限制Java NIO通道（文件或套接字）中可用的内容

Question

I'm pretty new to NIO and wanted to implement some feature with it, instead of typical Streams (which can do all sort of things).

What I'm not sure I can get is reading from a file into a buffer and limiting the content that I will transfer. Let's say from position 100 to 200 (even if file length is 1000). It also would be nice to do on network sockets.

I know that NIO keeps things basic to leverage OS capabilities that's why I'm not sure it can be done.

I was thinking that a tricky way to do it would be a 'LimitedReadChannel' that when it's should return less than the available buffer size it uses another byte-buffer and then transfer to the original one (1). But seems more tricky than necessary. I also don't want to use anything related to streams because it would defeat the purpose of using NIO.

(1) So far....

LimitedChannel.read(buffer) {
  if (buffer.available?? > contentLeft) {
    delegateChannel.read(smallerBuffer);
    // transfer from smallerBuffer to buffer
  } else {
    delegateChannel.read(buffer);
  }

}

Answer 1

I've found that Buffers admit to ask for the current limit or set a new one. So that wrapper channel (the one that limits the effective number of bytes read) could modify the buffer limit to avoid reading more...

Something like:

// LimitedChannel.java
// private int bytesLeft; // remaining amount of bytes to read
public int read(ByteBuffer buffer) {
  if (bytesLeft <= 0) {
    return -1;
  }
  int oldLimit = buffer.limit();
  if (bytesLeft < buffer.remaining()) {
    // ensure I'm not reading more than allowed
    buffer.limit(buffer.position() + bytesLeft);
  }
  int bytesRead = delegateChannel.read(buffer);
  bytesLeft -= bytesRead;
  buffer.limit(oldLimit);
  return bytesRead;
}

Anyway not sure if this already exists somewhere. It's difficult to find documentation about this use case...