使用java套接字通过ObjectOutputStream和ObjectInputStream将数组发送到android时为null指针

Question

Hi I have been trying to pass an array into a list view using ObjectOutput/InputStream and everything works fine - I can print out the array on the server side with no problem - up until the point where I try to insert the array into the view, I get a nullpointerexception - I can insert the preloaded 'animals' array and no errors are thrown. Any help would be greatly appreciated as I have been stuck for weeks, thanks.

DatabaseConnection.java (server side)

 public class DatabaseConnection implements Serializable {

/**
 * 
 */
private static final long serialVersionUID = 1L;

/**
 * @param args the command line arguments
 */

private static Connection connect;

static String sqlQuery;
private static PreparedStatement statement;
private static ResultSet result;
private static String username = "rachaelwaddington";
private static String password = "*****";

private static ServerSocket serverSocket;
private static Socket clientSocket;
private static InputStreamReader input;
private static ObjectOutputStream output;
private static BufferedReader bufferedReader;
private static int port = 10000;


static ArrayList<String> catArray = new ArrayList<String>();


public static void connect() {
    try {


        Class.forName("oracle.jdbc.driver.OracleDriver").newInstance();
        connect = DriverManager.getConnection("jdbc:oracle:thin:@tom.uopnet.plymouth.ac.uk:1521:orcl",
                username, password);
        System.out.println("Connected to Oracle!");


    } catch (Exception e) {
        e.printStackTrace();
        System.out.println("Error!");
    }
}

public void disconnect() throws SQLException, ClassNotFoundException {
    connect.close();
}

public static void getCats() throws SQLException, ClassNotFoundException{
    sqlQuery = "SELECT * FROM cat_lookup";
    statement = connect.prepareStatement(sqlQuery);
    result = statement.executeQuery();


    try {
        while (result.next()) {
            String cats = result.getObject(2).toString();



            catArray.add(cats);


        }

        //System.out.println(catArray);

    } catch (SQLException e) {
        e.printStackTrace();
    }
}
public static void main(String[] args) {
    connect();



    try {
        serverSocket = new ServerSocket(port); //Server socket
        System.out.println("Listening on port");

        } catch (IOException e) {
        System.out.println("Could not listen on port");
        }

    while (true) {
        try {

        clientSocket = serverSocket.accept(); //accept the client connection
        input = new InputStreamReader(clientSocket.getInputStream());
        bufferedReader = new BufferedReader(input); //get the client message
        String message = bufferedReader.readLine();
        input.close();

        System.out.println(message);

        if (message.equals("animal"))
        {
        getCats();
        }


        output = new ObjectOutputStream(clientSocket.getOutputStream());
        output.writeObject(catArray);  

        System.out.println(catArray);

        output.flush();
        output.close();
        clientSocket.close();



        } catch (IOException ex) {
        System.out.println("Problem in message reading");
        }

        catch (SQLException e)
        {
            System.out.println("SQLException Error!");
        }
        catch (ClassNotFoundException f){
            System.out.println("ClassNotFoundException Error!");
        }
      }

   }

 }

AnimalFragment.Java

 /** This is a listfragment class */
 public class AnimalFragment extends ListFragment {

/** An array of items to display in ArrayList */
String animals[] = new String[]{
        "Snail",
        "Fox",
        "Rabbit",
        "Woodpecker",
        "Mouse"     
    };
String[] cats = new String[57];

private Socket client;
private ObjectInputStream input;


    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        new SendDataTask().execute();

        /** Creating array adapter to set data in listview */
        ArrayAdapter<String> adapter = new ArrayAdapter<String>(getActivity().getBaseContext(), android.R.layout.select_dialog_item, cats);

        /** Setting the array adapter to the listview */
        setListAdapter(adapter);

        return super.onCreateView(inflater, container, savedInstanceState);




    }

    @Override
    public void onListItemClick(ListView l, View v, int position, long id) {
        // TODO Auto-generated method stub
        //super.onListItemClick(l, v, position, id);
        startActivity(new Intent (getActivity(), SpeciesSheetActivity.class));
    }

    @Override
    public void onStart() {
        super.onStart();


        getListView();


    }

    private class SendDataTask extends AsyncTask<Void,Void,Void>{

        @Override
        protected void onPreExecute() {
            // TODO Auto-generated method stub
            super.onPreExecute();
        }

        @Override
        protected Void doInBackground(Void... arg0) {
            try {



                 client = new Socket("192.168.1.70", 10000);  //connect to server
                 PrintWriter printwriter = new PrintWriter(client.getOutputStream(),true);
                 printwriter.write("animal");  //write the message to output stream

                 printwriter.flush();
                 printwriter.close();

                 input = new ObjectInputStream(client.getInputStream());
                 cats = (String[]) input.readObject();
                 input.close();


                 client.close();   //closing the connection

                } catch (UnknownHostException e) {
                 e.printStackTrace();
                } catch (IOException e) {
                 e.printStackTrace();
                } catch (ClassNotFoundException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }

            return null;
        }

        @Override
        protected void onPostExecute(Void result) {
            // TODO Auto-generated method stub
            super.onPostExecute(result);
                // Handle/Update UI Part
        }


    }

}

Answer 1

当您关闭打印编写器时，您正在关闭套接字，然后从同一套接字读取时必须得到一个被忽略的异常。