当调用selector.select()时会阻塞：
This method performs a blocking selection operation. It returns only after at least one channel is
selected, this selector's wakeup method is invoked, or the current thread is interrupted, whichever
comes first.
其返回值有可能为0：
The number of keys, possibly zero,
whose ready-operation sets were updated
另请见，解释了select后返回值为0，但selector.selectedKeys()不为空：Java NIO Selector select() returns 0 although channels are ready
selector.selectedKeys()返回的Set<SelectionKey>需要删除SelectionKey，原因请见：Why the key should be removed in `selector.selectedKeys().iterator()` in java nio?
另外，Set<SelectionKey>删除SelectionKey需要使用iterator，这个在第一个链接里也有原因