jboss的VFS是为了解决什么问题，他为什么有用呢
在jboss中有很多类似的资源操作的代码都分散在程序的各个地方，大多数情况下代码首先确定操作的资源的类型，比如是文件或者是文件夹，通过URL加载的资源等，在不同的代码中其中包含了相同的代码。
例如
public static URL[] search(ClassLoader cl, String prefix, String suffix)
throws IOException {
Enumeration[] e = new Enumeration[] { cl.getResources(prefix),
cl.getResources(prefix + "MANIFEST.MF") };
Set all = new LinkedHashSet();
URL url;
URLConnection conn;
JarFile jarFile;
for (int i = 0, s = e.length; i < s; ++i) {
while (e[i].hasMoreElements()) {
url = (URL) e[i].nextElement();
conn = url.openConnection();
conn.setUseCaches(false);
conn.setDefaultUseCaches(false);
if (conn instanceof JarURLConnection) {
jarFile = ((JarURLConnection) conn).getJarFile();
} else {
jarFile = getAlternativeJarFile(url);
}
if (jarFile != null) {
searchJar(cl, all, jarFile, prefix, suffix);
} else {
boolean searchDone = searchDir(all, new File(URLDecoder
.decode(url.getFile(), "UTF-8")), suffix);
if (searchDone == false) {
searchFromURL(all, prefix, suffix, url);
}
}
}
}
return (URL[]) all.toArray(new URL[all.size()]);
}
private static boolean searchDir(Set result, File file, String suffix)
throws IOException {
if (file.exists() && file.isDirectory()) {
File[] fc = file.listFiles();
String path;
for (int i = 0; i < fc.length; i++) {
path = fc[i].getAbsolutePath();
if (fc[i].isDirectory()) {
searchDir(result, fc[i], suffix);
} else if (path.endsWith(suffix)) {
result.add(fc[i].toURL());
}
}
return true;
}
return false;
}
上面的代码在多个地方都会包含这样的代码。各自进行文件的处理，但是这种方式有一个很大的问题，就是热发布，需要将文件覆盖已经发布锁定的文件。为了解决文件锁的问题，可能需要将操作文件的代码进行集中处理，正式因为这个，VFS工程出现了。
VFS发布的API
VFS的基本使用包括二部分内容
- 简单的资源导航
- 访问者模式的API
综上所属，jdk自身提供的资源导航，需要判定文件的类型，操作比较繁琐。在VFS中我们将所有的类型抽象为一个类型-VirtualFile
public final class VirtualFile implements Serializable{
/**
* Get the simple VF name (X.java)
*
* @return the simple file name
*/
public String getName();
/**
* Get the simple VF name mapped to lowercase (x.java) (used by case-insensitive filesystems like ZIP).
*
* @return the lowercase simple file name
*/
public String getLowerCaseName();
/**
* Get the absolute VFS full path name (/xxx/yyy/foo.ear/baz.jar/org/jboss/X.java)
*
* @return the VFS full path name
*/
public String getPathName();
/**
* Get the path name relative to a parent virtual file. If the given virtual file is not a parent of
* this virtual file, then an {@code IllegalArgumentException} is thrown.
*
* @param parent the parent virtual file
* @return the relative path name as a string
* @throws IllegalArgumentException if the given virtual file is not a parent of this virtual file
*/
public String getPathNameRelativeTo(VirtualFile parent) throws IllegalArgumentException ;
/**
* Get the absolute VFS full path name. If this is a URL then directory entries will have a trailing slash.
*
* @param url whether or not this path is being used for a URL
*
* @return the VFS full path name
*/
String getPathName(boolean url);
/**
* When the file was last modified
*
* @return the last modified time
*/
public long getLastModified();
/**
* Get the size
*
* @return the size
*/
public long getSize();
/**
* Tests whether the underlying implementation file still exists.
*
* @return true if the file exists, false otherwise.
*/
public boolean exists();
/**
* Determines whether this virtual file represents a true root of a file system.
* On UNIX, there is only one root "/". Howevever, on Windows there are an infinite
* number of roots that correspond to drives, or UNC paths.
*
* @return {@code true} if this represents a root.
*/
public boolean isRoot();
/**
* Determine whether the named virtual file is a plain file.
*
* @return {@code true} if it is a plain file, {@code false} otherwise
*/
public boolean isFile();
/**
* Determine whether the named virtual file is a directory.
*
* @return {@code true} if it is a directory, {@code false} otherwise
*/
public boolean isDirectory() ;
/**
* Access the file contents.
*
* @return an InputStream for the file contents.
*
* @throws IOException for any error accessing the file system
*/
public InputStream openStream();
/**
* Delete this virtual file
*
* @return {@code true} if file was deleted
*/
public boolean delete();
/**
* Get a physical file for this virtual file. Depending on the underlying file system type, this may simply return
* an already-existing file; it may create a copy of a file; or it may reuse a preexisting copy of the file.
* Furthermore, the retured file may or may not have any relationship to other files from the same or any other
* virtual directory.
*
* @return the physical file
*
* @throws IOException if an I/O error occurs while producing the physical file
*/
public File getPhysicalFile() ;
/**
* Get a {@code VirtualFile} which represents the parent of this instance.
*
* @return the parent or {@code null} if there is no parent
*/
public VirtualFile getParent();
/**
* Get the children. This is the combined list of real children within this directory, as well as virtual children
* created by submounts.
*
* @return the children
*/
public List<VirtualFile> getChildren();
/**
* Visit the virtual file system
*
* @param visitor the visitor
*
* @throws IOException for any problem accessing the virtual file system
* @throws IllegalArgumentException if the visitor is null
* @throws IllegalStateException if the file is closed
*/
public void visit(VirtualFileVisitor visitor) throws IOException;
......
}
正与以前的对与只读文件的操作，只需要添加一些选项区清楚或者删除资源，有时候清楚或者删除资源需要处理一些临时文件，比如嵌套的jar等。
转换jdk或者RUL资源到VirtualFile，虚拟文件需要一个root，VFS类知道如何根据一个URL取得虚拟文件
public class VFS
{
/**
* Get the virtual file system for a root uri
*
* @param rootURI the root URI
* @return the virtual file system
* @throws IOException if there is a problem accessing the VFS
* @throws IllegalArgumentException if the rootURL is null
*/
static VFS getVFS(URI rootURI) throws IOException
/**
* Create new root
*
* @param rootURI the root url
* @return the virtual file
* @throws IOException if there is a problem accessing the VFS
* @throws IllegalArgumentException if the rootURL
*/
static VirtualFile createNewRoot(URI rootURI) throws IOException
/**
* Get the root virtual file
*
* @param rootURI the root uri
* @return the virtual file
* @throws IOException if there is a problem accessing the VFS
* @throws IllegalArgumentException if the rootURL is null
*/
static VirtualFile getRoot(URI rootURI) throws IOException
/**
* Get the virtual file system for a root url
*
* @param rootURL the root url
* @return the virtual file system
* @throws IOException if there is a problem accessing the VFS
* @throws IllegalArgumentException if the rootURL is null
*/
static VFS getVFS(URL rootURL) throws IOException
/**
* Create new root
*
* @param rootURL the root url
* @return the virtual file
* @throws IOException if there is a problem accessing the VFS
* @throws IllegalArgumentException if the rootURL
*/
static VirtualFile createNewRoot(URL rootURL) throws IOException
/**
* Get the root virtual file
*
* @param rootURL the root url
* @return the virtual file
* @throws IOException if there is a problem accessing the VFS
* @throws IllegalArgumentException if the rootURL
*/
static VirtualFile getRoot(URL rootURL) throws IOException
/**
* Get the root file of this VFS
*
* @return the root
* @throws IOException for any problem accessing the VFS
*/
VirtualFile getRoot() throws IOException
}
注：
这部分代码是老的vfs的代码，在新的vfs代码中，已经不再依靠URL，而是采用mount的方式，此处只是为了表明思路。
你可以采用不同的方式来取得VFS的实例，比如getVFS, createNewRoot and getRoot
由于VFS的新版本已经发生了变化，可以看新代码，此处不再详述。（由于此部分直接翻译，后续可以考虑按照新的版本重新写一份）
VFS架构
VFS的发布的API非常直观，他的实现非常复杂，这里进行一下阐述
每次创建一个VFS实例，配套的实例化一个VFSContext，他是根据VFSContextFactory生成的，他会根据不同的协议映射不同的实现类，比如文件系统映射为FileSystemContextFactory，Zip文件映射为ZipEntryContextFactory。
另外每次VFS创建的时候，匹配的VirtualFileHandler也会被生成,他知道如何处理不同的类型的资源
到这里基本上可以了解vfs的原理，后面不再进行描述，下面会学习vfs3的代码，查看新代码的原理
原文地址：http://netliving.iteye.com/blog/839938