Java NIO: reading variable-sized blocks

Question

I would like to read a string from a TCP stream that is given with a byte length followed by the actual data. In Python, I would do

length = ord(stream.read(1))
data = stream.read(length)

How do I do the same in Java NIO? What I have is a buffer (of capacity 257)

stream.read(buffer); // cannot specify a size here
int length = buffer.get();
byte[] data = new byte[length];
buffer.get(data);

Unfortunately, this doesn't work: the get() calls read past the data in the buffer :-(

I probably need some combination of flip, rewind, reset, etc., but I can't figure it out.

Answer 1

I suggest, you have a look into java.nio.channel.GatheringByteChannel and DatagramChannel. You'll need to specifiy a Datagram for your needs.

That's what you've done by your Python example (first byte indicates the payload length, read the playload using this length), which looks really fault-prone(!).

A datagram is a specification for a data stream. It divides your stream into packages, logically. So, a better strategy would be to send a payload-package first, that indicates the length of the payload (and the package size). Edit: Of course, both sides of the stream must speak the same protocol / datagram language.

Answer 2

if stream is a SocketChannel and buffer is a ByteBuffer, you can do the following:

//assuming buffer is a ByteBuffer
buffer.position(0);
buffer.limit(1);
while (buffer.hasRemaining()) { 
   stream.read(buffer);
}
buffer.rewind();

//get the byte and cast it into the range 0-255
int length = buffer.get() & 0xFF;
buffer.clear();
buffer.limit(length);
while (buffer.hasRemaining()) { 
   stream.read(buffer);
}
buffer.rewind();
//the buffer is now ready for reading the data from