What is the purpose of the return statement in ASynctask's doInBackground()?

问题

This aspect of my login system works just fine if I have the return statement set to 0 or 1, but fails if I use null. This is all adapted from http://256design.com/blog/android-login-asynctask/ where this particular return looks as listed below my own code.

public LoginTask(Polling activity, ProgressDialog progressDialog)
    {
        this.activity = activity;
        this.progressDialog = progressDialog;
    }    

protected Integer doInBackground(String... arg0) {
            EditText userName = (EditText)activity.findViewById(R.id.emailEditText);
            EditText passwordEdit = (EditText)activity.findViewById(R.id.passEditText);


            String email = userName.getText().toString();
            String password = passwordEdit.getText().toString();
            UserFunctions userFunction = new UserFunctions();
            JSONObject json = userFunction.loginUser(email, password);
            progressDialog.dismiss();
            // check for login response
            //Log.v("test", Integer.toString(jsonParser.getResponseCode()));
            try {
                if (json.getString(KEY_SUCCESS) != null) {
                    //loginErrorMsg.setText("");
                    //loginFragment.loginErrorMsg.setText("Success");
                    String res = json.getString(KEY_SUCCESS);

                    if(Integer.parseInt(res) == 1){
                        //user successfully logged in
                        // Store user details in SQLite Database
                        DatabaseHandler db = new DatabaseHandler(activity.getApplicationContext());
                        JSONObject json_user = json.getJSONObject("user");
                        //Log.v("name", json_user.getString(KEY_NAME));
                        // Clear all previous data in database
                        userFunction.logoutUser(activity.getApplicationContext());
                        db.addUser(json_user.getString(KEY_NAME), json_user.getString(KEY_EMAIL), 
                                json.getString(KEY_UID), json_user.getString(KEY_CREATED_AT));                        


                        // Close Login Screen
                        //finish();
                        //loginErrorMsg = (TextView)activity.findViewById(R.id.loginErrorMsg);
                        //loginErrorMsg.setText("logged in");
                        //passwordEdit.setText("");
                    }else{
                        // Error in login
                        //progressDialog.setMessage("Incorrect username or password");
                        //loginErrorMsg.setText("Incorrect username/password");
                    }

                }

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

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

            return 1;
        }

The tutorial that I used, take a look at responseCode:

protected Integer doInBackground(String... arg0) 
{
    String result = "";
    int responseCode = 0;
    try 
    {
        HttpClient client = new DefaultHttpClient();
        HttpPost httppost = new HttpPost("http://www.256design.com/projectTransparency/project/headerLogin.php");

        List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>();
            nameValuePairs.add(new BasicNameValuePair("emailAddress", arg0[0]));
            nameValuePairs.add(new BasicNameValuePair("password", arg0[1]));
        httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));

        int executeCount = 0;
        HttpResponse response;
        do
        {
            progressDialog.setMessage("Logging in.. ("+(executeCount+1)+"/5)");
            // Execute HTTP Post Request
            executeCount++;
            response = client.execute(httppost);
            responseCode = response.getStatusLine().getStatusCode();                        
            // If you want to see the response code, you can Log it
            // out here by calling:
            // Log.d("256 Design", "statusCode: " + responseCode)
        } while (executeCount < 5 && responseCode == 408);

        BufferedReader rd = new BufferedReader(new InputStreamReader(
                response.getEntity().getContent()));

        String line;
        while ((line = rd.readLine()) != null)
        {
            result = line.trim();
        }
        id = Integer.parseInt(result);
    }
    catch (Exception e) {
        responseCode = 408;
        e.printStackTrace();
    }
    return responseCode;
}

回答1:

The purpose is to pass the result of your job (which is executed on a worker thread) to onPostExecute, in order to process the result on the UI thread. This is required if you want to update the user interface in response to a successful job run.

回答2:

DoInBackground return value to postExecute method, and passing null does not validate condition:

if(headerCode == 202)
activity.login(id);

回答3:

Yes its the value you send to postExecute:

http://developer.android.com/reference/android/os/AsyncTask.html#onPostExecute(Result)

protected void onPostExecute(Result result)

Runs on the UI thread after doInBackground(Params...).

The specified result is the value returned by doInBackground(Params...).

This method won't be invoked if the task was cancelled.

Parameters: result The result of the operation computed by doInBackground(Params...).

回答4:

protected class InitTask extends AsyncTask<Context, Integer, Integer>

In the line above we define our sub-class and the three parameters that will be passed to the callbacks. The callbacks look like this:

doInBackground()

@Override
protected Integer doInBackground( Context... params )  {
     return super.doInBackground( params )
}

Anything processed in this method is handled in a sperate thread. Note that the data type of the return value is an Integer and corresponds to the type third parameter in the class definition. This value returned from this method is passed to the onPostExecute() method when this thread completes.

And if you pass null in return then it fails condition on method onPostExecuted()

来源：https://stackoverflow.com/questions/9938985/what-is-the-purpose-of-the-return-statement-in-asynctasks-doinbackground