Spring 资源文件处理

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。