當調用selector.select()時會阻塞：
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.
其返回值有可能為0：
The number of keys, possibly zero,
whose ready-operation sets were updated
另請見，解釋了select后返回值為0，但selector.selectedKeys()不為空：Java NIO Selector select() returns 0 although channels are ready
selector.selectedKeys()返回的Set<SelectionKey>需要刪除SelectionKey，原因請見：Why the key should be removed in `selector.selectedKeys().iterator()` in java nio?
另外，Set<SelectionKey>刪除SelectionKey需要使用iterator，這個在第一個鏈接里也有原因