Java服务器从客户端应用程序接收到一个连接，但是客户端在尝试再次连接后超时

Question

当前使用Dynu DDNS服务通过端口80连接到我的Java服务器。服务器从客户端接收一个连接，发送数据，但随后被卡住以等待客户端应用程序再次连接。然后，客户端从第一个连接接收返回的结果，但是在尝试第二次连接时超时。我不确定为什么从这里只传输一次，然后再传输一次，所以我很困惑。

这是我的服务器类定义：

public class QueryServer implements Runnable{

protected String sql = "";
protected int serverPort = 80;
protected ServerSocket serverSocket = null;
protected boolean isStopped = false;
protected Thread runningThread = null;

DataInputStream input = null;
DataOutputStream output = null;

public QueryServer(int port)
{
    this.serverPort = port;
}

public void run()
{
    synchronized(this) 
    {this.runningThread = Thread.currentThread();}
    openServerSocket();

    while(!isStopped())
    {
        Socket clientSocket = null;
        try
        {
            System.out.println("Waiting for connection...");
            clientSocket = this.serverSocket.accept();
        }
        catch(IOException e)
        {
            if(isStopped())
            {
                System.out.println("Server Stopped");
                return;
            }
            throw new RuntimeException("Error accepting client connection", e);
        }
        try
        {
            processQueryRequest(clientSocket);
        }
        catch(Exception e)
        {
            System.out.println("Log: " + e);
        }
    }

    System.out.println("Server Stopped");
}

private void processQueryRequest(Socket clientSocket) throws Exception
{
    input = new DataInputStream(new BufferedInputStream(clientSocket.getInputStream()));
    output = new DataOutputStream(clientSocket.getOutputStream());
    sql = input.readUTF();
    System.out.println(sql);

    stop(); // Check for stop command

    String outputLine;
    QueryingProtocol QP = new QueryingProtocol();

    outputLine = QP.processInput(sql);
    output.writeUTF(outputLine);
    output.flush();

    output.close();
    input.close();
    System.out.println("Request processed");
}

private synchronized boolean isStopped()
{
    return this.isStopped;
}

public synchronized void stop()
{
    System.out.println("Checking for server shutdown command");
    if(sql.equals("Shutdown"))
    {
        this.isStopped = true;
        try
        {
            this.serverSocket.close();
        }
        catch(IOException e)
        {
            throw new RuntimeException("Error closing server", e);
        }
        System.out.println("Server shutdown");
    }
}

private void openServerSocket()
{
    try
    {
        this.serverSocket  = new ServerSocket(this.serverPort);
    }
    catch(IOException e)
    {
        throw new RuntimeException("Cannot open port 80", e);
    }
}
public static void main(String[] args)
{
    QueryServer server = new QueryServer(80);
    new Thread(server).start();

    try
    {
        Thread.sleep(10*1000);
    }
    catch(InterruptedException e)
    {
        e.printStackTrace();
    }
}

}

QueryProtocols只是用于处理由客户端应用发送的sql查询的类