Spring的Resource接口为资源访问提供了统一的接口，不同的实现类实现了从不同上下文获取资源。下面是该接口的方法:
public interface Resource extends InputStreamSource {
/**
* Determine 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();
/**
* Indicate whether the contents of this resource can be read via
* {@link #getInputStream()}.
* <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();
/**
* Indicate whether this resource represents a handle with an open stream.
* If {@code 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
* @since 2.5
*/
URI getURI() throws IOException;
/**
* Return a File handle for this resource.
* @throws java.io.FileNotFoundException if the resource cannot be resolved as
* absolute file path, i.e. if the resource is not available in a file system
* @throws IOException in case of general resolution/reading failures
* @see #getInputStream()
*/
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();
}
下面列举几个常用的Resource实现类。
- AbstractResource提供了Resource的默认实现；
- UrlResource封装了一个java.net.URL对象，用来访问URL可以正常访问的任意对象，比如文件；
- 可以使用ClassPathResource来获取类路径上的资源ClassPathResource可以使用线程上下文的加载器、调用者提供的加载器或指定的类中的任意一个来加载资源；
- FileSystemResource是针对java.io.File提供的Resource实现；
- ServletContextResource为了获取web根路径的ServletContext资源而提供的Resource实现；
- 在确实没有找到其他合适的Resource实现时，才使用InputSteamResource；
- 当需要从字节数组加载内容时，ByteArrayResource是一个不错的选择，使用ByteArrayResource可以不用求助于InputStreamResource；
