尝试在ActionbarActivity的Fragment中为SharedPreferences的空对象引用调用虚方法

我的应用需要在后台访问共享首选项文件.自从我开始使用v21支持库的actionbaractivity来保存我的片段后,我的app力就关闭了.每当片段离开屏幕时强制关闭,但异步任务仍在运行并尝试访问该sharedpreferences文件.为什么要抛出这个错误,我该如何解决？

这是堆栈跟踪:

12-22 23:49:49.469: E/AndroidRuntime(23016): FATAL EXCEPTION: AsyncTask #4
12-22 23:49:49.469: E/AndroidRuntime(23016): Process: com.bernard.beaconportal.activities, PID: 23016
12-22 23:49:49.469: E/AndroidRuntime(23016): java.lang.RuntimeException: An error occured while executing doInBackground()
12-22 23:49:49.469: E/AndroidRuntime(23016):    at android.os.AsyncTask$3.done(AsyncTask.java:300)
12-22 23:49:49.469: E/AndroidRuntime(23016):    at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
12-22 23:49:49.469: E/AndroidRuntime(23016):    at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
12-22 23:49:49.469: E/AndroidRuntime(23016):    at java.util.concurrent.FutureTask.run(FutureTask.java:242)
12-22 23:49:49.469: E/AndroidRuntime(23016):    at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
12-22 23:49:49.469: E/AndroidRuntime(23016):    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
12-22 23:49:49.469: E/AndroidRuntime(23016):    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
12-22 23:49:49.469: E/AndroidRuntime(23016):    at java.lang.Thread.run(Thread.java:818)
12-22 23:49:49.469: E/AndroidRuntime(23016): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.SharedPreferences android.support.v4.app.FragmentActivity.getSharedPreferences(java.lang.String, int)' on a null object reference
12-22 23:49:49.469: E/AndroidRuntime(23016):    at com.bernard.beaconportal.activities.Due_Tommorow_Fragment$Update.doInBackground(Due_Tommorow_Fragment.java:815)
12-22 23:49:49.469: E/AndroidRuntime(23016):    at com.bernard.beaconportal.activities.Due_Tommorow_Fragment$Update.doInBackground(Due_Tommorow_Fragment.java:1)
12-22 23:49:49.469: E/AndroidRuntime(23016):    at android.os.AsyncTask$2.call(AsyncTask.java:288)
12-22 23:49:49.469: E/AndroidRuntime(23016):    at java.util.concurrent.FutureTask.run(FutureTask.java:237)
12-22 23:49:49.469: E/AndroidRuntime(23016):    ... 4 more

以下是发生错误的特定代码行:

    SharedPreferences userName = getActivity().getSharedPreferences(
            "Login_Info", Context.MODE_PRIVATE);

这是发生错误的异步任务:

public class Update extends AsyncTask {

        private final HttpClient Client = new DefaultHttpClient();

        @Override
        protected Void doInBackground(String... urls) {
            SharedPreferences bDay = getActivity().getSharedPreferences(
                    "Login_Info", Context.MODE_PRIVATE);

            String day1 = Integer.toString(bDay.getInt("Day", 0));

            String year1 = Integer.toString(bDay.getInt("Year", 0));

            String month1 = Integer.toString(1 + bDay.getInt("Month", 0));

            SharedPreferences userName = getActivity().getSharedPreferences(
                    "Login_Info", Context.MODE_PRIVATE);

            String day = day1.replaceFirst("^0+(?!$)", "");

            String month = month1.replaceFirst("^0+(?!$)", "");

            String year = year1.replaceFirst("^0+(?!$)", "");

            String birthday = month + "/" + day + "/" + year;

            System.out.println("Birthday = " + birthday);

            String user = userName.getString("username", "");

            // String user = (username).split("@")[0];

            System.out.println("Username = " + user);

            try {

                // HttpClient httpClient = new DefaultHttpClient();
                HttpContext localContext = new BasicHttpContext();
                // HttpGet httpGet = new HttpGet(
                // "http://www.beaconschool.org/~markovic/lincoln.php");

                HttpClient httpclient = new DefaultHttpClient();
                HttpPost httppost = new HttpPost(
                        "http://www.beaconschool.org/~markovic/lincoln.php");

                try {
                    // Add your data
                    List nameValuePairs = new ArrayList(
                            2);
                    nameValuePairs
                            .add(new BasicNameValuePair("username", user));
                    nameValuePairs.add(new BasicNameValuePair("birthday",
                            birthday));
                    httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));

                    // Execute HTTP Post Request
                    response = httpclient.execute(httppost);

                    Log.d("Http Response:", response.toString());

                } catch (ClientProtocolException e) {
                    // TODO Auto-generated catch block
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                }

                try {
                    Log.d("receiver", "animation stopped and downloaded file");

                    String homework = new Scanner(response.getEntity()
                            .getContent(), "UTF-8").useDelimiter("\\A").next();

                    // String homework =
                    // Html.fromHtml(duetommorow_html).toString();

                    SharedPreferences.Editor localEditor = getActivity()
                            .getSharedPreferences("homework",
                                    Context.MODE_PRIVATE).edit();

                    SimpleDateFormat dateFormat = new SimpleDateFormat(
                            "MM/dd hh:mm a");
                    Calendar cal = Calendar.getInstance();
                    String downloaded = dateFormat.format(cal.getTime());

                    localEditor.putString("homework_content", homework);

                    localEditor.putString("download_date", downloaded);

                    localEditor.apply();

                    localEditor.putString("homework_content", homework);

                    localEditor.apply();

                    Log.d("receiver", "information given to shared preferences");

                    due_tommorow_list.clear();

                    parse_due_tommorow_string();

                    parse_due_tommorow_content();

                } catch (IllegalStateException e) {

                    e.printStackTrace();
                } catch (IOException e) {

                    e.printStackTrace();
                } catch (NullPointerException e) {

                    due_tommorow_list.clear();

                    parse_due_tommorow_string();

                    parse_due_tommorow_content();

                    SharedPreferences.Editor localEditor = getActivity()
                            .getSharedPreferences("homework",
                                    Context.MODE_PRIVATE).edit();

                    localEditor.putString("download_error", "yes");

                    localEditor.apply();

                    e.printStackTrace();
                }

                catch (NoSuchElementException e) {

                    due_tommorow_list.clear();

                    parse_due_tommorow_string();

                    parse_due_tommorow_content();

                    SharedPreferences.Editor localEditor = getActivity()
                            .getSharedPreferences("homework",
                                    Context.MODE_PRIVATE).edit();

                    localEditor.putString("download_error", "yes");

                    localEditor.apply();

                    e.printStackTrace();
                }

                catch (RuntimeException e) {

                    due_tommorow_list.clear();

                    parse_due_tommorow_string();

                    parse_due_tommorow_content();

                    SharedPreferences.Editor localEditor = getActivity()
                            .getSharedPreferences("homework",
                                    Context.MODE_PRIVATE).edit();

                    localEditor.putString("download_error", "yes");

                    localEditor.apply();

                    e.printStackTrace();
                }

            } finally {

            }
            return null;

        }

        @Override
        protected void onPostExecute(Void result) {

            swipeLayout.setRefreshing(false);

            Log.d("sender", "Broadcasting message");

            Intent intent = new Intent("up_navigation");

            intent.putExtra("message", "This is my message!");
            LocalBroadcastManager.getInstance(getActivity()).sendBroadcast(
                    intent);

            Toast.makeText(getActivity(), "Refresh Finished", 4000).show();

            SharedPreferences download_error = getActivity()
                    .getSharedPreferences("homework", Context.MODE_PRIVATE);

            String error = download_error.getString("download_error", "no");

            String download_date = "Download error, refreshed homework using homework downloaded at "
                    + download_error.getString("download_date", "");

            if (error.equals("yes")) {

                SharedPreferences.Editor localEditor = getActivity()
                        .getSharedPreferences("homework", Context.MODE_PRIVATE)
                        .edit();

                Toast.makeText(getActivity(), download_date, Toast.LENGTH_LONG)
                        .show();

                localEditor.putString("download_error", "no");

                localEditor.commit();

            }

            adapter.notifyDataSetChanged();

            swipeLayout.setRefreshing(false);

        }

    }

Wesley.. 10

你做了一些耗时的事情AsyncTask,当getActivity()部件被执行时,Activity就被销毁了.所以引用getActivity()返回null,这是非常正确的.

你应该做的是在你的Thread或者AsyncTask你应该总是检查是否getActivity()为null,以防Activity你的主机Fragment被销毁.

我不认为@ I'm_With_Stupid的答案是一个很好的实践,因为Activity在你的Fragment片段中保留一个参考是不好的,片段已经为你做了.