Java NIO Channel to Channel Transfers
Jakob Jenkov |
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.
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Jakob Jenkov | |
