Java NIO 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(fromChannel, position, count);

The 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.