Java-套接字继承

Question

我编写了一个扩展Socket类的类-旨在保留常规套接字中的所有方法，同时添加自己的两个方法：

public class ClientSocket extends Socket{

private long lastOnline;

public ClientSocket() {
      super();
}

public long getLastOnline() {
    return lastOnline;
}

public void setLastOnline(long lastOnline) {
    this.lastOnline = lastOnline;
}

当我尝试初始化该类的新实例并向其添加打开的套接字时，我做错了（或者我做错了），它告诉我无法将套接字设置为ClientSocket对象（尽管它扩展了Socket）：

ClientSocket socket = serverSocket.accept();

我该如何解决？ 谢谢！

Answer 1

serverSocket.accept()返回一个普通的Socket ，而不是ClientSocket 。
ClientSocket 是 Socket ，反之亦然。

Answer 2

您可以使用composition ：

public class ClientSocket {

    private final Socket socket;
    private long lastOnline;

    public ClientSocket(Socket socket) {
        this.socket = socket;
    }

    public long getLastOnline() {
         return lastOnline;
    }

    public void setLastOnline(long lastOnline) {
        this.lastOnline = lastOnline;
    }

    ...
}

然后创建一个ClientSocket：

ClientSocket clientSocket = new ClientSocket(serverSocket.accept());

Answer 3

为了记录在案，我相信你的具体情况是最好的继承上解决组成 。 Aka，最好是包装通过accept检索的套接字，而不是像现在那样尝试子类化。 有一本很好的书（Joshua Bloch的Effective Java），描述了何时在继承上使用组合。 看完后，如果您决定继承是适合你的使用情况，您可以实现它的插座，如下图所示：

更专业的套接字实现：

public class AuditableClientSocket extends Socket {
   private long lastPacketReceived = -1;

   // Constructors removed for brevity. Add them back here

   public long getLastPacketReceived() {
      return lastPacketReceived;
   }

   protected void setLastPacketReceived(long lastPacketReceived) {
      this.lastPacketReceived = lastPacketReceived;
   }

   public String formatLastReceived() {
      if (lastPacketReceived < 0)
         return "Never";

      return new Date(lastPacketReceived).toString();
   }
}

能够创建专用套接字的服务器套接字实现：

public class AuditableClientServerSocket extends ServerSocket {

   // Constructors removed for brevity. Add them back here

   @Override
   public Socket accept() throws IOException {
      if (isClosed())
         throw new SocketException("Socket is closed");
      if (!isBound())
         throw new SocketException("Socket is not bound yet");
      final Socket s = new AuditableClientSocket((SocketImpl) null);
      implAccept(s);
      return s;
   }
}

测试类别：

public class AuditableClientServer implements Runnable {
   public static void main(String[] args) {
      AuditableClientServer server = new AuditableClientServer();
      new Thread(server).start();
   }

   private AuditableClientServerSocket serverSocket = null;

   public void run() {

      try {
         serverSocket = new AuditableClientServerSocket(5656);
         final AuditableClientSocket sock = (AuditableClientSocket) serverSocket.accept();
         System.out.printf(
               "Client connected at %s. Last received packet at %s",
               new Date(), sock.formatLastReceived());

         sock.close();
         serverSocket.close();
      } catch (Exception e) {
         e.printStackTrace();
      }
   }
}

public class AuditableClient {
   public static void main(String[] args) {
      try {
         Socket s = new Socket("localhost", 5656);
         s.close();
      } catch(IOException ioe) {
         ioe.printStackTrace();
      }
   }
}