MapReduce - Hadoop: The Definitive Guide - page 72

readFully() methods will read length bytes into the buffer (or buffer.length
bytes for the version that just takes a byte array buffer ), unless the end of the file is
reached, in which case an EOFException is thrown.
All of these methods preserve the current offset in the file and are thread safe (although
FSDataInputStream is not designed for concurrent access; therefore, it's better to
create multiple instances), so they provide a convenient way to access another part of the
file — metadata, perhaps — while reading the main body of the file.
Finally, bear in mind that calling seek() is a relatively expensive operation and should
be done sparingly. You should structure your application access patterns to rely on stream-
ing data (by using MapReduce, for example) rather than performing a large number of
seeks.
Writing Data
The FileSystem class has a number of methods for creating a file. The simplest is the
method that takes a Path object for the file to be created and returns an output stream to
write to:
public FSDataOutputStream create ( Path f ) throws IOException
There are overloaded versions of this method that allow you to specify whether to forcibly
overwrite existing files, the replication factor of the file, the buffer size to use when writ-
ing the file, the block size for the file, and file permissions.
WARNING
The create() methods create any parent directories of the file to be written that don't already exist.
Though convenient, this behavior may be unexpected. If you want the write to fail when the parent dir-
ectory doesn't exist, you should check for the existence of the parent directory first by calling the ex-
ists() method. Alternatively, use FileContext , which allows you to control whether parent direct-
ories are created or not.
There's also an overloaded method for passing a callback interface, Progressable , so
your application can be notified of the progress of the data being written to the datanodes:
package org . apache . hadoop . util ;
public interface Progressable {
public void progress ();
}