/**
* return connected tracker server
* @return connected tracker server, null for fail
*/
    public TrackerServer getConnection(int serverIndex) throws IOException
    {
        Socket sock = new Socket();
        sock.setReuseAddress(true);
        sock.setSoTimeout(ClientGlobal.g_network_timeout);
        //将此套接字连接到服务器，并指定一个超时值。
        sock.connect(this.tracker_servers[serverIndex], ClientGlobal.g_connect_timeout);
        return new TrackerServer(sock, this.tracker_servers[serverIndex]);
    }

/**
* return connected tracker server
* @return connected tracker server, null for fail
*/
    public TrackerServer getConnection() throws IOException
    {
        int current_index;

        synchronized(this.lock)
        {
            this.tracker_server_index++;
            if (this.tracker_server_index >= this.tracker_servers.length)
            {
                this.tracker_server_index = 0;
            }

            current_index = this.tracker_server_index;
        }

        try
        {
            return this.getConnection(current_index);
        }
        catch(IOException ex)
        {
            System.err.println("connect to server " + this.tracker_servers[current_index].getAddress().getHostAddress() + ":" + this.tracker_servers[current_index].getPort() + " fail");
            ex.printStackTrace(System.err);
        }

      for (int i=0; i<this.tracker_servers.length; i++)
      {
          if (i == current_index)
          {
              continue;
          }

            try
            {
                TrackerServer trackerServer = this.getConnection(i);

                synchronized(this.lock)
                {
                    if (this.tracker_server_index == current_index)
                    {
                        this.tracker_server_index = i;
                    }
                }

                return trackerServer;
          }
          catch(IOException ex)
          {
              System.err.println("connect to server " + this.tracker_servers[i].getAddress().getHostAddress() + ":" + this.tracker_servers[i].getPort() + " fail");
              ex.printStackTrace(System.err);
          }
      }

      return null;
    }