Java 如何检测远程端套接字关闭？

How to detect a remote side socket close?

提问by Kevin Wong

How do you detect if Socket#close()has been called on a socket on the remote side?

你如何检测是否Socket#close()在远程端的套接字上被调用？

采纳答案by WMR

The isConnectedmethod won't help, it will return trueeven if the remote side has closed the socket. Try this:

该isConnected方法无济于事，true即使远程端关闭了套接字，它也会返回。尝试这个：

public class MyServer {
    public static final int PORT = 12345;
    public static void main(String[] args) throws IOException, InterruptedException {
        ServerSocket ss = ServerSocketFactory.getDefault().createServerSocket(PORT);
        Socket s = ss.accept();
        Thread.sleep(5000);
        ss.close();
        s.close();
    }
}

public class MyClient {
    public static void main(String[] args) throws IOException, InterruptedException {
        Socket s = SocketFactory.getDefault().createSocket("localhost", MyServer.PORT);
        System.out.println(" connected: " + s.isConnected());
        Thread.sleep(10000);
        System.out.println(" connected: " + s.isConnected());
    }
}

Start the server, start the client. You'll see that it prints "connected: true" twice, even though the socket is closed the second time.

启动服务器，启动客户端。您会看到它打印了两次“connected: true”，即使套接字第二次关闭。

The only way to really find out is by reading (you'll get -1 as return value) or writing (an IOException(broken pipe) will be thrown) on the associated Input/OutputStreams.

真正找出答案的唯一方法是IOException在关联的输入/输出流上读取（您将获得 -1 作为返回值）或写入（将抛出（损坏的管道））。

回答by Sangamesh

You can also check for socket output stream error while writing to client socket.

您还可以在写入客户端套接字时检查套接字输出流错误。

out.println(output);
if(out.checkError())
{
    throw new Exception("Error transmitting data.");
}

回答by Thorsten Niehues

Since the answers deviate I decided to test this and post the result - including the test example.

由于答案有所不同，我决定对此进行测试并发布结果 - 包括测试示例。

The server here just writes data to a client and does not expect any input.

这里的服务器只是将数据写入客户端，不需要任何输入。

The server:

服务器：

ServerSocket serverSocket = new ServerSocket(4444);
Socket clientSocket = serverSocket.accept();
PrintWriter out = new PrintWriter(clientSocket.getOutputStream(), true);
while (true) {
  out.println("output");
  if (out.checkError()) System.out.println("ERROR writing data to socket !!!");
  System.out.println(clientSocket.isConnected());
  System.out.println(clientSocket.getInputStream().read());
        // thread sleep ...
  // break condition , close sockets and the like ...
}

• clientSocket.isConnected() returns always true once the client connects (and even after the disconnect) weird !!
• getInputStream().read()
  • makes the thread wait for input as long as the client is connected and therefore makes your program not do anything - except if you get some input
  • returns -1 if the client disconnected
• out.checkError() is true as soon as the client is disconnected so I recommend this

• 一旦客户端连接（甚至在断开连接之后），clientSocket.isConnected() 总是返回 true 很奇怪！
• getInputStream().read()
  • 只要客户端连接，线程就会等待输入，因此使您的程序不执行任何操作 - 除非您获得一些输入
  • 如果客户端断开连接，则返回 -1
• 一旦客户端断开连接，out.checkError() 就为真，所以我推荐这个

回答by DumbestGuyOnSaturn

The method Socket.Available will immediately throw a SocketException if the remote system has disconnected/closed the connection.

如果远程系统断开/关闭连接，方法 Socket.Available 将立即抛出 SocketException。