目的：研究生高階計算機網路課程大作業--實現DV演算法的router編寫（JAVA）

問題描述：

      使用UDP協議進行通訊，好不容易使執行緒Thread安全地關閉，卻無法再次獲取 同一個（IP，Port） 上的連線。

      關於執行緒的安全終止這裡再說兩句，stop(), interrupt() ,destroy() 都是被廢棄的、不安全的，最好讓執行緒自動執行完畢。作者這裡是通過直接向socket發出stop命令 實現的，阻塞的執行緒受到stop命令後設置 stop = true 然後可結束執行緒。

解決辦法：

      呼叫 socket.close() 之後必須呼叫 serversocket.close()

      部分程式碼如下：（關鍵在t.close()和l.close() ）

public void run() { // socket to listen on the command port ServerSocket l = null; // socket to represent the accepted connection Socket t = null; // the input and output streams associated with the connection ObjectOutputStream oos; ObjectInputStream ois; try { // listen on the command port l = new ServerSocket(port); System.out.println("CommandThread: Listening for commands on port " + port + "/n"); } catch (Exception e) { System.out.println("CommandThread: Unable to open a listening socket on port: " + port + "/n"); System.out.println("Quitting./n"); return; } while (!stop) { try { // accept a connection t = l.accept(); } catch (Exception e) { System.out.println("CommandThread: An I/O error occured while accepting a connection/n"); System.out.println(e.getMessage()); try { l.close(); return; } catch (IOException e1) { e1.printStackTrace(); } } try { // open the streams oos = new ObjectOutputStream(t.getOutputStream()); ois = new ObjectInputStream(t.getInputStream()); // read the command String in = (String) ois.readObject(); String out; if(in.equals("stop")){ this.stop = true; out = "receive the stop command!"; } else{ // execute the command and send the response out = r.executeCommand(in); } oos.writeObject(out); // close the streams and the connection ois.close(); oos.close(); t.close(); } catch (Exception e) { System.out.println("CommandThread: An I/O error occured while communicating with COMMAND/n"+e.getMessage()+"/n"); continue; } } try { l.close(); } catch (IOException e) { System.out.println(e.getMessage()); } System.out.println("CommandThread is stopped!/n"); }