Java NIO FileChannel

A Java NIO FileChannel is a channel that is connected to a file. Using a file channel you can read data from a file, and write data to a file. The Java NIO FileChannel class is NIO's an alternative to reading
files with the standard Java IO API.

A 

FileChannel

 cannot be set into non-blocking mode. It always runs in blocking mode.

Opening a FileChannel

Before you can use a 

FileChannel

 you must open it. You cannot open a FileChannel directly. You need to obtain a FileChannel via an InputStream, OutputStream, or a RandomAccessFile. Here is how you open a FileChannel via a
RandomAccessFile:

RandomAccessFile aFile     = new RandomAccessFile("data/nio-data.txt", "rw");
FileChannel      inChannel = aFile.getChannel();

Reading Data from a FileChannel

To read data from a 

FileChannel

 you call one of the 

read()

 methods. Here is an example:

ByteBuffer buf = ByteBuffer.allocate(48);

int bytesRead = inChannel.read(buf);

First a 

Buffer

 is allocated. The data read from the 

FileChannel

 is read into the 

Buffer

.

Second the 

FileChannel.read()

 method is called. This method reads data from the 

FileChannel

 into the

Buffer

. The 

int

 returned
by the 

read()

 method tells how many bytes were witten into the 

Buffer

. If -1 is returned, the end-of-file is reached.

Writing Data to a FileChannel

Writing data to a 

FileChannel

 is done using the 

FileChannel.write()

 method, which takes a 

Buffer

 as parameter. Here is an example:

String newData = "New String to write to file..." + System.currentTimeMillis();

ByteBuffer buf = ByteBuffer.allocate(48);
buf.clear();
buf.put(newData.getBytes());

buf.flip();

while(buf.hasRemaining()) {
channel.write(buf);
}

Notice how the 

FileChannel.write()

 method is called inside a while-loop. There is no guarantee of how many bytes the 

write()

 method writes to the 

FileChannel.

 Therefore
we repeat the 

write()

 call until the

Buffer

 has no further bytes to write.

Closing a FileChannel

When you are done using a 

FileChannel

 you must close it. Here is how that is done:

channel.close();

FileChannel Position

When reading or writing to a 

FileChannel

 you do so at a specific position. You can obtain the current position of the 

FileChannel

 object by calling the 

position()

 method.

You can also set the position of the 

FileChannel

 by calling the 

position(long pos)

 method.

Here are two examples:

long pos channel.position();

channel.position(pos +123);

If you set the position after the end of the file, and try to read from the channel, you will get -1 - the end-of-file marker.

If you set the position after the end of the file, and write to the channel, the file will be expanded to fit the position and written data. This may result in a "file hole", where the physical file on the disk has gaps in the written data.

FileChannel Size

The 

size()

 method of the 

FileChannel

 object returns the file size of the file the channel is connected to. Here is a simple example:

long fileSize = channel.size();

FileChannel Truncate

You can truncate a file by calling the 

FileChannel.truncate()

 method. When you truncate a file, you cut it off at a given length. Here is an example:

channel.truncate(1024);

This example truncates the file at 1024 bytes in length.

FileChannel Force

The 

FileChannel.force()

 method flushes all unwritten data from the channel to the disk. An operating system may cache data in memory for performance reasons, so you are not guaranteed that data written to the channel is actually
written to disk, until you call the 

force()

 method.

The 

force()

 method takes a boolean as parameter, telling whether the file meta data (permission etc.) should be flushed too.

Here is an example which flushes both data and meta data:

channel.force(true);