SharedPreferences getString returns null though set by editor in AsyncTask

问题

I have a LoginActivity which calls an AsyncTask to post username and password to the server and on response, it will write username to SharedPreferences (can retrieve the username from SP here) and return to may app's MainActivity. However, I retrieve the username from SP in MainActivity, it is null. Where did I do wrong?

AsyncTask code:

//activity is the context passed from LoginActivity (using "this")
protected void onPostExecute(String result) {
    if(result!=null){
        try {
            JSONTokener jsonParser = new JSONTokener(result);  
            JSONObject msg = (JSONObject) jsonParser.nextValue();
            String data=msg.getString("data");
            int code=Integer.parseInt(msg.getString("code"));
            if(code==0){
                //Write to local storage
                SharedPreferences settings = activity.getPreferences(Context.MODE_PRIVATE);
                SharedPreferences.Editor editor = settings.edit();
                JSONTokener dataParser = new JSONTokener(data);  
                JSONObject dataJSON=(JSONObject) dataParser.nextValue();
                editor.putString("acc_username", dataJSON.getString("username"));
                if(dataJSON.getString("token")!=null){
                    editor.putString("acc_token", dataJSON.getString("token"));
                }
                editor.commit();
                //Log.d("TEST",activity.getPreferences(Context.MODE_PRIVATE).getString("acc_username", null));//I can get the username here -- means the post and login request and response are correct.
                Intent intent=new Intent(activity, MainActivity.class);
                intent.setFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP);
                activity.startActivity(intent);     
            }
        } catch (JSONException e) {
            e.printStackTrace();
        } 
    }
}

MainActivity code:

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    //read login info from device
    SharedPreferences settings = getPreferences(MODE_PRIVATE);
    String accUsername = settings.getString("acc_username", null);
    Log.d("TEST","accUsername="+accUsername);//it is null!
}

Manifest permissions:

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.GET_ACCOUNTS" />

回答1:

The problem is that you use the wrong method: getPreferences (int mode). Please refer to the document: http://developer.android.com/reference/android/app/Activity.html#getPreferences(int)

"This simply calls the underlying getSharedPreferences(String, int) method by passing in this activity's class name as the preferences name."

So, in your code, the Preferences file name you used to save your data may be "LoginActivity" because you save it in your LoginActivity class. Then when you get that data in your MainActivity class by using getPreferences (int mode) means you want to get data from a Preferences file named "MainAcitity". So, you must get a null.

How to solve your problem: Use the getSharedPreferences(String, int) method instead of getPreferences (int mode) and give the save Preferences file name in your two Activity class.

回答2:

Use getSharedPreference method instead of getPreference.

来源：https://stackoverflow.com/questions/25968482/sharedpreferences-getstring-returns-null-though-set-by-editor-in-asynctask

标签

android

sharedpreferences