java - 连接android到wamp服务器，出现空指针异常

Question

我正在开发第一个连接到外部数据库的应用程序。我目前正在使用 WAMP 作为数据库，并且正在完成一个为应用程序进行非常基本的登录的教程。我遇到的问题是寄存器 Activity 中的空指针异常。这是我的日志:

09-03 14:13:00.133  14945-14945/? E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.gps.gpsgametest2, PID: 14945
java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.gps.gpsgametest2.app.AppController.addToRequestQueue(com.android.volley.Request, java.lang.String)' on a null object reference
        at com.example.gps.gpsgametest2.RegisterActivity.registerUser(RegisterActivity.java:185)
        at com.example.gps.gpsgametest2.RegisterActivity.access$300(RegisterActivity.java:31)
        at com.example.gps.gpsgametest2.RegisterActivity$1.onClick(RegisterActivity.java:81)
        at android.view.View.performClick(View.java:5191)
        at android.view.View$PerformClick.run(View.java:20916)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:145)
        at android.app.ActivityThread.main(ActivityThread.java:5972)
        at java.lang.reflect.Method.invoke(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:372)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)

这是我收到错误的类，我评论了显示错误的 3 行:

public class RegisterActivity extends Activity { //LINE 31, GETTING AN ERROR
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);
    StrictMode.enableDefaults();

    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);
            } // ^^ THAT IS LINE 81 WHERE THERE IS AN ERROR
            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;
        }

    };

    //BELOW IS LINE 185 WHERE THERE IS AN ERROR
    AppController.getInstance().addToRequestQueue(strReq, tag_string_req);
}

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

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

我不知道为什么这个结果为空。当我第一次开始本教程时，我挑选了其中的某些部分。现在这几乎是逐字逐句的，但仍然不起作用。我什至尝试下载 tut 代码并运行它，它也会引发相同的异常。我已经盯着这个问题看了大约两天了，我想我可能需要一双新的眼睛来告诉我我做错了什么。欢迎任何提示!提前致谢!

编辑:这是AppController类

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);
    }
}

}

Answer 1

我通常会将 get Instance 类写为

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

我会在方法中添加一个写入日志来查看它是否为空