jboss的VFS是為了解決什么問題，他為什么有用呢

在jboss中有很多類似的資源操作的代碼都分散在程序的各個地方，大多數情況下代碼首先確定操作的資源的類型，比如是文件或者是文件夾，通過URL加載的資源等，在不同的代碼中其中包含了相同的代碼。

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;
?}

上面的代碼在多個地方都會包含這樣的代碼。各自進行文件的處理，但是這種方式有一個很大的問題，就是熱發布，需要將文件覆蓋已經發布鎖定的文件。為了解決文件鎖的問題，可能需要將操作文件的代碼進行集中處理，正式因為這個，VFS工程出現了。

?

VFS發布的API

VFS的基本使用包括二部分內容

• 簡單的資源導航
• 訪問者模式的API

綜上所屬，jdk自身提供的資源導航，需要判定文件的類型，操作比較繁瑣。在VFS中我們將所有的類型抽象為一個類型-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;

......

}

?

?正與以前的對與只讀文件的操作，只需要添加一些選項區清楚或者刪除資源，有時候清楚或者刪除資源需要處理一些臨時文件，比如嵌套的jar等。

轉換jdk或者RUL資源到VirtualFile，虛擬文件需要一個root，VFS類知道如何根據一個URL取得虛擬文件

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：

這部分代碼是老的vfs的代碼，在新的vfs代碼中，已經不再依靠URL，而是采用mount的方式，此處只是為了表明思路。

你可以采用不同的方式來取得VFS的實例，比如getVFS, createNewRoot and getRoot

由于VFS的新版本已經發生了變化，可以看新代碼，此處不再詳述。（由于此部分直接翻譯，后續可以考慮按照新的版本重新寫一份）

?

VFS架構

VFS的發布的API非常直觀，他的實現非常復雜，這里進行一下闡述

每次創建一個VFS實例，配套的實例化一個VFSContext，他是根據VFSContextFactory生成的，他會根據不同的協議映射不同的實現類，比如文件系統映射為FileSystemContextFactory，Zip文件映射為ZipEntryContextFactory。

另外每次VFS創建的時候，匹配的VirtualFileHandler也會被生成,他知道如何處理不同的類型的資源

?

到這里基本上可以了解vfs的原理，后面不再進行描述，下面會學習vfs3的代碼，查看新代碼的原理?

原文地址：http://netliving.iteye.com/blog/839938

轉載于:https://www.cnblogs.com/davidwang456/p/4027077.html

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