HDFS声明及构造函数
@InterfaceAudience.Private
@InterfaceStability.Evolving
public class Hdfs extends AbstractFileSystem { DFSClient dfs;
final CryptoCodec factory;
private boolean verifyChecksum = true; static {
HdfsConfiguration.init();
} /**
* This constructor has the signature needed by
* {@link AbstractFileSystem#createFileSystem(URI, Configuration)}
*
* @param theUri which must be that of Hdfs
* @param conf configuration
* @throws IOException
*/
Hdfs(final URI theUri, final Configuration conf) throws IOException, URISyntaxException {
super(theUri, HdfsConstants.HDFS_URI_SCHEME, true, NameNode.DEFAULT_PORT); if (!theUri.getScheme().equalsIgnoreCase(HdfsConstants.HDFS_URI_SCHEME)) {
throw new IllegalArgumentException("Passed URI's scheme is not for Hdfs");
}
String host = theUri.getHost();
if (host == null) {
throw new IOException("Incomplete HDFS URI, no host: " + theUri);
} this.dfs = new DFSClient(theUri, conf, getStatistics());
this.factory = CryptoCodec.getInstance(conf);
}
Hdfs继承了AbstractFileSystem这个抽象类，其中有一个静态块，执行HdfsConfiguration的初始化方法,我们先来看下这个方法
/**
* This method is here so that when invoked, HdfsConfiguration is class-loaded if
* it hasn't already been previously loaded. Upon loading the class, the static
* initializer block above will be executed to add the deprecated keys and to add
* the default resources. It is safe for this method to be called multiple times
* as the static initializer block will only get invoked once.
*
* This replaces the previously, dangerous practice of other classes calling
* Configuration.addDefaultResource("hdfs-default.xml") directly without loading
* HdfsConfiguration class first, thereby skipping the key deprecation
*/
public static void init() {
}
init是一个空函数，它存在的作用，只是为了类加载，当类加载的时候，静态块中的方法将会执行从而增加过时的Key和Resource。因为它本身是一个空函数，它被反复调用时安全的，因为静态块中的方法只会被调用一次，使用这个方法来替代之前不安全的直接类调用Configuration.addDefaultResource("hdfs-default.xml")方法。所以，他的静态块中肯定会包含这个方法，我们来看下是不是这样.
static {
addDeprecatedKeys();
// adds the default resources
Configuration.addDefaultResource("hdfs-default.xml");
Configuration.addDefaultResource("hdfs-site.xml");
}
看完这些，回到HDFS的父类AbstractFileSystem
/**
* This class provides an interface for implementors of a Hadoop file system
* (analogous to the VFS of Unix). Applications do not access this class;
* instead they access files across all file systems using {@link FileContext}.
*
* Pathnames passed to AbstractFileSystem can be fully qualified URI that
* matches the "this" file system (ie same scheme and authority)
* or a Slash-relative name that is assumed to be relative
* to the root of the "this" file system .
*/
@InterfaceAudience.Public
@InterfaceStability.Evolving /*Evolving for a release,to be changed to Stable */
public abstract class AbstractFileSystem {
AbstractFileSystem提供了一个实现Hadoop 文件系统的接口，应用访问文件通过FileContext而不需要访问这个类，路径名称一旦匹配了这个文件系统，就会视为合法的URI,否则的会认为是该文件系统根目录的相对路径？（这个不太确定）
来看下它的构造函数：
/**
* Constructor to be called by subclasses.
*
* @param uri for this file system.
* @param supportedScheme the scheme supported by the implementor
* @param authorityNeeded if true then theURI must have authority, if false
* then the URI must have null authority.
*
* @throws URISyntaxException <code>uri</code> has syntax error
*/
public AbstractFileSystem(final URI uri, final String supportedScheme,
final boolean authorityNeeded, final int defaultPort)
throws URISyntaxException {
myUri = getUri(uri, supportedScheme, authorityNeeded, defaultPort);
statistics = getStatistics(uri);
}
/**
* Get the URI for the file system based on the given URI. The path, query
* part of the given URI is stripped out and default file system port is used
* to form the URI.
*
* @param uri FileSystem URI.
* @param authorityNeeded if true authority cannot be null in the URI. If
* false authority must be null.
* @param defaultPort default port to use if port is not specified in the URI.
*
* @return URI of the file system
*
* @throws URISyntaxException <code>uri</code> has syntax error
*/
private URI getUri(URI uri, String supportedScheme,
boolean authorityNeeded, int defaultPort) throws URISyntaxException {
checkScheme(uri, supportedScheme);
// A file system implementation that requires authority must always
// specify default port
if (defaultPort < 0 && authorityNeeded) {
throw new HadoopIllegalArgumentException(
"FileSystem implementation error - default port " + defaultPort
+ " is not valid");
}
String authority = uri.getAuthority();
if (authority == null) {
if (authorityNeeded) {
throw new HadoopIllegalArgumentException("Uri without authority: " + uri);
} else {
return new URI(supportedScheme + ":///");
}
}
// authority is non null - AuthorityNeeded may be true or false.
int port = uri.getPort();
port = (port == -1 ? defaultPort : port);
if (port == -1) { // no port supplied and default port is not specified
return new URI(supportedScheme, authority, "/", null);
}
return new URI(supportedScheme + "://" + uri.getHost() + ":" + port);
}
getUri主要是通过给定的URI生成FS的URI