运行约5-10秒后，Asnctask失败

Question

我正在尝试构建一个应用程序，在该应用程序中，我尝试永久获取同一应用程序中的系统时间和ntp时间，并将其写入excel文件。虽然这可以正常工作约5-10s。异步任务会在大约5s和随机10s后失败。我仍然试图将其范围缩小到asynctask，因为我已经在服务中实现了它。请让我知道我在哪里出错，以便我可以解决它。

MainActivity.java

public class MainActivity extends AppCompatActivity {
    static excel excel;
    static int index = 1;
    int cell = 0;
    EditText server;
    Intent intent;
    static TextView system1;
    static TextView ntp;
    Button start1,stop;
    static String server_name;
    NtpV3Packet message;
    long diff;
    String time2;
    Date time;
    DateFormat dateFormat  = new SimpleDateFormat("yyyy-mm-dd hh:mm:ss_SSS");

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    intent = new Intent(MainActivity.this, NtpUpdate.class);
    server = (EditText)findViewById(R.id.Server12);
    ntp = (TextView)findViewById(R.id.ntp_time);
    system1 = (TextView)findViewById(R.id.System_Time);
    start1 = (Button)findViewById(R.id.start);
    stop = (Button)findViewById(R.id.stop);
    server_name = "0.north-america.pool.ntp.org";
    excel = new excel();
    excel.createsheet();
    excel.createrow(0);
    excel.createcell(0,"NTP Time");
    excel.createcell(1,"System Time");
    excel.createcell(2,"Difference");
    excel.write();
}

public static String getservername(){
    return server_name;
}
public static void updateui(String data){
    String[] parts = data.split(";;;;");
    String part1 = parts[0]; // 004
    String part2 = parts[1];
    String part3 = parts[2];
    ntp.setText(part1);
    system1.setText(part2);
    excel.createrow(index);
    excel.createcell(0,part1);
    excel.createcell(1,part2);
    excel.createcell(2,part3);
    excel.write();
    index++;
}
public void start1(View view){
    intent.putExtra("start",1);
    startService(intent);
}
public void stop1(View view){
    intent.putExtra("start",0);
    stopService(intent);
}
}

class excel{
    HSSFWorkbook workbook;
    static HSSFSheet firstSheet;
    static HSSFCell cellA;
    static HSSFRow rowA;
    File file = new 

File(String.valueOf(Environment.getExternalStorageDirectory()) + File.separator + "data.xls");
        public void createsheet(){
            workbook = new HSSFWorkbook();
            firstSheet = workbook.createSheet("NtpLog");

        }
        public static void createrow(int index){
            rowA = firstSheet.createRow(index);
        }

        public static void createcell(int index, String text){
            cellA = rowA.createCell(index);
            cellA.setCellValue(text);
        }
        public void write(){
            FileOutputStream fos = null;
            try {
                fos = new FileOutputStream(file);
                workbook.write(fos);
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
                if (fos != null) {
                    try {
                        fos.flush();
                        fos.close();
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
            }
        }
    }

NTpUpdate.java(service)

public class NtpUpdate extends Service {
    String server_name = "0.north-america.pool.ntp.org";
    Timer myTimer = new Timer("MyTimer", true);
    int start2;
    int i = 0;
    NtpV3Packet message;
    Date time;

    public NtpUpdate() {
    }
    class MyTask extends TimerTask {
        public void run(){
            new LongOperation().execute("");
        }
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        //TODO do something useful
        myTimer.scheduleAtFixedRate(new MyTask(), 0, 2000);
        return Service.START_STICKY;
    }

    @Override
    public void onDestroy(){
        myTimer.cancel();
        super.onDestroy();
    }
    @Override
    public IBinder onBind(Intent intent) {
        // TODO: Return the communication channel to the service.
        return null;
    }
    public int start1(){
        return start2;
    }
}
class LongOperation extends AsyncTask<String, Void, String> {
    String server_name = MainActivity.getservername();
    TimeInfo timeInfo;
    long currentTime;
    InetAddress inetAddress;
    NtpV3Packet message;
    long serverTime,diff;
    Date time;
    Date currentTime1;
    String time1,time2,time3,differene;
    DateFormat dateFormat  = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss_SSS");


    @Override
    protected String doInBackground(String... params) {
        NTPUDPClient timeClient = new NTPUDPClient();
        try {
            inetAddress = InetAddress.getByName(server_name);

        } catch (UnknownHostException e) {
            e.printStackTrace();
        }
            try {
                timeInfo = timeClient.getTime(inetAddress);
                Log.d("NTPLOG","timeinfo: " + timeInfo);
                message = timeInfo.getMessage();
                serverTime = message.getTransmitTimeStamp().getTime();
                time = new Date(serverTime);
                currentTime = System.currentTimeMillis();
                currentTime1 = new Date(Long.parseLong(Long.toString(currentTime)));
                Log.d("NTPLOG","timeinfo: " + time);
                time1 = dateFormat.format(time);
                time2 = dateFormat.format(currentTime);
                diff = time.getTime() - currentTime1.getTime();
                differene = Long.toString(diff);
                time3 = time1 + ";;;;" +  time2 + ";;;;" + differene;
                Log.d("NTPLOG","timeinfo: " + time1);
            } catch (IOException e) {
                e.printStackTrace();
            }
        return time3;
    }

    @Override
    protected void onPostExecute(String result) {
        MainActivity.updateui(result);
    }

    @Override
    protected void onPreExecute() {
    }

    @Override
    protected void onProgressUpdate(Void... values) {
    }
}

谢谢