When does java nio selector unblock on select() call?

This. Accept(key) calls serverSocketChannel.accept() which returns a new socket channel for communication with the client. It's this channel that is registered with the selector for "read" operations, i.e.

The selector now has two registrations: the original ServerSocketChannel, with OP_ACCEPT the SocketChannel for the new client, with OP_READ If a write cannot complete due to the buffer filling up, the corresponding SocketChannel remains registered with OP_WRITE. Once the client reads some data off the other end, the channel will be selected again, allowing us to write the remaining data before flipping the interest set back to OP_READ.

OP_WRITE triggers when then there is room in the socket send buffer. NB getting a zero length write() result is the only occasion for using OP_WRITE. Most of the time there is room, so OP_WRITE will keep triggering.

You don't want this, so you normally don't have OP_WRITE registered for a channel: only when it has just returned zero from write; and you deregister it when that write eventually completes via being re-triggered after an OP_WRITE.

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