RAFs, Random Access Files permit asynchronous (random) access to a file contents. To access a file randomly we open the file, seek a particular position, and then we read or write to that file.

Java NIO.2 introduces a new interface – SeekableByteChannel for working with Random Access Files. Also improves the well-known FileChannel class by implementing this interface.

Before we start to talk about about FileChannel and SeekableByteChannel it is advisable to first talk a little bit about ByteBuffers and Channels.


A byte-buffer is an in-memory array of bytes. It usually contains data that was recently read, or that will be written from/to a destination.

A buffer has three important properties:

  • The buffer’s capacity represents the “maximum amount of information” that can be stored in the buffer.
  • The buffer’s position represents how much data has been read or written. The position is an index in the buffer’s array, and cannot have a negative value or a value bigger than the buffer’s capacity.
  • The buffer’s limit is the difference between buffer’s capacity and the buffer’s position.


Channels are in a way similar with the classical I/O streams, the difference is that while streams are one-way directed (read or write), channels can support both operations in the same time. Also Channels allow you to write and read asynchronously.

Using SeekableByteChannel to write / read to / from files

The SeekableByteChannel interface has 6 methods:

  • position(): Returns the channels current position.
  • position(long): Sets the channel position to the specified value. The value needs to be a positive number.
  • truncate(long): Truncates the entity connected to the SeekableByteChannel to the specified value.
  • read(ByteBuffer): Reads into the buffer (from the channe).
  • write(ByteBuffer): Writes bytes from the buffer to the channel.
  • size(): Returns the current size of the entity to which the channel is connected.

To open a SeekableByteChannel we will need to use the the two methods from the java.nio.file.Files class:


As you can see to open a seekable channel you need to supply as input the Path you want to open (basically the file) and also open options, which are enum constants.

The possible enum options are as follow:

READ The file is opened with READ access.
WRITE The file is opened with WRITE access.
CREATE Creates the file if the file does not already exist.
CREATE_NEW Creates the file if the file does not already exist. If the file exists throw an exception.
APPPEND Appends to the file. It is used in conjunction with CREATE and WRITE.
DELETE_ON_CLOSE Deletes the file after the channel is closed. Use this when creating and editing / reading from temporary files.
TRUNCATE_EXISTING Truncates the file to size 0. It is used in conjunction with WRITE and it’s useful when you want to clean the contents of a file.
SPARSE Usually used in conjunction with CREATE or CREATE_NEW. On some file systems large files that have big “data gaps” are stored in a more efficient way.
SYNC File content + metadata is synchronized with the underlying file system.
DSYNC File content is synchronized with the underlying file system.

Reading a file using a SeekableByteChannel

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Writing a file using SeekableByteChannel

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If we are going to open the file called “file2.txt” from our home folder the content will be:

If we run multiple times the program the content of the file will look like this (the text is appended):

Using the SeekableByteChannel to read characters from different file locations

Given the following acrostic:

We want to read the first 3 letters at the beginning of every word and print on the standard output.

Code example:

And the output of the code: