Android Asynctask在更改为第二个活动并返回第一个活动时等于null

我有个问题

我有两项活动。在第一个活动中，我使用asynctask从服务器下载文件，没问题。我有一个取消下载的按钮。下载时，我单击按钮“取消”“取消下载，我使用方法“myAsyntask.Cancel（true）”进行此操作。它做得很好，但当我切换到第二个活动时，然后我返回到第一个活动，并单击按钮“取消””->我的应用程序崩溃。我调试并发现更改活动时myAsyntask为空

我怎样才能解决这个问题当我的应用程序更改为第二个活动并返回第一个活动时，我想取消任务

多谢各位

这是我的任务

 public class AsynDownload extends AsyncTask<GetParams, Integer, String> {

    private Context context;


    public AsynDownload(Context context) {
        this.context = context;
    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        llDownloadItem.setVisibility(View.VISIBLE);
    }

    @Override
    protected String doInBackground(GetParams... params) {
        FunctionHelper.UpdateCacheDownBook(context, true);
        checkDown = true;
        String domain = params[0].domain;
        String token = params[0].token;
        String codeDevice = params[0].codeDevice;

        String strPdf = "";
        String strJsonPdf = "";
        String strZip = "";
        String strJsonZip = "";


        Log.d("getdata", token + " - " + codeDevice);
        try {
            long total = 0;
            long total_pdf = 0;

            InputStream input_pdf = null;
            OutputStream output_pdf = null;
            HttpURLConnection connection_pdf = null;

            InputStream input_zip = null;
            OutputStream output_zip = null;
            HttpURLConnection connection_zip = null;

            JSONParser jsonParser = new JSONParser();


            strPdf = domain + DOWNLOAD_BOOK + token + "/" + pdfFileId + "/" + codeDevice;


            strJsonPdf = jsonParser.getJSONFromUrl(strPdf, METHOD_GET, null);

            JSONObject object_Pdf = new JSONObject(strJsonPdf);
            String path_pdf = object_Pdf.getString("DownloadsResult").toString();

            URL url_pdf = new URL(path_pdf);
            connection_pdf = (HttpURLConnection) url_pdf.openConnection();
            connection_pdf.setRequestMethod("GET");
            connection_pdf.connect();

            if (connection_pdf.getResponseCode() != HttpURLConnection.HTTP_OK) {
                FunctionHelper.UpdateCacheDownBook(context, false);
                btnDownload.setVisibility(View.VISIBLE);
                btnXemsach.setVisibility(View.GONE);
                return "Server returned HTTP " + connection_pdf.getResponseCode()
                        + " " + connection_pdf.getResponseMessage() + ". Vui lòng tải lại";

            }

            int fileLength_pdf = connection_pdf.getContentLength();

            input_pdf = connection_pdf.getInputStream();

            File root = new File(LIBOL_ROOT_PATH + proCode + "/");
            if (!root.exists()) {
                root.mkdirs();
            } else {
                FunctionHelper.DeleteRecursive(root);
                root.mkdirs();
            }

            File pdf = new File(root, proCode + ".pdf");

            if (!pdf.exists()) {
                pdf.createNewFile();
            }


            long total_zip = 0;
            strZip = domain + DOWNLOAD_BOOK + token + "/" + zipFileId + "/" + codeDevice;


            strJsonZip = jsonParser.getJSONFromUrl(strZip, METHOD_GET, null);

            JSONObject object_zip = new JSONObject(strJsonZip);
            String path_zip = object_zip.getString("DownloadsResult").toString();

            URL url_zip = new URL(path_zip);
            connection_zip = (HttpURLConnection) url_zip.openConnection();
            connection_zip.setRequestMethod("GET");
            connection_zip.connect();
            if (connection_zip.getResponseCode() != HttpURLConnection.HTTP_OK) {
                btnDownload.setVisibility(View.VISIBLE);
                btnXemsach.setVisibility(View.GONE);
                FunctionHelper.UpdateCacheDownBook(context, false);
                return "Server returned HTTPZip " + connection_zip.getResponseCode()
                        + " " + connection_zip.getResponseMessage() + ". Vui lòng tải lại";

            }

            int fileLength_zip = connection_zip.getContentLength();

            long lengOfFile = (long) fileLength_pdf + fileLength_zip;


            output_pdf = new FileOutputStream(pdf, true);

            byte data_pdf[] = new byte[4096];

            int oldPercent = 0;
            int count;

            while ((count = input_pdf.read(data_pdf)) != -1) {
                if (isCancelled()) {
                    input_pdf.close();
                    return null;
                }
                total += count;
                output_pdf.write(data_pdf, 0, count);
                int currentPercent = (int) (total * 100 / lengOfFile);
                if (currentPercent > oldPercent) {
                    oldPercent = currentPercent;

                    Intent i = new Intent();
                    i.setAction(ACTION_FILTER);
                    i.putExtra("action", ACTION_SHOW_NOTIFICATION);
                    i.putExtra("BookId", bookId);
                    i.putExtra("BookName", bookName);
                    i.putExtra("LibraryId", libId);
                    i.putExtra("LibraryDomain", libDomain);
                    i.putExtra("LibraryName", libName);
                    if (isCancelled()) {
                        i.putExtra("action", ACTION_CANCEL_NOTIFICATION);
                        btnDownload.setVisibility(View.VISIBLE);
                        btnXemsach.setVisibility(View.GONE);
                        llDownloadItem.setVisibility(View.GONE);
                    } else {
                        i.putExtra("action", ACTION_SHOW_NOTIFICATION);
                    }
                    i.putExtra("percent", oldPercent);
                    if (oldPercent == 100) {
                        i.putExtra("show_percent", false);
                    } else {
                        i.putExtra("show_percent", true);
                    }

                    context.sendBroadcast(i);
                }

            }
            Log.d("duythole3", "totalPDF =" + total_pdf);

            output_pdf.flush();

            output_pdf.close();
            input_pdf.close();


            input_zip = connection_zip.getInputStream();
            File rootFile_zip = new File(LIBOL_ROOT_PATH);
            File zipFile = new File(rootFile_zip, proCode + ".zip");

            if (!rootFile_zip.exists()) {
                rootFile_zip.mkdirs();
            }

            if (!zipFile.exists()) {
                zipFile.createNewFile();
            } else {
                Boolean deletezip = zipFile.delete();
                zipFile.createNewFile();
            }


            output_zip = new FileOutputStream(zipFile, true);

            byte data_zip[] = new byte[1024 * 1024];

            while ((count = input_zip.read(data_zip)) != -1) {
                if (isCancelled()) {
                    input_zip.close();
                    return null;
                }

                total += count;
                output_zip.write(data_zip, 0, count);

                int currentPercent = (int) (total * 100 / lengOfFile);
                if (currentPercent > oldPercent) {
                    oldPercent = currentPercent;

                    Intent i = new Intent();
                    i.setAction(ACTION_FILTER);
                    i.putExtra("action", ACTION_SHOW_NOTIFICATION);
                    i.putExtra("BookId", bookId);
                    i.putExtra("BookName", bookName);
                    i.putExtra("LibraryId", libId);
                    i.putExtra("LibraryDomain", libDomain);
                    i.putExtra("LibraryName", libName);
                    if (isCancelled()) {

                        i.putExtra("action", ACTION_CANCEL_NOTIFICATION);
                        btnDownload.setVisibility(View.VISIBLE);
                        btnXemsach.setVisibility(View.GONE);
                        llDownloadItem.setVisibility(View.GONE);
                    } else {
                        i.putExtra("action", ACTION_SHOW_NOTIFICATION);
                    }
                    i.putExtra("percent", oldPercent);
                    if (oldPercent == 100) {
                        i.putExtra("show_percent", false);
                    } else {
                        i.putExtra("show_percent", true);
                    }

                    context.sendBroadcast(i);
                }

            }

            output_zip.flush();

            output_zip.close();
            input_zip.close();

            Thread.sleep(500);

            FunctionHelper.LibolUnzip(zipFile, zipFile.getParent(), proCode);

        } catch (Exception e) {
            return e.toString();
        }
        return null;
    }

    @Override
    protected void onPostExecute(String result) {
        if (DetailBookActivity.this.isDestroyed()) {
            String countTimeEnd = String.valueOf(history.getRemainDate());
            modelLibrary.OpenConnectionSQL(ctx);
            modelLibrary.AddBookInfo(idBook, bookName, cover, proCode, author, publisher, libDomain, libId, libName, user, countTimeEnd);
            modelLibrary.CloseConnection();
            return;
        }
        if (result != null) {
            FunctionHelper.UpdateCacheDownBook(context, false);
            checkDown = false;
            btnDownload.setText("Tải lại");
            btnDownload.setVisibility(View.VISIBLE);
            btnXemsach.setVisibility(View.GONE);
            llDownloadItem.setVisibility(View.GONE);
            File root = new File(LIBOL_ROOT_PATH + proCode);
            FunctionHelper.DeleteRecursive(root);
            Toast.makeText(context, "Có lỗi trong quá trình tải. Vui lòng tải lại", Toast.LENGTH_LONG).show();

            Intent intent = new Intent();
            intent.setAction(ACTION_FILTER);
            intent.putExtra("action", ACTION_LIBOL_DOWNLOAD_FAIL);
            intent.putExtra("BookId", bookId);
            context.sendBroadcast(intent);

        } else {
            FunctionHelper.UpdateCacheDownBook(context, false);
            Log.d("tt", "tai thanh cong 1");
            checkDown = false;
            btnDownload.setVisibility(View.GONE);
            btnXemsach.setVisibility(View.VISIBLE);
            llDownloadItem.setVisibility(View.GONE);
            Log.d("tt", "tai thanh cong 3");

            String countTime = String.valueOf(history.getRemainDate());
            modelLibrary.OpenConnectionSQL(ctx);
            Boolean result_addbook = modelLibrary.AddBookInfo(idBook, bookName, cover, proCode, author, publisher, libDomain, libId, libName, user, countTime);
            Log.d("addbook", result_addbook.toString());
            modelLibrary.CloseConnection();

            Toast.makeText(context, "Tải sách thành công", Toast.LENGTH_SHORT).show();

            Intent intent = new Intent();
            intent.setAction(ACTION_FILTER);
            intent.putExtra("action", ACTION_LIBOL_DOWNLOAD_COMPLETE);
            intent.putExtra("BookId", bookId);
            intent.putExtra("LibraryId", libId);
            intent.putExtra("LibraryDomain", libDomain);
            intent.putExtra("LibraryName", libName);
            context.sendBroadcast(intent);
        }
    }


}

 private void CancelDownload() {
    AlertDialog.Builder builder = new AlertDialog.Builder(ctx);
    builder.setCancelable(true);
    builder.setTitle("Thông báo");
    builder.setMessage("Bạn có muốn hủy tải cuốn sách này không?");

    builder.setPositiveButton("Có", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {


            Intent i = new Intent();
            i.setAction(ACTION_FILTER);
            i.putExtra("action", ACTION_CANCEL_NOTIFICATION);
            i.putExtra("BookId", bookId);
            ctx.sendBroadcast(i);

            //  llDownloadItem.setVisibility(View.GONE);
            Boolean checkCache = FunctionHelper.GetCacheDownBook(ctx);
            Log.d("checkCache", checkCache.toString());
            if (checkCache) {

                asynDownload.cancel(true);

                FunctionHelper.UpdateCacheDownBook(ctx, false);
                llDownloadItem.setVisibility(View.GONE);

            }
            File root = new File(LIBOL_ROOT_PATH + proCode);
            FunctionHelper.DeleteRecursive(root);
            modelLibrary.OpenConnectionSQL(ctx);
            Boolean resultDelete = modelLibrary.DeleteBookSQlite(idBook);
            Log.d("delete sqlite", resultDelete.toString());
            modelLibrary.CloseConnection();

            btnDownload.setVisibility(View.VISIBLE);
            btnXemsach.setVisibility(View.GONE);


            Toast.makeText(ctx, "Hủy tải sách thành công", Toast.LENGTH_SHORT).show();

        }
    });

    builder.setNegativeButton("Không", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {

        }
    });

    AlertDialog dialog = builder.create();
    dialog.show();

}

这是我用来取消异步任务的方法

 private void CancelDownload() {
    AlertDialog.Builder builder = new AlertDialog.Builder(ctx);
    builder.setCancelable(true);
    builder.setTitle("Thông báo");
    builder.setMessage("Bạn có muốn hủy tải cuốn sách này không?");

    builder.setPositiveButton("Có", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {


            Intent i = new Intent();
            i.setAction(ACTION_FILTER);
            i.putExtra("action", ACTION_CANCEL_NOTIFICATION);
            i.putExtra("BookId", bookId);
            ctx.sendBroadcast(i);

            //  llDownloadItem.setVisibility(View.GONE);
            Boolean checkCache = FunctionHelper.GetCacheDownBook(ctx);
            Log.d("checkCache", checkCache.toString());
            if (checkCache) {

                asynDownload.cancel(true);

                FunctionHelper.UpdateCacheDownBook(ctx, false);
                llDownloadItem.setVisibility(View.GONE);

            }
            File root = new File(LIBOL_ROOT_PATH + proCode);
            FunctionHelper.DeleteRecursive(root);
            modelLibrary.OpenConnectionSQL(ctx);
            Boolean resultDelete = modelLibrary.DeleteBookSQlite(idBook);
            Log.d("delete sqlite", resultDelete.toString());
            modelLibrary.CloseConnection();

            btnDownload.setVisibility(View.VISIBLE);
            btnXemsach.setVisibility(View.GONE);


            Toast.makeText(ctx, "Hủy tải sách thành công", Toast.LENGTH_SHORT).show();

        }
    });

    builder.setNegativeButton("Không", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {

        }
    });

    AlertDialog dialog = builder.create();
    dialog.show();

}

这是进入第二个活动的按钮

btnBack.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            if (TAG.equals("BOOK_IN_LIBRARY") || TAG.equals("LOGIN") || TAG.equals("SEARCH")) {

                Intent iBookinLib = new Intent(ctx, BookInLibraryActivity.class);
                iBookinLib.setFlags(Intent.FLAG_ACTIVITY_NO_HISTORY);
                iBookinLib.putExtra("LibraryId", libId);
                iBookinLib.putExtra("LibraryName", libName);
                iBookinLib.putExtra("LibraryDomain", libDomain);
                PendingIntent pendingIntent = PendingIntent.getActivity(ctx, 0, iBookinLib, PendingIntent.FLAG_UPDATE_CURRENT);
                try {
                    pendingIntent.send();
                } catch (PendingIntent.CanceledException e) {
                    e.printStackTrace();
                }

            } else if (TAG.equals("BOOK_SHELF")) {
                Intent iBookShelf = new Intent(ctx, BookShelfActivity.class);
                iBookShelf.setFlags(Intent.FLAG_ACTIVITY_NO_HISTORY);
                PendingIntent pendingIntent = PendingIntent.getActivity(ctx, 0, iBookShelf, PendingIntent.FLAG_UPDATE_CURRENT);
                try {
                    pendingIntent.send();
                } catch (PendingIntent.CanceledException e) {
                    e.printStackTrace();
                }
            }
        }
    });

---

检查空指针是最简单的解决方案

if (checkCache && asynDownload != null) {

                asynDownload.cancel(true);

                FunctionHelper.UpdateCacheDownBook(ctx, false);
                llDownloadItem.setVisibility(View.GONE);

            }

编辑：

---

将asyctask声明为静态对象，以便在从其他屏幕返回时将该对象保留在内存中。

检查空指针是最简单的解决方案

if (checkCache && asynDownload != null) {

                asynDownload.cancel(true);

                FunctionHelper.UpdateCacheDownBook(ctx, false);
                llDownloadItem.setVisibility(View.GONE);

            }

编辑：

---

---

将asyctask声明为静态对象，以便在从其他屏幕返回时将该对象保留在内存中。

在取消任务之前，您可以检查异步任务是否为null检查您传递的参数是否为null也显示一些活动代码，以便我们可以获取您的案例在

cancelButton.setOnClickListener{if（asyncDownload！=null）{CancelDownload（）；}否则{//可能是错误处理}

@sharatikka我将我的按钮更改发布到第二个活动中，谢谢您在取消任务之前检查异步任务是否为空检查您传递的参数是否为空也显示一些活动代码以便我们可以获取您的案例在

cancelButton.setOnClickListener{if（asyncDownload！=null）{CancelDownload（）中尝试此操作；}否则{//可能是错误处理}

@Sharatika我将我的按钮更改发布到第二个活动，谢谢。但是如果他想取消异步任务，空检查在这种情况下不会有帮助。谢谢，但我的意思是，我想在更改到第二个活动并返回第一个活动时取消下载。你的代码无法取消我的异步任务，我以前是这样做的。你能尝试将异步任务声明为静态对象谢谢，我尝试将asynctask声明为静态对象，但没有效果。也许您可以为此目的使用应用程序类。您应该首先创建自定义应用程序类，然后为asynctask创建公共对象，并创建getter和setter。之后，您可以在每次需要访问asynctask时都使用它但是如果他想取消AsyntTask，空检查在这种情况下不会有帮助谢谢，但我的意思是，我想取消下载，同时更改为第二个活动并返回第一个活动。您的代码不能取消我的AsyntTask，我曾经这样做过。您可以尝试将asynctask声明为静态对象吗谢谢，我尝试将asynctask声明为static对象，但没有效果可能您可以为此目的使用应用程序类。您应该首先创建自定义应用程序类，然后为asynctask创建公共对象，并创建getter和setter。之后，您可以在每次需要访问asynctask实例时都使用它。