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Java NIO Selector select() returns 0 although channels are ready
Selector selector = Selector.open();

SocketChannel channel;

for (...) { // for each node
  // Create and connect channels...
  ...

  channel.configureBlocking(false);
  channel.register(selector, SelectionKey.OP_READ, someRelatedObject);
}

int ready;
Set<SelectionKey> readyKeys;
while (true) {
  ready = selector.select();
  readyKeys = selector.selectedKeys();

  System.out.println("Ready channels: " + ready);
  System.out.println("Selected channels: " + readyKeys.size());

  if (ready == 0) {
    continue;
  }

  for (SelectionKey key : readyKeys) {
    if (key.isValid() && key.isReadable()) {
      // Take action...
    }

    readyKeys.remove(key);
  }
}

for (SelectionKey key : readyKeys) {
    if (key.isValid() && key.isReadable()) {
      // Take action...
    }

    readyKeys.remove(key);
  }

for (SelectionKey key : readyKeys) {
    readyKeys.remove(key);

    if (key.isValid() && key.isReadable()) {
      // Take action...
    }
  }