MapReduce - Hadoop: The Definitive Guide - page 69

user )
throws IOException
A Configuration object encapsulates a client or server's configuration, which is set
using configuration files read from the classpath, such as etc/hadoop/core-site.xml . The
first method returns the default filesystem (as specified in core-site.xml , or the default loc-
al filesystem if not specified there). The second uses the given URI 's scheme and author-
ity to determine the filesystem to use, falling back to the default filesystem if no scheme is
specified in the given URI . The third retrieves the filesystem as the given user, which is
important in the context of security (see Security ) .
In some cases, you may want to retrieve a local filesystem instance. For this, you can use
the convenience method getLocal() :
public static LocalFileSystem getLocal ( Configuration conf ) throws
IOException
With a FileSystem instance in hand, we invoke an open() method to get the input
stream for a file:
public FSDataInputStream open ( Path f ) throws IOException
public abstract FSDataInputStream open ( Path f , int bufferSize )
throws IOException
The first method uses a default buffer size of 4 KB.
Putting this together, we can rewrite Example 3-1 as shown in Example 3-2 .
Example 3-2. Displaying files from a Hadoop filesystem on standard output by using the
FileSystem directly
public class FileSystemCat {
public static void main ( String [] args ) throws Exception {
String uri = args [ 0 ];
Configuration conf = new Configuration ();
FileSystem fs = FileSystem . get ( URI . create ( uri ), conf );
InputStream in = null ;
try {
in = fs . open ( new Path ( uri ));
IOUtils . copyBytes ( in , System . out , 4096 , false );
} finally {
IOUtils . closeStream ( in );
}
}
}