see ContextResource
* @see FileSystemResource
* @see ClassPathResource
* @see UrlResource
* @see ByteArrayResource
* @see InputStreamResource
* @see PathResource
*/
public interface Resource extends InputStreamSource { /**
* Return whether this resource actually exists in physical form.
* <p>This method performs a definitive existence check, whereas the
* existence of a {@code Resource} handle only guarantees a
* valid descriptor handle.
*/
boolean exists(); /**
* Return whether the contents of this resource can be read,
* e.g. via {@link #getInputStream()} or {@link #getFile()}.
* <p>Will be {@code true} for typical resource descriptors;
* note that actual content reading may still fail when attempted.
* However, a value of {@code false} is a definitive indication
* that the resource content cannot be read.
* @see #getInputStream()
*/
boolean isReadable(); /**
* Return whether this resource represents a handle with an open
* stream. If true, the InputStream cannot be read multiple times,
* and must be read and closed to avoid resource leaks.
* <p>Will be {@code false} for typical resource descriptors.
*/
boolean isOpen(); /**
* Return a URL handle for this resource.
* @throws IOException if the resource cannot be resolved as URL,
* i.e. if the resource is not available as descriptor
*/
URL getURL() throws IOException; /**
* Return a URI handle for this resource.
* @throws IOException if the resource cannot be resolved as URI,
* i.e. if the resource is not available as descriptor
*/
URI getURI() throws IOException; /**
* Return a File handle for this resource.
* @throws IOException if the resource cannot be resolved as absolute
* file path, i.e. if the resource is not available in a file system
*/
File getFile() throws IOException; /**
* Determine the content length for this resource.
* @throws IOException if the resource cannot be resolved
* (in the file system or as some other known physical resource type)
*/
long contentLength() throws IOException; /**
* Determine the last-modified timestamp for this resource.
* @throws IOException if the resource cannot be resolved
* (in the file system or as some other known physical resource type)
*/
long lastModified() throws IOException; /**
* Create a resource relative to this resource.
* @param relativePath the relative path (relative to this resource)
* @return the resource handle for the relative resource
* @throws IOException if the relative resource cannot be determined
*/
Resource createRelative(String relativePath) throws IOException; /**
* Determine a filename for this resource, i.e. typically the last
* part of the path: for example, "myfile.txt".
* <p>Returns {@code null} if this type of resource does not
* have a filename.
*/
String getFilename(); /**
* Return a description for this resource,
* to be used for error output when working with the resource.
* <p>Implementations are also encouraged to return this value
* from their {@code toString} method.
* @see Object#toString()
*/
String getDescription(); }
对于不同来源的资源文件都有相应的Resource实现：
1. 文件（FileSystemResource）
2. Classpath资源（ClassPathResource）
3. URL资源（UrlResource）
4. InputStream资源（InputStreamResource）
5. Byte数组资源（ByteArrayResource）等。
ClassPathResource实现：
this.class.getResourceAsStream(this.path);
FileSystemResource实现：
new FileInputStream(this.file);
... ...
Resource实现返回InputStream，后续由XmlBeanDefinitionReader进行操作，如下：
配置文件加载流程
ResourceLoader：资源加载器，根据资源地址返回Resource。EncodedResource封装（处理编码）
↓↓
↓↓委托
↓↓
DocumentLoader（DefaultDocumentLoader）：转换Resource为Document。SAX读取XML文件获取InputStream构造解析InputSource返回Documet。
↓↓
↓↓
↓↓
BeanDefinitionDocumentReader（DefaultBeanDefinitionDocumentReader）（单一职责应用）：解析Document，注册bean。registerBeanDefinitions（doRegisterBeanDefinitions）
DefaultBeanDefinitionDocumentReader：包含preXmlProcess和postXmlProcess两个空的模板方法，供子类做相应处理。