AsyncTask有时只在Android中起作用

我有一个从url返回数组的异步任务。它80%的执行时间都能正常工作，但另20%的执行时间会使应用程序崩溃。当我点击活动并点击后退按钮10次时，应用程序第5次开始崩溃。以下是相关代码和日志cat

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.friend_picker);

        ActionBar mainBar = getActionBar();
        mainBar.hide();

        arraylist = new ArrayList<FacebookFriend>();

        OpenSession();

        checkButtonClick();
        pickAllButton();
    }

public void OpenSession() {
        Session.openActiveSession(this, true, new Session.StatusCallback() {

            // Calls whenever it changes state.
            @SuppressWarnings("deprecation")
            @Override
            public void call(Session session, SessionState state,
                    Exception exception) {
                if (session.isOpened()) {
                    Request.executeMeRequestAsync(session, new Request.GraphUserCallback() {
                        @Override
                        public void onCompleted(GraphUser user, Response response)
                        {
                            getOwnId = user.getId();  
                        }
                    });

                    Request.executeMyFriendsRequestAsync(session, new Request.GraphUserListCallback() {

                        @Override
                        public void onCompleted(List<GraphUser> users, Response response) {

                                ArrayList<String> checkedFriend = null;
                                try {
                                    checkedFriend = new getFriendAsync().execute().get();
                                } catch (InterruptedException e) {
                                    // TODO Auto-generated catch block
                                    e.printStackTrace();
                                } catch (ExecutionException e) {
                                    // TODO Auto-generated catch block
                                    e.printStackTrace();
                                }

                            if (users != null) {
                                for (GraphUser user : users) {
                                    String userURL = "http://graph.facebook.com/"+user.getId().toString()+"/picture?width=100&height=100";

                                    fbFriend = new FacebookFriend(user.getId().toString(), user.getName().toString(), userURL, false);

                                    if(checkedFriend.contains(user.getId().toString()))
                                        fbFriend.setSelected(true);

                                    arraylist.add(fbFriend);
                                }
                                Collections.sort(arraylist);

                                listview = (ListView) findViewById(R.id.mainListView);
                                adapter = new ListViewAdapterFb(FriendPicker.this,R.layout.simplerow,arraylist);
                                listview.setAdapter(adapter);  
                            }
                        }
                    });
                }
            }
        });

private class getFriendAsync extends AsyncTask<Void, Void, ArrayList<String>> {

        @Override
        protected void onPreExecute() {
        }

        @Override
        protected ArrayList<String> doInBackground(Void... params) {  

            String jsonobject;
            JSONArray jsonA;
            ArrayList<String> list = null;
            try {
                jsonobject = JSONfunctions.getJSONfromURL("http://tikipass.com/get_followed.php?id=" + getOwnId);
                jsonA = new JSONArray(jsonobject);
                list = new ArrayList<String>();
                for(int i = 0; i < jsonA.length(); i++) {
                    list.add(jsonA.getString(i));
                }
            } catch (JSONException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            return list;
        }

        protected void onPostExecute(ArrayList<String> args) {
        }
    }

---

首先，

AsyncTask

的全部目的在这里被破坏了

new getFriendAsync().execute().get();

您只需调用

newgetFriendAsync（）.execute（）

并将剩余代码移动到

onPostExecute（）

其次，

onCompleted（）

可能在

Activity

不再处于活动状态时被调用，因此您可以在尝试访问GUI时获得空指针异常。 因此，只需在访问任何UI元素之前添加空检查：

listview = (ListView) findViewById(R.id.mainListView);
if(listview != null){    
    adapter = new ListViewAdapterFb(FriendPicker.this,R.layout.simplerow,arraylist);
    listview.setAdapter(adapter);  
}

listview = (ListView) findViewById(R.id.mainListView);
if(listview != null){    
    adapter = new ListViewAdapterFb(FriendPicker.this,R.layout.simplerow,arraylist);
    listview.setAdapter(adapter);  
}