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 aFileChannel. The FileChannel class has atransferTo() 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 theSocketChannel has ready in its internal buffer here and now - even if theSocketChannel may later have more data available. Thus, it may not transfer the entire data requested (count) from theSocketChannel 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 theFileChannel until the send buffer is full, and then stop. 

本文转自：http://tutorials.jenkov.com/java-nio/channel-to-channel-transfers.html