Java NIO Channel to Channel Transfers

Java Nio

1	Java NIO Tutorial
2	Java NIO Overview
3	Java NIO Channel
4	Java NIO Buffer
5	Java NIO Scatter / Gather
6	Java NIO Channel to Channel Transfers
7	Java NIO Selector
8	Java NIO FileChannel
9	Java NIO SocketChannel
10	Java NIO ServerSocketChannel
11	Java NIO DatagramChannel
12	Java NIO Pipe
13	Java NIO vs. IO

Java NIO Channel to Channel Transfers

By Jakob Jenkov Connect with me:
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Table of ContentstransferFrom()
transferTo()

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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