Volley AppController class object returning null

问题

I am making an app that makes a JsonObjectRequest and retrieves a JSON data from an URL using the Volley Networking Library for android.

AppController.java

public class AppController extends AppCompatActivity {

    public static final String TAG = AppController.class.getSimpleName();

    private RequestQueue mRequestQueue;

    private static AppController mInstance;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mInstance = this;
    }

    public static synchronized AppController getInstance(){
        return mInstance;
    }

    public RequestQueue getRequestQueue(){
        if(mRequestQueue == null){
            mRequestQueue = Volley.newRequestQueue(getApplicationContext());
        }

        return mRequestQueue;
    }

    public <T> void addToRequestQueue(Request<T> req) {
        req.setTag(TAG);
        getRequestQueue().add(req);
    }
}

Method in MainActivity.class

private void makeJSONObjectRequest() {

        showDialog();

        JsonObjectRequest jsonObjReq = new JsonObjectRequest(Request.Method.GET,
                urlJsonObj, (String) null, new Response.Listener<JSONObject>() {

            @Override
            public void onResponse(JSONObject response) {
                Log.d(TAG, response.toString());

                try {

                    //Parsing JSON Object response
                    String name = response.getString("name");
                    String email = response.getString("email");
                    JSONObject phone = response.getJSONObject("phone");
                    String home = phone.getString("home");
                    String mobile = phone.getString("mobile");

                    jsonResponse = "";
                    jsonResponse += "Name: " + name + "\n\n";
                    jsonResponse += "Email: " + email + "\n\n";
                    jsonResponse += "Home: " + home + "\n\n";
                    jsonResponse += "Mobile: " + mobile + "\n\n";

                    txtResponse.setTag(jsonResponse);

                }

                catch (JSONException e) {
                    e.printStackTrace();
                    Toast.makeText(getApplicationContext(), "Error: " + e.getMessage(), Toast.LENGTH_SHORT).show();
                }

                hideDialog();
            }
        },

        new ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError volleyError) {
                VolleyLog.d(TAG+"Error:"+ volleyError.getMessage());
                Toast.makeText(getApplicationContext(), volleyError.getMessage(), Toast.LENGTH_SHORT).show();
                hideDialog();
            }
        });

        /*THE ERROR OCCURS HERE! */
        //adding request to the RequestQueue
        AppController.getInstance().addToRequestQueue(jsonObjReq);
    }

It gives the following error:

java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.h8pathak.volleyjson.AppController.addToRequestQueue(com.android.volley.Request)' on a null object reference

How can I rectify this code?

回答1:

Your AppController class needs to extend Application class instead of the AppCompatActivity class.

And remember to update your Manifest as well. ie. Add this class in your AndroidManifest.xml using name attribute for <application> tag.

<application
        android:name=".AppController"/>

回答2:

you can't use an Activity like a Singleton. An Activity is a screen of your app and it could be in different states during the usage of your app. You are also leaking it, since you keep a static reference to it. For your purpose, if you need a Context, extend Application instead of AppCompatActivity, and register it in your Manifest.

回答3:

i think you should create the "AppController" like this :

public class AppController {

    private static AppController mInstance;

    private RequestQueue mRequestQueue;

    private static Context mCtx;

    private AppController(Context context){
        mCtx = context;
        mRequestQueue = getRequestQueue();
    }

    public static synchronized AppController getInstance(Context context) {
        if (mInstance == null) {
            mInstance = new AppController(context);
        }
        return mInstance;
    }

    public RequestQueue getRequestQueue() {
        if (mRequestQueue == null) {
            mRequestQueue = Volley.newRequestQueue(mCtx.getApplicationContext());
        }
        return mRequestQueue;
    }

    public <T> void addToRequestQueue(@NonNull final Request<T> request) {
        getRequestQueue().add(request);
    }

    public <T> void addToRequestQueueWithTag(@NonNull final Request<T> request, String tag) {
        request.setTag(tag);
        getRequestQueue().add(request);
    }
}

and the MainActivity.class

//adding request to the RequestQueue
AppController.getInstance(this).addToRequestQueue(jsonObjReq);

回答4:

Don't you forget to initialize the RequestQueue Object. You need to initialize the RequestQueue inside the onCreate method, like you can see in the example:
(Else when you call request.add(jsonObjectRequest) the application will try to reference the null object)

RequestQueue request;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    //request qwe
    request= Volley.newRequestQueue(this);
}

来源：https://stackoverflow.com/questions/34479808/volley-appcontroller-class-object-returning-null