Selector is a mutiplexer. Basically aggregate multiple channel into one bundle so we don’t have to switch between different thread. By doing so, we can use one thread to handle multiple channel.


Seems like there is only one way to create selector.

final Selector selector =;

register channel

Each channel must register to selector is order to subscribe to coming event.

// channel must be configured as unblocking
// thus FileChannel can not be registered is selector
channel.register(selector, SelectionKey.OP_ACCEPT);

The second parameter is interest operations, which is a list of operations that this channel is interested in.

  1. SelectionKey.OP_CONNECT As a client, waiting for server to accept my connection request
  2. SelectionKey.OP_ACCEPT As a server, waiting for client connection request
  3. SelectionKey.OP_READ Waiting for data reading
  4. SelectionKey.OP_WRITE Waiting for data writing

These interest operations are just integer number, so they can be combined using bitwise operator like A|B, to indicate multiple interests for single channel.


Now that we have channel registered, let us waiting for incoming request.

// this will block current thread until at least one channel is ready
// it return an integer to indicate the number of channel that is ready
// `select` will only return those that are ready between last and this `select` call
int n =;

get inbound

Then we need to know which exact channel get ready.

// `selectedKeys` to get a list of ready channel, we call it `selectedKey`
for (SelectionKey key : selector.selectedKeys()) {
// `selectedKey` contains a lot of objects inside.
// including, the corresponding channel, selector and its interest operations
  if (key.isAcceptable()) {
    // process channel that listen to SelectionKey.OP_ACCEPT
    // which is incoming client connection request
  if (key.isReadable()) {
    // process channel that listen to SelectionKey.OP_READ
    // which is incoming request to read
  // ...

close channel

When want to close channel connection, we may use cancel or close This will deregister itself from channel as well.

final SocketChannel client = (SocketChannel);

close selector

Shutdown entire selector by closing it.



08 March 2020