NullPointerException happens in an Android code sn

Here's the code which deals with member registration.

public class RegisterActivity extends Activity {
    private static final String TAG = RegisterActivity.class.getSimpleName();
    private Button btnRegister;
    private Button btnLinkToLogin;
    private EditText inputFullName;
    private EditText inputEmail;
    private EditText inputPassword;
    private ProgressDialog pDialog;
    private SessionManager session;
    private SQLiteHandler db;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_register);

        inputFullName = (EditText) findViewById(R.id.name);
        inputEmail = (EditText) findViewById(R.id.email);
        inputPassword = (EditText) findViewById(R.id.password);
        btnRegister = (Button) findViewById(R.id.btnRegister);
        btnLinkToLogin = (Button) findViewById(R.id.btnLinkToLoginScreen);

        // Progress dialog
        pDialog = new ProgressDialog(this);
        pDialog.setCancelable(false);

        // Session manager
        session = new SessionManager(getApplicationContext());

        // SQLite database handler
        db = new SQLiteHandler(getApplicationContext());

        // Check if user is already logged in or not
        if (session.isLoggedIn()) {
            // User is already logged in. Take him to main activity
            Intent intent = new Intent(RegisterActivity.this,
                MainActivity.class);
            startActivity(intent);
            finish();
        }

        // Register Button Click event
        btnRegister.setOnClickListener(new View.OnClickListener() {
            public void onClick(View view) {
                String name = inputFullName.getText().toString();
                String email = inputEmail.getText().toString();
                String password = inputPassword.getText().toString();

                if (!name.isEmpty() && !email.isEmpty() && !password.isEmpty()) {
                    registerUser(name, email, password);
                 } else {
                    Toast.makeText(getApplicationContext(),
                            "Please enter your details!", Toast.LENGTH_LONG)
                        .show();
                }
            }
        });

        // Link to Login Screen
        btnLinkToLogin.setOnClickListener(new View.OnClickListener() {

            public void onClick(View view) {
                Intent i = new Intent(getApplicationContext(),
                        LoginActivity.class);
                startActivity(i);
                finish();
            }
        });

    }

    /**
     * Function to store user in MySQL database will post params(tag, name,
     * email, password) to register url
     * */
    private void registerUser(final String name, final String email,
                          final String password) {
        // Tag used to cancel the request
        String tag_string_req = "req_register";

        pDialog.setMessage("Registering ...");
        showDialog();

        StringRequest strReq = new StringRequest(Method.POST,
                AppConfig.URL_REGISTER, new Response.Listener<String>() {

            @Override
            public void onResponse(String response) {
                Log.d(TAG, "Register Response: " + response.toString());
                hideDialog();

                try {
                    JSONObject jObj = new JSONObject(response);
                    boolean error = jObj.getBoolean("error");
                    if (!error) {
                        // User successfully stored in MySQL
                        // Now store the user in sqlite
                        String uid = jObj.getString("uid");

                        JSONObject user = jObj.getJSONObject("user");
                        String name = user.getString("name");
                        String email = user.getString("email");
                        String created_at = user
                            .getString("created_at");

                        // Inserting row in users table
                        db.addUser(name, email, uid, created_at);

                        // Launch login activity
                        Intent intent = new Intent(
                            RegisterActivity.this,
                            LoginActivity.class);
                        startActivity(intent);
                        finish();
                    } else {

                        // Error occurred in registration. Get the error
                        // message
                        String errorMsg = jObj.getString("error_msg");
                        Toast.makeText(getApplicationContext(),
                            errorMsg, Toast.LENGTH_LONG).show();
                    }
                } catch (JSONException e) {
                    e.printStackTrace();
                }

            }
        }, new Response.ErrorListener() {

            @Override
            public void onErrorResponse(VolleyError error) {
                Log.e(TAG, "Registration Error: " + error.getMessage());
                Toast.makeText(getApplicationContext(),
                    error.getMessage(), Toast.LENGTH_LONG).show();
                hideDialog();
            }
        }) {

            @Override
            protected Map<String, String> getParams() {
                // Posting params to register url
                Map<String, String> params = new HashMap<String, String>();
                params.put("tag", "register");
                params.put("name", name);
                params.put("email", email);
                params.put("password", password);

                return params;
            }

        };

        // Adding request to request queue
        AppController.getInstance().addToRequestQueue(strReq, tag_string_req);
    }

    private void showDialog() {
        if (!pDialog.isShowing())
            pDialog.show();
    }

    private void hideDialog() {
        if (pDialog.isShowing())
            pDialog.dismiss();
    }
    }

When I run the code, it throws a NullPointerException, showing the following logs.

07-03 16:13:33.061 24078-24078/com.marshall.gruppo E/AndroidRuntime﹕ FATAL EXCEPTION: main Process: com.marshall.gruppo, PID: 24078 java.lang.NullPointerException at com.marshall.gruppo.ui.RegisterActivity.registerUser(RegisterActivity.java:184) at com.marshall.gruppo.ui.RegisterActivity.access$300(RegisterActivity.java:31) at com.marshall.gruppo.ui.RegisterActivity$1.onClick(RegisterActivity.java:80) at android.view.View.performClick(View.java:4633) at android.view.View$PerformClick.run(View.java:19270) at android.os.Handler.handleCallback(Handler.java:733) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:146) at android.app.ActivityThread.main(ActivityThread.java:5602) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:515) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1283) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1099) at dalvik.system.NativeStart.main(Native Method)

According to the log, the snippet that seems to have the problem is:

AppController.getInstance().addToRequestQueue(strReq, tag_string_req);

Which is the last line of the registerUser method.

What do you think the problem is? If there is any information that you should know further about this code, please let me know.

The AppController class is as follows.

public class AppController extends Application {

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

    private RequestQueue mRequestQueue;

    private static AppController mInstance;

    @Override
    public void onCreate() {
        super.onCreate();
        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, String tag) {
        req.setTag(TextUtils.isEmpty(tag) ? TAG : tag);
        getRequestQueue().add(req);
    }

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

    public void cancelPendingRequests(Object tag) {
        if (mRequestQueue != null) {
            mRequestQueue.cancelAll(tag);
        }
    }
}

Manifest.xml:

<uses-permission android:name="android.permission.INTERNET" />

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name=".ui.LoginActivity"
        android:label="@string/app_name"
        android:launchMode="singleTop"
        android:windowSoftInputMode="adjustPan" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity
        android:name=".ui.RegisterActivity"
        android:label="@string/app_name"
        android:launchMode="singleTop"
        android:windowSoftInputMode="adjustPan" />
    <activity
        android:name=".ui.MainActivity"
        android:label="@string/app_name"
        android:launchMode="singleTop" />
</application>

回答1:

the problem is you didn't declare your Subclass of Application to the AndroidManifest. Add android:name="path.to.AppController" to the Application tag, otherwise your subclass of Application is not instantiated and mInstance remains not initialized (null by default). And that explains the NPE

 <application
    android:name="path.to.AppController"
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >

回答2:

As you can see, AppController is not added to your manifest:

<application
        android:name=".AppController" //this one
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme">

So onCreate never get called and mInstance is null.