Java NIO(6-Channel to Channel Transfers)

In Java NIO you can transfer data directly from one channel to another, if one of the channels is a

FileChannel

. The

FileChannel

class has a

transferTo()

and
a

transferFrom()

method which does this for you.

transferFrom()

The

FileChannel.transferFrom()

method transfers data from a source channel into the

FileChannel

. Here is a simple example:

RandomAccessFile fromFile = new RandomAccessFile("fromFile.txt", "rw");
FileChannel      fromChannel = fromFile.getChannel();

RandomAccessFile toFile = new RandomAccessFile("toFile.txt", "rw");
FileChannel      toChannel = toFile.getChannel();

long position = 0;
long count    = fromChannel.size();

toChannel.transferFrom(position, count, fromChannel);

The two first parameters, position and count, tell where in the destination file to start writing (

position

), and how many bytes to transfer maximally (

count

).
If the source channel has fewer than

count

bytes, less is transfered.

Additionally, some

SocketChannel

implementations may transfer only the data the

SocketChannel

has ready in its internal buffer here and now - even
if the

SocketChannel

may later have more data available. Thus, it may not transfer the entire data requested (

count

) from the

SocketChannel

into

FileChannel

.

transferTo()

The

transferTo()

method transfer from a

FileChannel

into some other channel. Here is a simple example:

RandomAccessFile fromFile = new RandomAccessFile("fromFile.txt", "rw");
FileChannel      fromChannel = fromFile.getChannel();

RandomAccessFile toFile = new RandomAccessFile("toFile.txt", "rw");
FileChannel      toChannel = toFile.getChannel();

long position = 0;
long count    = fromChannel.size();

fromChannel.transferTo(position, count, toChannel);

Notice how similar the example is to the previous. The only real difference is the which

FileChannel

object the method is called on. The rest is the same.

The issue with

SocketChannel

is also present with the

transferTo()

method. The

SocketChannel

implementation
may only transfer bytes from the

FileChannel

until the send buffer is full, and then stop.