Java ByteBuffer : how does this work?

The first time you encounter the ByteBuffer, you may run into some surprises. The function that flips most folks is in fact ByteBuffer.flip(). To understand the flip() and to be not flipped by it and other such idioms, we will look at what this class is and how it should be used.

Basically, a ByteBuffer allows us to read data repeatedly from some input, like a non-blocking socket. ByteBuffer keeps track of the position of the last byte written, so you don't need to. You can keep writing to the same ByteBuffer and rest assured that previous data will not be over-written.

This is pretty handy in asynchronous I/O (using Java NIO package) as data from asynchronous sockets don't always arrive all at once. We need to map buffers to sockets and keep reading until there is no more data from the remote end.

So what about this flip()? Well, the way the ByteBuffer class was designed, data is read to the buffer starting at position and upto limit. Data is written starting at position and upto limit as well. So, if you followed that, after reading some data from a socket, the ByteBuffer position would be advanced, and reading now will not get any data as position is at the end of the buffer. So flip() basically sets the position to 0 (start), and limit to the position (previous position to be exact, or rather the end of useful input).

So think about read and write operations on the ByteBuffer manipulating data within position and limit and you will see more clearly the need to flip once in a while.

Happy flipping!