Hadoop源码阅读-HDFS-day2

昨天看到了AbstractFileSystem，也知道应用访问文件是通过FileContext这个类，今天来看这个类的源代码，先看下这个类老长的注释说明

/**
* Generic helper function overridden on instantiation to perform a
* specific operation on the given file system using the given path
* which may result in an UnresolvedLinkException.
* @param fs AbstractFileSystem to perform the operation on.
* @param p Path given the file system.
* @return Generic type determined by the specific implementation.
* @throws UnresolvedLinkException If symbolic link <code>path</code> could
*           not be resolved
* @throws IOException an I/O error occurred
*/
abstract public T next(final AbstractFileSystem fs, final Path p)
throws IOException, UnresolvedLinkException;

/**
* Performs the operation specified by the next function, calling it
* repeatedly until all symlinks in the given path are resolved.
* @param fc FileContext used to access file systems.
* @param path The path to resolve symlinks on.
* @return Generic type determined by the implementation of next.
* @throws IOException
*/
public T resolve(final FileContext fc, final Path path) throws IOException {
int count = 0;
T in = null;
Path p = path;
// NB: More than one AbstractFileSystem can match a scheme, eg
// "file" resolves to LocalFs but could have come by RawLocalFs.
AbstractFileSystem fs = fc.getFSofPath(p);

// Loop until all symlinks are resolved or the limit is reached
for (boolean isLink = true; isLink;) {
try {
in = next(fs, p);
isLink = false;
} catch (UnresolvedLinkException e) {
if (!fc.resolveSymlinks) {
throw new IOException("Path " + path + " contains a symlink"
+ " and symlink resolution is disabled ("
+ CommonConfigurationKeys.FS_CLIENT_RESOLVE_REMOTE_SYMLINKS_KEY + ").", e);
}
if (!FileSystem.areSymlinksEnabled()) {
throw new IOException("Symlink resolution is disabled in"
+ " this version of Hadoop.");
}
if (count++ > FsConstants.MAX_PATH_LINKS) {
throw new IOException("Possible cyclic loop while " +
"following symbolic link " + path);
}
// Resolve the first unresolved path component
p = qualifySymlinkTarget(fs.getUri(), p, fs.getLinkTarget(p));
fs = fc.getFSofPath(p);
}
}
return in;
}

View Code

next 是一个一般的helper函数，需要被实例重写，从而在给定路径的文件系统上执行特定的操作，可能会抛UnresolvedLinkException异常

resolve 通过next执行特定的操作，反复的调用next函数，知道路径上所有的符号链接被解析