NIO Reactor Pattern: Receiving asynchronous callbacks inside the selector loop every N miliseconds

Question

I was wondering what is the standard/best implementation of that in Java NIO. This is fundamental to implement something like heartbeats every N seconds, etc. Note: For obvious reasons (threads are evil and context switches are slow) everything must always happens inside the selector loop.

Note1: Answering Apache MINA does not count, unless you and the framework can demonstrate a clear scenario where this is done in a KISS (Keep It Simple Stupid) way.

Note2: Pipes require threads.

Answer 1

Not sure why you wouldn't have a background thread for sending heartbeats or timing out connections. Heartbeats are not generally considered performance critical.

You can have the selector wait a specific amount of time and send heartbeats and check time outs at intervals.

Do you mean like

selector.select(timeout);

if (System.currentTimeMS() > sendHeartbeatTime) {
    for(Connection conn: connections) 
        conn.checkAndSendHeartbeat();
}

// in Connection
private long lastSend = System.currentTimeMS();
private long lastRead = System.currentTimeMS();

public void writeData() {
   lastSend = System.currentTimeMS();
   // write data.
}

public void checkAndSendHeartbeat() {
   long now = System.currentTimeMS();
   if (now - lastRead > HEARTBEAT_TIMEOUT) {
      closeConnection();
   else if (now - lastSend > HEATBEAT_INTERVAL)
      writeHeartBeatData();
}