异步数据未被清除

Question

所以我目前有3个异步任务正在运行Login，Registration和GetInstitutes。我首先使Login类功能，我知道它工作正常，但添加其他2导致它无法正常工作。因此，当应用程序启动时，它会执行GetInstitutes并返回

{"institute":[{"id":"1","name":"plymouh university"},{"id":"2","name":"University Of Bradford"},{"id":"3","name":"University of West England"}],"success":1}

如果我注册它返回：

{"message":"Invalid email","success":0}

这些运行正常并且JSON对象很好，但是当登录而不是获取登录的JSON对象时，我得到之前的登录之一

class loginAttempt extends AsyncTask<String, String, String>{

    JSONObject json = null;  //reseting the JSON Object to null to try and fix

    @Override
    protected void onPreExecute() {
        //Show progress Dialog
    }

    @Override
    protected String doInBackground(String... params) {

        String usernameSTR = username.getText().toString();
        String passwordSTR = password.getText().toString();


        // Building Parameters
        List<NameValuePair> params1 = new ArrayList<NameValuePair>();
        params1.add(new BasicNameValuePair("username", usernameSTR));
        params1.add(new BasicNameValuePair("password", passwordSTR));



         Log.d("Create Response", json.toString());    //outputs null


         // getting JSON Object
         json = jsonParser.makeHttpRequest(url_login,
                "POST", params1);                      //Performs Login request



        Log.d("Create Response", json.toString());  //outputs original JSON from register or Get Institute not from the JSON above


       //manipulate results
        return null;
    }
    protected void onPostExecute(String file_url) {
                //Dismiss Dialouge and show results
    }
 }