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.